View source for Draft:Original research/Planets

[[Image:Heic0821e.jpg|thumb|right|250px|This annotated image shows key features of the Fomalhaut system, including the newly discovered planet Fomalhaut b, and the dust ring. Credit: Credit: NASA, ESA, and Z. Levay (STScI).]]
A '''planet''' is often thought of as a celestial body moving in an elliptical orbit around a star.

The "annotated image [at right] shows key features of the Fomalhaut system, including the newly discovered planet Fomalhaut b, and the dust ring. Also included are a distance scale and an insert, showing how the planet has moved around its parent star over the course of 21 months. The Fomalhaut system is located approximately 25 light-years from the Earth."<ref name=Levay>{{ cite book
|author=Z. Levay
|title=Annotated illustration of Fomalhaut system
|publisher=NASA
|location=Washington, DC USA
|date=November 13, 2008
|url=http://www.spacetelescope.org/images/heic0821e/
|accessdate=2013-08-29 }}</ref>

The extrasolar planet is in orbit around Fomalhaut and is estimated "to be no more than three times Jupiter's mass ... In 2004, the coronagraph in the High Resolution Camera on Hubble's Advanced Camera for Surveys produced the first-ever resolved visible-light image of a large dust belt surrounding Fomalhaut. It clearly showed that this structure is in fact a ring of protoplanetary debris approximately 21.5 billion miles across with a sharp inner edge. This large debris disk is similar to the Kuiper Belt, which encircles the solar system and contains a range of icy bodies from dust grains to objects the size of dwarf planets, such as Pluto."<ref name=Harrington>{{ cite book
|author=J.D. Harrington
|author2=Ray Villard
|title=Hubble Directly Observes Planet Orbiting Fomalhaut
|publisher=Hubblesite
|location=Baltimore, Maryland USA
|date=November 13, 2008
|url=http://hubblesite.org/newscenter/archive/releases/2008/39/full/
|accessdate=2013-08-29 }}</ref>

"Observations taken 21 months apart by Hubble's Advanced Camera for Surveys' coronagraph show that the object is moving along a path around the star and therefore is gravitationally bound to it. The planet is 10.7 billion miles from the star, or about 10 times the distance of the planet Saturn from the sun."<ref name=Harrington/>

"A follow-up image in 2006 showed that one of the objects is moving through space with Fomalhaut but changed position relative to the ring since the 2004 exposure. The amount of displacement between the two exposures corresponds to an 872-year-long orbit as calculated from Kepler's laws of planetary motion."<ref name=Harrington/>

"The planet mysteriously dimmed by half a stellar magnitude between the 2004 and 2006 observations."<ref name=Harrington/>
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==Plasma objects==
{{main|Plasmas/Plasma objects|Plasma objects}}
[[Image:Solar flare photographed by the third crew 19 December 1973 in the light of ionized helium, using the extreme-ultraviolet spectroheliograph of the U.S. Naval Research Laboratory.jpg|thumb|right|250px|A coronal mass ejection occurs in the upper left from the Sun. Credit: Skylab, NASA.]]
A '''magnetic cloud''' is a transient event observed in the solar wind. It was defined in 1981 by Burlaga et al. 1981 as a region of enhanced magnetic field strength, smooth rotation of the magnetic field vector and low proton temperature <ref name=Burlaga>Burlaga, L. F., E. Sittler, F. Mariani, and R. Schwenn, "Magnetic loop behind an interplanetary shock: Voyager, Helios and IMP-8 observations" in "Journal of Geophysical Research", 86, 6673, 1981</ref>. Magnetic clouds are a possible manifestation of a Coronal Mass Ejection (CME). The association between CMEs and magnetic clouds was made by Burlaga et al. in 1982 when a magnetic cloud was observed by Helios-1 two days after being observed by [Solar Maximum Mission] SMM<ref name=Burlaga82>Burlaga, L. F. et al., "A magnetic cloud and a coronal mass ejection" in "Geophysical Research Letter"s, 9, 1317-1320, 1982</ref>. However, because observations near Earth are usually done by a single spacecraft, many CMEs are not seen as being associated with magnetic clouds. The typical structure observed for a fast CME by a satellite such as [Advanced Composition Explorer] ACE is a fast-mode shock wave followed by a dense (and hot) sheath of plasma (the downstream region of the shock) and a magnetic cloud.
{{clear}}

==Gaseous objects==
{{main|Gases/Gaseous objects|Gaseous objects}}
"The atmospheric flow [of a hot Jupiter] is characterized by a super-rotating equatorial jet, transonic wind speeds, and eastward advection of heat away from the dayside. [There is] a dynamically induced temperature inversion ("stratosphere") on the planetary dayside and ... temperatures at the planetary limb [which] differ systematically from local radiative equilibrium values, a potential source of bias for transit spectroscopic interpretations."<ref name=Rauscher>{{ cite journal
|author=Emily Rauscher
|author2=Kristen Menou
|title=Three-dimensional Modeling of Hot Jupiter Atmospheric Flows
|journal=The Astrophysical Journal
|month=April
|year=2010
|volume=714
|issue=2
|pages=1334-42
|url=http://iopscience.iop.org/0004-637X/714/2/1334
|arxiv=
|bibcode=
|doi=10.1088/0004-637X/714/2/1334
|pmid=
|accessdate=2012-01-17 }}</ref>

Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random.  However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and they are polarized.<ref name=Schmid>{{ cite journal
| author=Schmid, H. M.
|author2=Beuzit, J.-L.
|author3=Feldt, M.
| display-authors=etal
| title=Search and investigation of extra-solar planets with polarimetry
| journal=Direct Imaging of Exoplanets: Science & Techniques. Proceedings of the IAU Colloquium #200
| year=2006
| volume= 1
| issue= C200
| pages=165–170
| bibcode=2006dies.conf..165S
| doi=10.1017/S1743921306009252 }}</ref>

==Aeronomy==
{{main|Atmospheres/Astronomy/Aeronomy|Aeronomy}}
[[Image:Upperatmoslight1.jpg|thumb|right|250px|The composite shows upper atmospheric lightning and electrical discharge phenomena. Credit: [[commons:User:Abestrobi|Abestrobi]].]]
'''Def.''' "the atmosphere of the Earth and the other planets with reference to their chemical composition, physical properties, relative motion, and responses to radiation from space"<ref name=AeronomyWikt>{{ cite book
|title=aeronomy
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=March 26, 2012
|url=http://en.wiktionary.org/wiki/aeronomy
|accessdate=2013-02-22 }}</ref> is called '''aeronomy'''.

Aeronomy is the study of the upper layers of the atmosphere, where dissociation and ionization are important.

'''Aeronomy''' is the science of the upper region of the atmosphere, where dissociation and ionization are important.<ref name=Chapman>{{cite book
|author=Sydney Chapman
|title=The Thermosphere - the Earth's Outermost Atmosphere
|series= Physics of the Upper Atmosphere
|date=1960
|publisher=Academic Press
|isbn=978-0125820509
|pages=4 }}</ref>
{{clear}}

==Liquid objects==
{{main|Liquids/Liquid objects|Liquid objects}}
[[Image:Earth Pacific jul 30 2010.jpg|thumb|right|250px|This is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Credit: Robert Simmon and Marit Jentoft-Nilsen, NASA.]]
The image at right is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.

"Viewed from space, the most striking feature of our planet is the water. In both liquid and frozen form, it covers 75% of the Earth’s surface. It fills the sky with clouds. Water is practically everywhere on Earth, from inside the rocky crust to inside our cells."<ref name=Simmon>{{ cite book
|author=Robert Simmon
|author2=Marit Jentoft-Nilsen
|title=The Water Planet
|publisher=NASA
|location=Washington, DC USA
|date=October 2, 2010
|url=http://earthobservatory.nasa.gov/IOTD/view.php?id=46209
|accessdate=2013-05-29 }}</ref>
{{clear}}

==Rocky objects==
{{main|Rocks/Rocky objects|Rocky objects}}
[[Image:Moon Farside LRO.jpg|thumb|right|250px|This full disk is nearly featureless, a uniform grey surface with almost no dark mare. There are many bright overlapping dots of impact craters. Credit: NASA/GSFC/ASU LRO.]]
'''Def.''' a natural object in the sky especially at night is called an '''astronomical object'''.

'''Def.'''
# a "naturally occurring aggregate of minerals"<ref name=RockWikt>{{ cite book
|author=[[wikt:User:193.203.81.129|193.203.81.129]]
|title=rock
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=11 July 2005
|url=http://en.wiktionary.org/wiki/rock
|accessdate=2012-10-23 }}</ref> or
# any "natural material with a distinctive composition of minerals"<ref name=RockWikt/>
is called a '''rock'''.

'''Def.''' full "of, or abounding in, rocks; consisting of rocks... [l]ike a rock"<ref name=RockyWikt>{{ cite book
|title=rocky
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=August 29, 2012
|url=http://en.wiktionary.org/wiki/rocky
|accessdate=2012-10-23 }}</ref> is called '''rocky'''.

'''Def.''' a rocky, natural object in the sky especially at night is called an '''astronomical rocky object''', or a '''rocky object'''.

Rocky objects are astronomical objects with solid surfaces.

The [[Draft:Moon|Moon]], shown in the image at right, is a rocky object.

==Theoretical planets==

In these clouds, and smaller versions of them, stars and planets are formed. One of the primary fields of study of molecular astrophysics then, is star and planet formation. Molecules may be found in many environments, however, from stellar atmospheres to those of planetary satellites.

'''Def.'''
# any "secondary body in the Solar system that is geologically differentiated or in hydrostatic equilibrium and thus has a planet-like geology"<ref name=PlanetaryObjectWikt>{{ cite book
|title=planetary object
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 18, 2013
|url=https://en.wiktionary.org/wiki/planetary_object
|accessdate=2013-08-30 }}</ref>,
# any "substantial body in orbit around the Sun or other star"<ref name=PlanetaryObjectWikt/>, or
# any "substantial body associated with the development of planets"<ref name=PlanetaryObjectWikt/>
is called a '''planetary object''', or '''planetary body'''.

'''Usage note:'''

"The phrase is often used for an unidentified body known to have complex geology, such as the unknown parent body of a meteorite."<ref name=PlanetaryObjectWikt/>

'''Def.''' "a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit" is called a '''planet'''.<ref name=Christensen>{{ cite book
|author=Lars Lindberg Christensen
|title=IAU 2006 General Assembly: Result of the IAU Resolution votes
|publisher=International Astronomical Union
|date=August 24, 2006
|url=http://www.iau.org/public_press/news/detail/iau0603/
|accessdate=2011-10-30 }}</ref>

The proposed more general definition for a planet in orbit around another star substitutes "a star" for "the Sun" in part (a), keeps part (b), does not contain part (c), and adds "is neither a star nor a satellite of a planet."<ref name=Christensen1/>

'''Def.''' "[a]ll other objects [not a planet or dwarf planet], except satellites, orbiting the Sun" are called collectively '''Small Solar-System Bodies'''.<ref name=Christensen/>

'''Def.''' a wanderer that is a moving light in the sky is called a '''planet'''.<ref name=Christensen/> This is the original description meant by the word "planet".<ref name=Christensen/>

'''Def.''' a "planet which exists outside Earth's solar system"<ref name=ExoplanetWikt>{{ cite book
|title=exoplanet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=August 16, 2013
|url=https://en.wiktionary.org/wiki/exoplanet
|accessdate=2013-08-30 }}</ref> is called an '''exoplanet''', '''exosolar planet''', or '''extrasolar planet'''.

"More such announcements will likely come in the months to follow, as the first space observatory dedicated to hunting exoplanets, called COROT, begins full operation and researchers complete their calculations."<ref name=Hellemans>{{ cite journal
|author=Alexander Hellemans
|title=Dangling a COROT
|journal=Scientific American
|month=
|year=2007
|volume=297
|issue=3
|pages=32
|url=http://www.sciam.com/article.cfm?id=dangling-a-corot
|arxiv=
|bibcode=
|doi=
|pmid=
|accessdate=2013-08-30 }}</ref>

'''Def.''' any "planet of the [[Solar System]] whose orbit is located between the asteroid belt and the [[Stars/Sun|sun]]"<ref name=InnerPlanetWikt>{{ cite book
|title=inner planet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 21, 2013
|url=https://en.wiktionary.org/wiki/inner_planet
|accessdate=2013-08-30 }}</ref> is called an '''inner planet'''.

'''Def.''' any "planet of the [[Solar System]] whose orbit is located beyond the asteroid belt"<ref name=OuterPlanetWikt>{{ cite book
|title=outer planet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 16, 2013
|url=https://en.wiktionary.org/wiki/outer_planet
|accessdate=2013-08-30 }}</ref> is called an '''outer planet'''.

'''Def.'''
# "a planet with a [[Draft:Moon|moon]] large enough to be considered a planet, were the planet not there, having a substantial portion of the system's overall mass"<ref name=DoublePlanetWikt>{{ cite book
|title=double planet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 10, 2012
|url=https://en.wiktionary.org/wiki/double_planet
|accessdate=2013-08-30 }}</ref>,
# "a pair of [[wikt:planemo|planemos]] in orbit around each other, where the [[wikt:barycenter|barycenter]] of their orbits lies outside of both planemos, which in turn is in orbit about a star"<ref name=DoublePlanetWikt/>,
# "a pair of planemos in orbit around each other, where the barycenter of their orbits lies outside of both planemos"<ref name=DoublePlanetWikt/>,
# "a planemo with a moon large enough to be considered a planemo, having a substantial portion of the system's overall mass"<ref name=DoublePlanetWikt/>,
is called a '''double planet'''.

'''Def.''' an "astronomical object, approximately the size of the [[Draft:Moon|Moon]], formed from the mutual gravitational attraction of planetesimals"<ref name=ProtoplanetWikt>{{ cite book
|title=protoplanet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=July 31, 2013
|url=https://en.wiktionary.org/wiki/protoplanet
|accessdate=2013-08-30 }}</ref> is called a '''protoplanet'''.

==Pebble accretion theory==

Leftover "star-forming material assembles as loose collections of dust and pebbles. Already-formed planetesimals swim among them, and then grow quickly by accumulating more pebbles, much as a snowball gets bigger as it rolls downhill. In this scenario, a planet would start out at the edges of a star’s natal disk and gather up pebbles as it migrates inwards. Depending on gravitational interactions between worlds, it could end up either very close to its host star, or far removed from it."<ref name=Johansen>{{ cite web
|author=Anders Johansen
|author2=Michiel Lambrechts
|title=These dusty young stars are changing the rules of planet-building
|publisher=Nature
|location=
|date=4 December 2018
|url=https://www.nature.com/articles/d41586-018-07591-8?utm_source=Nature+Briefing&utm_campaign=560158b08c-briefing-dy-20190808&utm_medium=email&utm_term=0_c9dfd39373-560158b08c-43855389
|accessdate=10 August 2019 }}</ref>

"Jupiter and Saturn might have undergone such a migration early in the life of the Solar System."<ref name=Johansen/>

"These dark rings probably have young planets in them. This has been really inspirational for us. It’s confirmation, in a way, that planets grow really fast."<ref name=Clement>{{ cite web
|author=Matthew Clement
|title=These dusty young stars are changing the rules of planet-building
|publisher=Nature
|location=
|date=4 December 2018
|url=https://www.nature.com/articles/d41586-018-07591-8?utm_source=Nature+Briefing&utm_campaign=560158b08c-briefing-dy-20190808&utm_medium=email&utm_term=0_c9dfd39373-560158b08c-43855389
|accessdate=10 August 2019 }}</ref>

==Brown dwarfs==
{{main|Stars/Dwarfs/Browns|Brown dwarfs}}
[[Image:Brown Dwarf Gliese 229B.jpg|thumb|250px|right|This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. The brown dwarf, called Gliese 229B, is about 20 to 50 times the mass of Jupiter. Credit: .]]
'''Brown dwarfs''' are sub-stellar objects that have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest-mass stars; this upper limit is between 75<sup>[1]</sup> and 80 Jupiter masses (<math>M_J</math>).

Astronomers have reported that spectral class T brown dwarves (the ones with the coolest temperatures) are colored magenta because of absorption by sodium and potassium atoms of light in the green portion of the spectrum.<ref name=darkstar>[http://www.darkstar1.co.uk/ds3.htm Brown Dwarves (go halfway down the website to see a picture of a magenta brown dwarf):]</ref><ref name=Burrows>Burrows et al. The theory of brown dwarfs and extrasolar giant planets. Reviews of Modern Physics 2001; 73: 719-65</ref><ref name=Hurt>http://spider.ipac.caltech.edu/staff/davy/2mass/science/comparison.html > "An Artist's View of Brown Dwarf Types" Dr. Robert Hurt of the Infrared Processing and Analysis Center</ref>

"2MASS J10475385+2124234 [is] a brown dwarf more than 33 light-years away in the constellation Leo. The dwarf ... has a surface temperature of just ... 900 Kelvin ... Jupiter's lights are linked to its rapid rotation ... Since brown dwarfs are comparable in size to Jupiter, the brown dwarf flare mechanisms might arise similarly. ... [When] first examined using the ... fixed radio dish at Arecibo Observatory in Puerto Rico In several observations, ... flares of radio activity [occurred] ... [Using the] Karl G. Jansky Very Large Array (VLA) of telescopes ... The radio waves emanating ... are about 4.5 times fainter than the previous record ... observing ... LPP 944-20."<ref name=Howell>{{ cite book
|author=Elizabeth Howell
|title=Faint Radio Signals Reveal Secrets of Failed Stars
|publisher=Yahoo! News
|location=
|date=January 31, 2013
|url=http://news.yahoo.com/faint-radio-signals-reveal-secrets-failed-stars-120226595.html;_ylt=AsmcA384JRfb2fZ.CJP.EUeHgsgF;_ylu=X3oDMTRlZTAybzNsBG1pdANUb3BTdG9yeSBTY2llbmNlU0YgU3BhY2VBc3Ryb25vbXlTU0YEcGtnAzJkMWRmMTExLWUxYTItMzI0Yi04OTk1LTMxZDY1ZTcyMjM1OQRwb3MDNwRzZWMDdG9wX3N0b3J5BHZlcgM2ZTMwZjMxMC02YjlmLTExZTItYmZlOS0yNzM5NDllNmRiNWQ-;_ylg=X3oDMTI1MG9icjRhBGludGwDdXMEbGFuZwNlbi11cwRwc3RhaWQDBHBzdGNhdANzY2llbmNlfHNwYWNlLWFzdHJvbm9teQRwdANzZWN0aW9ucw--;_ylv=3
|accessdate=2013-01-31 }}</ref>

Some of the incontrovertible brown dwarf substellar objects are identified by the presence of the 670.8&nbsp;nm lithium [I] line. The most notable of these objects was [[w:Gliese 229|Gliese 229]]B, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of [[Saturn]]'s moon, [[Draft:Titan|Titan]]. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype. [[w:Lithium|Lithium]] is generally present in brown dwarfs and not in low-mass stars. [T]he presence of the lithium [[w:spectral line|line]] in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the '''lithium test''' Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planetlike temperatures (under 1000&nbsp;K).
{{clear}}

==Sub-brown dwarfs==
{{main|Stars/Dwarfs/Sub-browns|Sub-brown dwarfs}}
A '''sub-brown dwarf''' is an astronomical object of [[w:planemo|planetary mass]] that is not orbiting a star and is not considered to be a [[w:brown dwarf|brown dwarf]] because its mass is below the limiting mass of about 13 [[w:Jupiter mass|Jupiter masses]]).<ref name=Working>[http://www.dtm.ciw.edu/boss/definition.html Working Group on Extrasolar Planets - Definition of a "Planet"] POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)</ref> Sub-brown dwarfs are formed in the manner of stars, through the collapse of a [[w:nebula|gas cloud]] (perhaps with the help of [[w:photo-erosion|photo-erosion]]), and not through accretion or core collapse from a [[w:protoplanetary disk|circumstellar disc]] although not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.<ref name=Britt>[http://www.space.com/1023-fresh-debate-photo-extrasolar-planet.html Fresh Debate over First Photo of Extrasolar Planet], by Robert Roy Britt, 30 April 2005</ref> The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 M<sub>J</sub>.<ref name=Boss>[http://adsabs.harvard.edu/full/2003IAUS..211..529B Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?], Brown Dwarfs, Proceedings of IAU Symposium #211, held 20–24 May 2002 at University of Hawaii, Honolulu, Boss, A. P., Basri, G., Kumar, S. S., Liebert, J., Martín, E. L., Reipurth, B</ref> This is because to collapse by [[w:gravity|gravitational]] contraction requires radiating away energy as heat and this is limited by the opacity of the gas.<ref name=Scholz>[http://arxiv.org/abs/0907.2243v1 SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC): THE BOTTOM OF THE INITIAL MASS FUNCTION IN NGC 1333], Alexander Scholz, Vincent Geers, Ray Jayawardhana, Laura Fissel, Eve Lee, David Lafreni`ere, Motohide Tamura</ref>

==Jupiter-like planets==
[[Image:2M1207b - First image of an exoplanet.jpg|thumb|right|250px|This composite image shows an exoplanet (2M1207b, the red spot on the lower left), orbiting the brown dwarf 2M1207 (centre). 2M1207b is a Jupiter-like planet. It orbits the brown dwarf at a distance nearly twice as far as [[Neptune]] is from the [[Stars/Sun|Sun]]. Credit: ESO.]]
'''Def.''' a "planet whose characteristics are similar to [[Jupiter]], but which orbits its parent star much more closely and thus has higher temperatures"<ref name=HotJupiterWikt>{{ cite book
|title=hot Jupiter
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 20, 2013
|url=https://en.wiktionary.org/wiki/hot_Jupiter
|accessdate=2013-08-30 }}</ref> is called a '''hot Jupiter'''.

'''2M1207b''' is a [[w:planetary-mass object|planetary-mass object]] [[w:planetary orbit|orbiting]] the brown dwarf 2M1207, in the constellation [[w:Centaurus|Centaurus]], approximately 170 light-years from Earth.<ref name=mamajek>[http://www.cfa.harvard.edu/~emamajek/memo_2m1207.html "The Distance to the 2M1207 System"], Eric Mamajek, November 8, 2007.  Accessed on line June 15, 2008.</ref> Notable as one of the first candidate extrasolar planets to be directly observed (by infrared imaging), it was discovered in April 2004 by the [[w:Very Large Telescope|Very Large Telescope]] (VLT) at the [[w:Paranal Observatory|Paranal Observatory]] in [[w:Chile|Chile]] by a team from the [[w:European Southern Observatory|European Southern Observatory]] led by Gaël Chauvin.<ref name=chauvin04>A giant planet candidate near a young brown dwarf. Direct VLT/NACO observations using IR wavefront sensing, G. Chauvin, A.-M. Lagrange, C. Dumas, B. Zuckerman, D. Mouillet, I. Song, J.-L. Beuzit, P. Lowrance, ''Astronomy and Astrophysics'', '''425''' (October 2004), pp. L29–L32.  |Bibcode=2004A&A...425L..29C
|doi=10.1051/0004-6361:200400056 }}.</ref> It is believed to be from 3 to 10 times the mass of [[Jupiter]] and may orbit 2M1207 at a distance roughly as far from the brown dwarf as [[Draft:Pluto]] is from the Earth's [[Sun (star)|Sun]].<ref name=encyc>[http://exoplanet.eu/star.php?st=2M1207 Star: 2M1207], ''Extrasolar Planets Encyclopaedia''. Accessed on line June 15, 2008.</ref>

The object is a very hot [[w:gas giant|gas giant]]; the estimated surface temperature is roughly 1600 [[w:kelvin|K]] (1300 °C or 2400 °F), mostly due to gravitational contraction.<ref name=mohanty>The Planetary Mass Companion 2MASS 1207-3932B: Temperature, Mass, and Evidence for an Edge-on Disk, Subhanjoy Mohanty, Ray Jayawardhana, Nuria Huelamo, and Eric Mamajek, ''Astrophysical Journal'' '''657''', #2 (March 2007), pp. 1064–1091
|Bibcode=2007ApJ...657.1064M
|doi=10.1086/510877 }}</ref> Its [[w:mass|mass]] is well below the calculated limit for [[w:deuterium|deuterium]] [[w:nuclear fusion|fusion]] in brown dwarfs, which is 13 [[w:Jupiter mass|Jupiter mass]]es. The projected distance between 2M1207b and its primary is around 40 [[w:astronomical unit|AU]] (similar to the mean distance between [[Draft:Pluto]] and the Sun).<ref name=estsep>Estimated observed projected separation from observed angular separation and estimated distance.</ref> Its infrared spectrum indicates the presence of [[w:water|water]] molecules in its atmosphere.<ref name=esopr>[http://www.eso.org/public/news/eso0515/ Yes, it is the Image of an Exoplanet:  Astronomers Confirm the First Image of a Planet Outside of Our Solar System], ESO Press Release 12/05, April 30, 2005, European Southern Observatory.  Accessed on line July 10, 2010.</ref>

'''UGPS J072227.51-054031.2''' (designation often abbreviated to '''UGPS 0722-05''') is a [[w:brown dwarf|brown dwarf]] of late T type, located approximately {{convert|4.1|pc|ly|abbr=off}} from Earth.<ref name="Lucas2010v2"/> The [[w:astronomical object|astronomical object]] was discovered by Philip Lucas at the [[w:University of Hertfordshire|University of Hertfordshire]] and announced in 2010. The discovery image was taken on 28 November 2006 by the [[w:UKIRT Infrared Deep Sky Survey|UKIRT Infrared Deep Sky Survey]] (UKIDSS) with a recovery image confirming the object's [[w:proper motion|proper motion]] on 2 March 2010.<ref name="Lucas2010v2"/> The reported distance is derived from the current measured [[w:parallax|parallax]] of 246 [[w:milliarcsecond|milliarcsecond]]s. The object was initially reported to be at an even closer distance of 2.9 parsecs, which would have placed it among the ten nearest stars to the Sun<ref name="Lucas2010v1">{{cite journal
  |last=Lucas |first=Philip W.
  |coauthors=Tinney; Burningham; Leggett; Pinfield; Smart
  |display-authors=etal
  |title=Discovery of a very cool brown dwarf amongst the ten nearest stars to the Solar System
  |year=2010
  |arxiv=1004.0317
  |version=v1
  |class=astro-ph.SR
  }}</ref> but later measurements revealed that the object was in fact located at a greater distance than initially thought, at 4.1<sup>+0.6</sup><sub>-0.5</sub> parsecs.<ref name="Lucas2010v2">{{ cite arXiv
|author=Philip W. Lucas
|author2=C.G. Tinney
|author3=Ben Burningham
|author4=S. K. Leggett
|author5=David J. Pinfield
|author6=Richard Smart
|author7=Hugh R.A. Jones
|author8=Federico Marocco
|author9=Robert J. Barber
|author10=Sergei N. Yurchenko
|author11=Jonathan Tennyson
|author12=Miki Ishii
|author13=Motohide Tamura
|author14=Avril C. Day-Jones
|author15=Andrew Adamson
|author16=France Allard
|author17=Derek Homeier
|title=The discovery of a very cool, very nearby brown dwarf in the Galactic plane
|eprint=1004.0317
|version=v2
|class=astro-ph.SR
}}</ref>

In June 2010, the authors updated their [[w:arXiv|arXiv]] paper, noting that new parallax measurements were significantly different from earlier estimates, and that they had withdrawn their Nature submission. The revised data gives a distance of 4.1<sup>+0.6</sup><sub>-0.5</sub> parsecs. The latest version of the discovery paper has been submitted to the journal Monthly Notices of the Royal Astronomical Society.<ref name="Lucas2010v2"/>

The object is roughly the volume of [[Jupiter]], but is estimated to have 5–40 [[w:Jupiter mass|Jupiter mass]]es (M<sub>J</sub>).<ref name="Lucas2010v2"/> This would make it less massive than [[w:Epsilon Indi#Companions|ε Indi Ba]]. Planets have a mass of less than about 13 Jupiter masses. [[w:Infrared spectroscopy|Infrared spectra]] shows the object contains water vapor and [[w:methane|methane]] and has a surface temperature of approximately 480–560 Kelvin.<ref name="Lucas2010v2"/>

'''NGTS-10b''': "Perhaps 1,060 light-years away, a gas giant called NGTS-10b is whipping around its star so closely, it completes an entire orbit in just 18.4 hours."<ref name=Starr>{{ cite web
|author=Michelle Starr
|title=Astronomers Detect a 'Hot Jupiter' With a Staggering 18-Hour-Short Orbit
|publisher=Science Alert
|location=
|date=1 October 2019
|url=https://www.sciencealert.com/extreme-hot-jupiter-orbits-its-star-at-an-absolutely-breakneck-18-hours
|accessdate=2 October 2019 }}</ref>

"That's nearly as close as the planet can get to the host star without being ripped apart by gravitational forces. But it will get closer."<ref name=Starr/>

The "exoplanet is [estimated to be] spiralling in towards the star, and will cross that ripping-apart point - called the Roche limit - in just 38 million years."<ref name=Starr/>

"Hot Jupiters are fascinating exoplanets. As the name suggests, they are gas giants like Jupiter; unlike Jupiter, however, they orbit very closely to their host stars, with orbital periods of less than 10 days."<ref name=Starr/>

"According to current models of planet formation, technically hot Jupiters shouldn't exist. A gas giant can't form that close to their star, because the gravity, radiation, and intense stellar winds ought to keep the gas from clumping together."<ref name=Starr/>

"However, they do exist; of the over 4,000 confirmed exoplanets discovered to date, up to 337 could be hot Jupiters. It's thought that they form farther out in their planetary systems, then migrate inwards towards the star."<ref name=Starr/>

"Until this latest breakneck discovery, only six of these enigmatic gas giants had ever been detected with an orbital period of less than one day - WASP-18b (22.6 hours), WASP-19b (18.9 hours), WASP-43b (19.5 hours), WASP-103b (22.2 hours), HATS-18b (20.1 hours) and KELT-16b (23.3 hours)."<ref name=Starr/>

"NGTS-10b, discovered using the ground-based Next-Generation Transit Survey in Paranal, Chile, marks the seventh of these ultra-close hot Jupiters, and it has the shortest orbital period of them all."<ref name=Starr/>

"Between 21 September 2015 and 14 May 2016, a single telescope observed the star now known as NGTS-10 over 237 nights. The survey wasn't officially operational yet, but it captured 220,918 10-second exposures of the star during this commissioning phase."<ref name=Starr/>

"NGTS-10b is just over 1.2 times the size of Jupiter, and just over 2.1 times its mass. And it's orbiting the star at 1.46 times the Roche radius - meaning it's right on the verge (in cosmic time) of tidal devastation."<ref name=Starr/>
{{clear}}

==Rogue planets==

A '''rogue planet'''  — also known as an '''interstellar planet''', '''nomad planet''', '''free-floating planet''' or '''orphan planet''' — is a [[w:planetary-mass object|planetary-mass object]] which has either been ejected from its system or was never gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the [[galaxy]] directly.<ref>[http://www.space.com/searchforlife/seti_orphan_planets_050224.html ''Orphan Planets: It's a Hard Knock Life''], [http://www.space.com/ Space.com], 24 Feb 2005, retrieved 5 Feb 2009.</ref><ref>[http://web.archive.org/web/20010521192001/http://www.space.com/scienceastronomy/astronomy/free_floaters_010403-1.html ''Free-Floating Planets – British Team Restakes Dubious Claim''], Space.com, 18 Apr 2001, retrieved 5 Feb 2009.</ref><ref>[http://astrobiology.arc.nasa.gov/news/expandnews.cfm?id=783 ''Orphan 'planet' findings challenged by new model''], [http://astrobiology.nasa.gov/ NASA Astrobiology], 18 Apr 2001, retrieved 5 Feb 2009.</ref> Astronomers agree that either way, the [[w:definition of planet|definition of planet]] should depend on its current observable state and not its origin.

Larger planetary-mass objects which were not ejected, but have always been free-floating, are thought to have formed in a similar way to stars, and the [[w:IAU|IAU]] has proposed that those objects be called sub-brown dwarfs<ref>[http://www.dtm.ciw.edu/boss/definition.html Working Group on Extrasolar Planets – Definition of a "Planet"] POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)</ref> (an example of this is [[w:Cha 110913-773444|Cha 110913-773444]], which may be an ejected rogue planet or may have formed on its own and be a sub-brown dwarf).<ref name="CNN">[http://archives.cnn.com/2000/TECH/space/10/06/space.planets.reut/index.html Rogue planet find makes astronomers ponder theory]</ref> The closest rogue planet to Earth yet discovered, [[w:CFBDSIR 2149-0403|CFBDSIR 2149-0403]], is around 100 light years away.<ref>[http://www.washingtonpost.com/national/health-science/astronomers-spy-a-planet-untethered-to-any-star-there-may-be-many-more/2012/11/17/ff15b274-2e89-11e2-beb2-4b4cf5087636_story.html?tid=ts_carousel Astronomers spy a planet untethered to any star; there may be many more], ''Washington Post'', 19 Nov 2012. Retrieved 20 Nov 2012.</ref>

When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as [[w:microlensing|microlensing]]. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand's [[w:Mount John Observatory|Mount John Observatory]] and the 1.3 meter [[w:University of Warsaw|University of Warsaw]] telescope at Chile's [[w:Las Campanas Observatory|Las Campanas Observatory]]. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two free-floaters for every star in our galaxy.<ref>[http://news.sciencemag.org/sciencenow/2011/05/homeless-planets-may-be-common.html?ref=hp Homeless' Planets May Be Common in Our Galaxy] by Jon Cartwright, Science Now ,18 May 2011, Accessed 20 may 2011
</ref><ref>[http://www.physorg.com/news/2011-05-class-planets.html Planets that have no stars: New class of planets discovered], Physorg.com, May 18, 2011. Accessed May 2011.</ref><ref> [{{cite arXiv
|last=T. Sumi
|display-authors=etal
|eprint=1105.3544
|class=astro-ph.EP
|title=Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing
|year=2011
|version=v1
}}</ref> Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way.<ref>{{cite book
|title=Researchers say galaxy may swarm with 'nomad planets'
|url=http://news.stanford.edu/news/2012/february/slac-nomad-planets-022312.html
|accessdate=29 February 2012
|publisher=Stanford University
}}</ref>

==Superplanets==
[[Image:Exoplanet Very Large Telescope.jpg|right|thumb|300px|The exoplanet (seen here as a bright splotch to the right of its host star, whose light has been blocked out) is a cloudy world several times more massive than Jupiter. Credit: M. Keppler.{{tlx|fairuse}}]]
'''Def.''' any planet larger than [[Jupiter/Keynote lecture|Jupiter]] is called a '''superplanet'''.

"New infrared telescope observations [such as in the image on the right] reveal an exoplanet [PDS 70b] that appears to be growing inside the disk of gas and dust around a star [PDS 70] about 370 light-years away."<ref name=Temming>{{ cite book
|author=Maria Temming
|title=Astronomers snap the first baby pictures of a planet
|publisher=Science News
|location=
|date=July 2, 2018
|url=https://www.sciencenews.org/article/astronomers-snap-first-baby-pictures-planet
|accessdate=29 June 2018 }}</ref>

"PDS 70b [is] a giant, cloudy world about as far from its host star as [[Uranus]] is from the [[Stars/Sun|sun]], but there’s still much we don’t know about the planet. It could be as cool as 730° Celsius or as hot as 1330° C, and its mass could be anywhere from twice to 17 times that of [[Planets/Jupiter|Jupiter]]."<ref name=Temming/>

"PDS 70b has blazed a trail through the disk of planet-making material around its star by packing on the gas and dust in its orbital path. PDS 70b’s position inside this empty track, as well as the young age of its star — just 5.4 million years old — indicate that the planet isn’t finished forming."<ref name=Keppler>{{ cite book
|author=M. Keppler
|title=Astronomers snap the first baby pictures of a planet
|publisher=Science News
|location=
|date=July 2, 2018
|url=https://www.sciencenews.org/article/astronomers-snap-first-baby-pictures-planet
|accessdate=29 June 2018 }}</ref>
{{clear}}

==Giant planets==

'''Def.''' a "planet larger than terrestrial planets, composed mostly of gases and astronomical ices"<ref name=GiantPlanetWikt>{{ cite book
|title=giant planet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 20, 2013
|url=https://en.wiktionary.org/wiki/giant_planet
|accessdate=2013-08-30 }}</ref> is called a '''giant planet'''.

'''Def.''' a "large planet composed mostly of gaseous hydrogen and helium, along with methane and ammonia; possibly with a solid core"<ref name=GasGiantWikt>{{ cite book
|title=gas giant
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=August 9, 2013
|url=https://en.wiktionary.org/wiki/gas_giant
|accessdate=2013-08-30 }}</ref> is called a '''gas giant'''.

"In the past two years, NASA’s Kepler Space Telescope has located nearly 3,000 exoplanet candidates ranging from sub-Earth-sized minions to gas giants that dwarf our own Jupiter."<ref name=Heng>{{ cite journal
|author=Kevin Heng
|title=Why Does Nature Form Exoplanets Easily?
|journal=American Scientist
|month=May-June
|year=2013
|volume=101
|issue=3
|pages=184
|url=http://www.americanscientist.org/issues/pub/2013/3/why-does-nature-form-exoplanets-easily
|arxiv=
|bibcode=
|doi=
|pmid=
|accessdate=2013-08-30 }}</ref>

'''Def.''' a "giant planet composed mostly of astronomical ices (condensed forms of volatile compounds) including water, methane, oxygen, carbon, nitrogen, and sulfur, and smaller than a gas giant"<ref name=IceGiantWikt>{{ cite book
|title=ice giant
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 20, 2013
|url=https://en.wiktionary.org/wiki/ice_giant
|accessdate=2013-08-30 }}</ref> is called an '''ice giant'''.

==Major planets==

'''Def.''' any "planet that is significantly larger"<ref name=SeaSkyGlossary>{{ cite book
|title=Astronomy glossary
|url=http://www.seasky.org/astronomy/astronomy_glossary.html }}</ref> in mean radius than [[Keynote lectures/Earth|Earth]] is called a '''major planet'''.

==Super Earths==

'''Def.''' any "extrasolar rocky planet several times larger than Earth, especially such a planet detected as it transits its sun"<ref name=SuperEarthWikt>{{ cite book
|title=super-Earth
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 20, 2013
|url=https://en.wiktionary.org/wiki/super-Earth
|accessdate=2013-08-30 }}</ref> is called a '''super-Earth'''.

The "detection of a transiting planet around π Mensae (HD 39091), [uses] data from the Transiting Exoplanet Survey Satellite (TESS). The solar-type host star is unusually bright (V=5.7) and was already known to host a Jovian planet on a highly eccentric, 5.7-year orbit. The newly discovered planet has a size of 2.14 ± 0.04 ~ R<sub>⊕</sub> and an orbital period of 6.27 days. Radial-velocity data from the HARPS and AAT/UCLES archives also displays a 6.27-day periodicity, confirming the existence of the planet and leading to a mass determination of 4.82 ± 0.85 ~ M<sub>⊕</sub>."<ref name=Huang>{{ cite journal
|author=Chelsea X. Huang
|author2=Jennifer Burt
|author3=Andrew Vanderburg
|author4=Maximilian N. Günther
|author5=Avi Shporer
|author6=Jason A. Dittmann
|author7=Joshua N. Winn
|author8=Rob Wittenmyer
|author9=Lizhou Sha
|author10=Stephen R. Kane
|author11=George R. Ricker
|author12=Roland Vanderspek
|author13=David W. Latham
|author14=Sara Seager
|author15=Jon Jenkins
|author16=Douglas A. Caldwell
|author17=Karen A. Collins
|author18=Natalia Guerrero
|author19=Jeffrey C. Smith
|author20=Sam Quinn
|author21=Stéphane Udry
|author22=Francesco Pepe
|author23=François Bouchy
|author24=Damien Sé gransan
|author25=Christophe Lovis
|author26=David Ehrenreich
|author27=Maxime Marmier
|author28=Michel Mayor
|author29=Bill Wohler
|author30=Kari Haworth
|author31=Edward Morgan
|author32=Michael Fausnaugh
|author33=David Charbonneau
|author34=Norio Narita
|author35=the TESS team
|title=TESS Discovery of a Transiting Super-Earth in the π Mensae System
|journal=
|date=16 September 2018
|volume=
|issue=
|pages=
|url=https://arxiv.org/abs/1809.05967
|arxiv=1809.05967v1 [astro-ph.EP]
|bibcode=2018arXiv180905967H
|doi=
|pmid=
|accessdate=20 September 2018 }}</ref>

"The M3V-type star [TOI-270] is transited by the Super-Earth-sized TOI-270 b (<math>1.247^{+0.089}_{-0.083}</math> R<sub>⊕</sub>) and the sub-Neptune-sized exoplanets TOI-270 c (2.42 ± 0.13 R<sub>⊕</sub>) and TOI-270 d (2.13 ± 0.12 R<sub>⊕</sub>)<sup>1</sup>."<ref name=Gunther>{{ cite journal
|author=Maximilian N. Günther
|author2=Francisco J. Pozuelos
|author3=Jason A. Dittmann
|author4=Diana Dragomir
|author5=Stephen R. Kane
|author6=Tansu Daylan
|author7=Adina D. Feinstein
|author8=Chelsea Huang
|author9=Timothy D. Morton
|author10=Andrea Bonfanti
|author11=L. G. Bouma
|author12=Jennifer Burt1
|author13=Karen A. Collins
|author14=Jack J. Lissauer
|author15=Elisabeth Matthews
|author16=Benjamin T. Montet
|author17=Andrew Vanderburg
|author18=Songhu Wang
|author19=Jennifer G. Winters
|author20=George R. Ricker
|author21=Roland K. Vanderspek
|author22=David W. Latham
|author23=Sara Seager
|author24=Joshua N. Winn
|author25=Jon M. Jenkins
|author26=James D. Armstrong
|author27=Khalid Barkaoui
|author28=Natalie Batalha
|author29=Jacob L. Bean
|author30=Douglas A. Caldwell
|author31=David R. Ciardi
|author32=Kevin I. Collins
|author33=Ian Crossfield
|author34=Michael Fausnaugh
|author35=Gabor Furesz
|author36=Tianjun Gan
|author37=Michäel Gillon
|author38=Natalia Guerrero
|author39=Keith Horne
|author40=Steve B. Howell
|author41=Michael Ireland
|author42=Giovanni Isopi
|author43=Emmanüel Jehin
|author44=John F. Kielkopf
|author45=Sebastien Lepine
|author46=Franco Mallia
|author47=Rachel A. Matson
|author48=Gordon Myers
|author49=Enric Palle
|author50=Samuel N. Quinn
|author51=Howard M. Relles
|author52=Bárbara Rojas-Ayala
|author53=Joshua Schlieder
|author54=Ramotholo Sefako
|author55=Avi Shporer
|author56=Juan C. Suárez
|author57=Thiam-Guan Tan
|author58=Eric B. Ting
|author59=Joseph D. Twicken
|author60=Ian A. Waite
|title=A Super-Earth and two sub-Neptunes transiting the bright, nearby, and quiet M-dwarf TOI-270
|journal=arXiv
|date=14 March 2019
|volume=1903
|issue=
|pages=28
|url=https://arxiv.org/pdf/1903.06107
|arxiv=1903.06107
|bibcode=
|doi=
|pmid=
|accessdate=31 July 2019 }}</ref>

"The three exoplanets are among the smallest and nearest transiting exoplanets known to date [...]. The radius of TOI-270 b places it in a planetary population distinct from planets c and d; the trio is separated by the planetary radius gap around 1.7–2.0 R<sub>⊕</sub> which divides two populations of planets, rocky super-Earths and gas-dominated sub-Neptunes (e.g. [8, 9]). TOI-270 b likely falls into the regime of Earth-like/rocky compositions, while planets c and d likely have rocky/icy compositions when employing statistically predicted masses of <math>1.9^{+1.5}_{-0.7}</math> M<sub>⊕</sub>, <math>6.6^{+5.2}_{-2.8}</math> M<sub>⊕</sub>, and <math>5.4^{+4.0}_{-2.1}</math> M<sub>⊕</sub>, respectively [10, 11]."<ref name=Gunther/>

==Terrestrial planets==

'''Def.''' any "planet of the [[Solar System|solar system]] or any exoplanet which is "[[Keynote lectures/Earth|Earth]]-like" in the sense that it is composed primarily of metals and rock, in contrast to a planet which is a gas giant"<ref name=TerrestrialPlanetWikt>{{ cite book
|title=terrestrial planet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 10, 2013
|url=https://en.wiktionary.org/wiki/terrestrial_planet
|accessdate=2013-08-30 }}</ref> is called a '''terrestrial planet'''.

'''Def.''' "a terrestrial planet composed mostly of silicate minerals"<ref name=SilicatePlanetWikt>{{ cite book
|title=silicate planet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 10, 2012
|url=https://en.wiktionary.org/wiki/silicate_planet
|accessdate=2013-08-30 }}</ref> is called a '''silicate planet'''.

==Dwarf planets==
[[Image:Ceres optimized.jpg|thumb|right|250px|Ceres is seen by the [[w:Hubble Space Telescope|Hubble Space Telescope]] ([[w:Advanced Camera for Surveys|ACS]]). The [[w:Contrast (vision)|contrast]] has been enhanced to reveal surface details. Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park).]]

'''Def.''' "a celestial body that

(a) is in orbit around the [[Sun (star)|Sun]] [or another star],

(b) has sufficient mass for its self-gravity to overcome rigid forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite" is called a '''dwarf planet'''.<ref name=Christensen>{{ cite book
|author=Lars Lindberg Christensen
|title=IAU 2006 General Assembly: Result of the IAU Resolution votes
|publisher=International Astronomical Union
|date=August 24, 2006
|url=http://www.iau.org/public_press/news/detail/iau0603/
|accessdate=2011-10-30 }}</ref>

'''Ceres''' is the smallest identified dwarf planet in the [[Solar System]].

'''Ceres''' is the only dwarf planet in the [[w:Asteroid belt|asteroid belt]].<ref name="NASADawnMission">{{ cite book
|title=NASA – Dawn at a Glance
|publisher=NASA
|accessdate=14 August 2011
|url = http://www.webcitation.org/62D66N3T7
 }}</ref>

'''Def.''' "a planet that is between the size of [[Keynote lectures/Mercury|Mercury]] and [[Draft:Ceres|Ceres]]"<ref name=MesoplanetWikt>{{ cite book
|title=mesoplanet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 20, 2013
|url=https://en.wiktionary.org/wiki/mesoplanet
|accessdate=2013-08-30 }}</ref> is called a '''mesoplanet'''.
{{clear}}

==Minor planets==
[[Image:Vesta_full_mosaic.jpg|thumb|right|200px|As NASA's Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA.]]

'''Def.''' an "astronomical object in direct orbit around the [[Stars/Sun|Sun]] [or another star] smaller than a planet and not classified as a comet"<ref name=MinorPlanetWikt>{{ cite book
|title=minor planet
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=August 29, 2013
|url=https://en.wiktionary.org/wiki/minor_planet
|accessdate=2013-08-30 }}</ref> is called a '''minor planet'''.

In the full image of Vesta at right, the rocky-object appears to have suffered from meteor damage.

'''Vesta''', [[w:minor-planet designation|minor-planet designation]] '''4 Vesta''', is one of the largest asteroids in the [[w:Solar System|Solar System]]. It lost some 1% of its mass less than a [[w:1,000,000,000 (number)|billion]] years ago in a collision that left an enormous crater occupying much of its southern hemisphere. Debris from this event has fallen to Earth as [[w:HED meteorite|howardite–eucrite–diogenite (HED) meteorites]], a rich source of information about the asteroid.<ref name="Kelley2003">{{ cite journal
  |author=Kelley, M. S.
  |display-authors=etal
  |title=Quantified mineralogical evidence for a common origin of 1929 Kollaa with 4 Vesta and the HED meteorites
  |journal=Icarus
  |volume=165
  |issue=1
  |page=215
  |year=2003
  |doi=10.1016/S0019-1035(03)00149-0
|bibcode=2003Icar..165..215K}}</ref><ref name="nasa2011-vesta">{{ cite book
  |date=12 July 2011
  |title= Vesta
  |publisher=NASA/JPL
  |accessdate=30 July 2011|url= http://web.archive.org/web/20110629153841/http://www.nasa.gov/mission_pages/dawn/ceresvesta/index.html
 }}</ref>

'''Def.'''
# an "asteroid of any size"<ref name=PlanetoidWikt/>,
# an "asteroid-like body in an orbit beyond the asteroid belt"<ref name=PlanetoidWikt>{{ cite book
|title=planetoid
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 18, 2013
|url=https://en.wiktionary.org/wiki/planetoid
|accessdate=2013-08-30 }}</ref>,
# a "larger, [[wikt:planetary|planetary]], body in orbit around the [[Stars/Sun|Sun]]"<ref name=PlanetoidWikt/>, or other star, or
# a "dwarf planet"<ref name=PlanetoidWikt/>,
is called a '''planetoid'''.

'''Usage notes:'''
"The term "planetoid" has never been precisely defined. At first, it was a synonym for [[wikt:asteroid|asteroid]]; whereas "asteroid" referred to the star-like image seen through a telescope, "planetoid" referred to its planet-like orbit. Though it approached the popularity of "asteroid" ca. 1915, this usage was never dominant, and largely ceased by ca. 1980. Even before then the etymology of the term was reanalyzed as meaning planet-like in form, and started being used for larger asteroids such as Vesta which had planet-like geologies (that is, were [[wikt:planetary object|planetary bodies]]). There was an increase in such usage after 2000 with the discovery of planetary bodies in the [[wikt:Kuiper belt|Kuiper belt]] and beyond, which many felt were not appropriately called "asteroids" and concomitant with doubts as to the appropriate definition of "planet". [[wikt:Sedna|Sedna]], for example, was called a "planetoid" in its discovery announcement."

"The group of small bodies that circle round the Sun, outside the orbit of Mars, are known under the designation of ''the '''planetoids'''''."<ref name=Mann>{{ cite book
|author=Robert James Mann
|title=A guide to astronomical science
|publisher=
|location=
|date=1856
|editor=
|page=231
|url=
|arxiv=
|bibcode=
|doi=
|pmid=
|isbn=
}}</ref> Bold added.

For a model on the generation of chondrites, "if one can argue for the early existence of a few largely molten planetoids with dimension of kilometers or tens of kilometers, one can quite as easily argue for very large numbers of such planetoids."<ref name=Zook>{{ cite journal
|author=H.A. Zook
|title=On a New Model for the Generation of Chondrites
|journal=Lunar and Planetary Science
|month=
|year=1979
|volume=12
|issue=
|page=1242
|url=
|arxiv=
|bibcode=
|doi=
|pmid=
}}</ref>

"Eris [...] was the upstart planetoid that knocked Pluto off the planetary lists"<ref name=Daniels>{{ cite book
|author=Patricia Daniels
|author2=Robert Burnham
|title=The New Solar System: Ice Worlds, Moons, and Planets Redefined
|publisher=National Geographic Books
|location=
|date=2009
|editor=
|page=173
|url=
|arxiv=
|bibcode=
|doi=
|pmid=
|isbn=
}}</ref>

'''Def.''' any "of many small, solid astronomical objects, that orbit a star and form protoplanets through mutual gravitational attraction"<ref name=PlanetesimalWikt>{{ cite book
|title=planetesimal
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=June 18, 2013
|url=https://en.wiktionary.org/wiki/planetesimal
|accessdate=2013-08-30 }}</ref> is called a '''planetesimal'''.
{{clear}}

==Protoplanetary disks==
{{main|Protoplanetary disks}}
[[Image:Egg Nebula HST.jpg|thumb|right|250px|This Hubble image of the Egg Nebula shows one of the best views to date of the brief but dramatic preplanetary, or protoplanetary nebula phase in a star’s life. Credit: ESA/Hubble & NASA.]]
[[Image:The Red Rectangle around the star HD 44179.jpg|thumb|right|250px|The Hubble Space Telescope Advanced Camera for Surveys (ACS) image has H-alpha emission of the Red Rectangle shown in blue. Credit: .]]
[[Image:PIA04533.jpg|thumb|250px|left|The Red Rectangle is a proto-planetary nebula. Here is the Hubble Space Telescope Advanced Camera for Surveys (ACS) image. Broadband red light is shown in red. Credit: .]]
The presence of a protoplanetary disk may be used to estimate a stars age.

In December 2006, seven papers were published in the scientific journal, ''Science'', discussing initial details of the sample analysis. Among the findings are: a wide range of organic compounds, including two that contain biologically usable nitrogen; indigenous aliphatic hydrocarbons with longer chain lengths than those observed in the diffuse [[interstellar medium]]; abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene, proving consistency with the mixing of solar system and interstellar matter, previously deduced spectroscopically from ground observations;<ref name=Nottingham>{{ cite journal
|title=The building blocks of planets within the `terrestrial' region of protoplanetary disks
|url=http://adsabs.harvard.edu/abs/2004Natur.432..479V
|journal=Nature
|month=11
|year=2004
|doi=10.1038/nature03088
|accessdate=2012-10-15 }}</ref> hydrous silicates and carbonate minerals were found to be absent, suggesting a lack of aqueous processing of the cometary dust; limited pure carbon (CHON) was also found in the samples returned; methylamine and ethylamine was found in the aerogel but was not associated with specific particles.

[[Infrared astronomy]] deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets and circumstellar disks. Longer infrared wavelengths can also penetrate clouds of dust that block visible light, allowing observation of young stars in molecular clouds and the cores of galaxies.<ref name=ESAfeature>{{ cite book
|author=Staff|date=11 September 2003
|title=Why infrared astronomy is a hot topic
|publisher=ESA
|url=http://www.esa.int/esaCP/SEMX9PZO4HD_FeatureWeek_0.html
|accessdate=11 August 2008}}</ref> Some molecules radiate strongly in the infrared. This can be used to study chemistry in space; more specifically it can detect water in comets.<ref name=NASAIpac>{{ cite book
|title=Infrared Spectroscopy – An Overview
|publisher=NASA/IPAC
|url=http://www.ipac.caltech.edu/Outreach/Edu/Spectra/irspec.html
|accessdate=11 August 2008 }}</ref>

"The NASA/ESA Hubble Space Telescope has been at the cutting edge of research into what happens to stars like our Sun at the ends of their lives ... One stage that stars pass through as they run out of nuclear fuel is the preplanetary, or protoplanetary nebula. This Hubble image [at right] of the Egg Nebula shows one of the best views to date of this brief but dramatic phase in a star’s life."<ref name=Hubble>{{ cite book
|author=ESA/Hubble
|author2=NASA
|title=Hubble images searchlight beams from a preplanetary nebula
|publisher=ESA/Hubble & NASA
|location=
|date=April 23, 2012
|url=http://www.spacetelescope.org/images/potw1217a/
|accessdate=2013-01-10 }}</ref>

"The preplanetary nebula phase is a short period in the cycle of stellar evolution — over a few thousand years, the hot remains of the star in the centre of the nebula heat it up, excite the gas, and make it glow as a planetary nebula. The short lifespan of preplanetary nebulae means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. The Egg Nebula, the first to be discovered, was first spotted less than 40 years ago, and many aspects of this class of object remain shrouded in mystery."<ref name=Hubble/>

"At the centre of this image, and hidden in a thick cloud of dust, is the nebula’s central star. While we can’t see the star directly, four searchlight beams of light coming from it shine out through the nebula. It is thought that ring-shaped holes in the thick cocoon of dust, carved by jets coming from the star, let the beams of light emerge through the otherwise opaque cloud. The precise mechanism by which stellar jets produce these holes is not known for certain, but one possible explanation is that a binary star system, rather than a single star, exists at the centre of the nebula."<ref name=Hubble/>

"The onion-like layered structure of the more diffuse cloud surrounding the central cocoon is caused by periodic bursts of material being ejected from the dying star. The bursts typically occur every few hundred years."<ref name=Hubble/>

"The distance to the Egg Nebula is only known very approximately, the best guess placing it at around 3000 light-years from Earth. This in turn means that astronomers do not have any accurate figures for the size of the nebula (it may be larger and further away, or smaller but nearer). This image is produced from exposures in visible and infrared light from Hubble’s Wide Field Camera 3."<ref name=Hubble/>

"The ERE was first recognized clearly in the peculiar reflection nebula called the Red Rectangle by Schmidt, Cohen, & Margon (1980)."<ref name=Witt>{{ cite journal
|author=Adolf N. Witt
|author2=Karl D. Gordon
|author3=Douglas G. Furton
|title=Silicon Nanoparticles: Source of Extended Red Emission?
|journal=The Astrophysical Journal Letters
|month=July 1,
|year=1998
|volume=501
|issue=1
|pages=L111-5
|url=http://iopscience.iop.org/1538-4357/501/1/L111
|arxiv=astro-ph/9805006
|bibcode=
|doi=10.1086/311453
|pmid=
|accessdate=2013-07-30 }}</ref>

The '''Red Rectangle Nebula''', so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated with the AFCRL Infrared Sky Survey called Hi Star.

In the Red Rectangle Nebula, "[d]iffraction-limited speckle images of it in visible and near infrared light reveal a highly symmetric, compact bipolar nebula with X-shaped spikes which imply toroidal dispersion of the circumstellar material. The central binary system is completely obscured, providing no direct light.<ref name=Menshchikov>{{ cite journal
|author=A. B. Men'shchikov
|author2=D. Schertl
|author3=P. G. Tuthill
|author4=G. Weigelt
|author5=L. R. Yungelson
|title=Properties of the close binary and circumbinary torus of the Red Rectangle
|journal=Astronomy and Astrophysics
|month=
|year=2002
|volume=393
|issue=
|pages=867-85
|url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002A%26A...393..867M
|arxiv=astro-ph/0206189
|bibcode=2002A&A...393..867M
|doi=10.1051/0004-6361:20020859
|pmid=
|accessdate=2013-07-30 }}</ref>
{{clear}}

==Orbits==
{{main|Orbits}}
[[Image:ISEE3-ICE-trajectory.gif|thumb|left|250px|ISEE-3 is inserted into a "halo" orbit on June 10, 1982. Credit: NASA.]]
[[Image:Kepler orbits.svg|thumb|right|250px|A hyperbolic pass is indicated by the blue line with an eccentricity of 1.3. A parabolic pass is the green line. The elliptical orbit in red has an eccentricity (''e'') of 0.7. Credit: [[commons:User:Stamcose|Stamcose]].]]
[[Image:Orbit3.gif|thumb|right|250px|Two bodies orbiting around a common barycenter (red cross) with circular orbits. Credit: [[commons:User:Zhatt|Zhatt]].]]
[[Image:Orbit5.gif|thumb|right|250px|Two bodies orbiting around a common barycenter (red cross) with elliptic orbits. Credit: [[commons:User:Zhatt|Zhatt]].]]
[[Image:Kepler laws diagram.svg|thumb|300px|The diagram illustrates Kepler's three laws using two planetary orbits. Credit: [[commons:User:Hankwang|Hankwang]].]]
'''Def.''' "[a] circular or elliptical path of one object around another object"<ref name=OrbitWikt>{{ cite book
|title=orbit
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=December 13, 2012
|url=http://en.wiktionary.org/wiki/orbit
|accessdate=2013-01-31 }}</ref> is called an '''orbit'''.

Historically, the apparent motions of the planets were first understood geometrically (and without regard to gravity) in terms of [[w:epicycles|epicycles]], which are the sums of numerous circular motions.<ref>''Encyclopaedia Britannica'', 1968, vol. 2, p. 645</ref> Theories of this kind predicted paths of the planets moderately well, until [[w:Johannes Kepler|Johannes Kepler]] was able to show that the motions of planets were in fact (at least approximately) elliptical motions.<ref name=Caspar>M Caspar, ''Kepler'' (1959, Abelard-Schuman), at pp.131–140; A Koyré, ''The Astronomical Revolution: Copernicus, Kepler, Borelli'' (1973, Methuen), pp. 277–279</ref>

In the [[w:geocentric model|geocentric model]] of the solar system, the [[w:celestial spheres|celestial spheres]] model was originally used to explain the apparent motion of the planets in the sky in terms of perfect spheres or rings, but after the planets' motions were more accurately measured, theoretical mechanisms such as [[w:deferent and epicycle|deferent and epicycle]]s were added. Although it was capable of accurately predicting the planets' position in the sky, more and more epicycles were required over time, and the model became more and more unwieldy.

In [[theoretical astronomy]], whether the [[Keynote lectures/Earth|Earth]] moves or not, serving as a fixed point with which to measure movements by objects or entities, or there is a [[w:solar system|solar system]] with the Sun near its center, is a matter of simplicity and calculational accuracy.

Copernicus's theory provided a strikingly simple explanation for the apparent retrograde motions of the planets&mdash;namely as [[w:parallax|parallactic]] displacements resulting from the Earth's motion around the Sun&mdash;an important consideration in [[w:Johannes Kepler|Johannes Kepler]]'s conviction that the theory was substantially correct.<ref name=Linton>{{ cite book
|author=Christopher M. Linton
|title=From Eudoxus to Einstein&mdash;A History of Mathematical Astronomy
|publisher=Cambridge University Press
|location=Cambridge
|date=2004
|editor=
|pages=
|url=
|bibcode=
|doi=
|pmid=
|isbn=978-0-521-82750-8
}}</ref>

Kepler knew that the tables constructed from the heliocentric theory were more accurate than those of Ptolemy"<ref name=Linton/> with the Earth at the center. Using a computer, this means that for competing programs, one written for each theory, the heliocentric program finishes first (for a mutually specified high degree of accuracy).

[[w:Kepler's laws of planetary motion|"Kepler's laws]]:
# The orbit of every planet is an [[w:ellipse|ellipse]] with the Sun at one of the two [[w:Focus (geometry)|foci]].
# A [[w:line (geometry)|line]] joining a planet and the Sun sweeps out equal [[w:area|area]]s during equal intervals of time.<ref name="Wolfram2nd">Bryant, Jeff; Pavlyk, Oleksandr. "[http://demonstrations.wolfram.com/KeplersSecondLaw/ Kepler's Second Law]", ''Wolfram Demonstrations Project''. Retrieved December 27, 2009.</ref>
# The [[w:square (algebra)|square]] of the [[w:orbital period|orbital period]] of a planet is directly [[w:Proportionality (mathematics)|proportional]] to the [[w:cube (arithmetic)|cube]] of the [[w:semi-major axis|semi-major axis]] of its orbit.

The diagram at the right illustrates Kepler's three laws of planetary orbits: (1) The orbits are ellipses, with focal points ''&fnof;''<sub>1</sub> and ''&fnof;''<sub>2</sub> for the first planet and ''&fnof;''<sub>1</sub> and ''&fnof;''<sub>3</sub> for the second planet. The Sun is placed in focal point ''&fnof;''<sub>1</sub>. (2) The two shaded sectors ''A''<sub>1</sub> and ''A''<sub>2</sub> have the same surface area and the time for planet 1 to cover segment ''A''<sub>1</sub> is equal to the time to cover segment ''A''<sub>2</sub>. (3) The total orbit times for planet 1 and planet 2 have a ratio ''a''<sub>1</sub><sup>3/2</sup>&nbsp;:&nbsp;''a''<sub>2</sub><sup>3/2</sup>.

The simplest description of the paths astronomical objects may take when passing each other includes a hyperbolic and parabolic pass. When capture occurs it usually produces an elliptical orbit.
{{clear}}

==Orbital theory==
[[Image:Classical Kepler orbit e0.6.gif|thumb|right|250px|The animated elliptical orbit has an eccentricity of 0.6. Credit: [[commons:User:WillowW|Willow]].]]
[[Image:Ellipse Properties of Directrix and String Construction.svg|thumb|left|250px|The ellipse and some of its mathematical properties is described. Credit: .]]
Orbits come in many shapes and motions. The simplest forms are a circle or an ellipse.

The [[w:focus (geometry)|foci]] of an ellipse are two special points ''F<sub>1</sub>'' and ''F<sub>2</sub>'' on the ellipse's major axis and are equidistant from the center point. The sum of the distances from any point P on the ellipse to those two foci is constant and equal to the major axis ( ''PF<sub>1</sub>''&nbsp;+&nbsp;''PF<sub>2</sub>''&nbsp;=&nbsp;2''a'' ). Each of these two points is called a '''[[w:focus (geometry)|focus]]''' of the ellipse.

In the gravitational two-body problem, if the two bodies are bound to each other (i.e., the total energy is negative), their orbits are similar ellipses with the common barycenter being one of the foci of each ellipse. The other focus of either ellipse has no known physical significance. Interestingly, the orbit of either body in the reference frame of the other is also an ellipse, with the other body at one focus.

Ideally, the motion of two oppositely charged particles in empty space would also be an ellipse.

A real orbit (and its elements) changes over time due to gravitational perturbations by other objects and the effects of relativity. A Keplerian orbit is merely an idealized, mathematical approximation at a particular time.
{{clear}}

==Conic sections==
{{main|Conic sections}}
[[Image:Conic Sections.svg|thumb|right|250px|alt=Diagram of conic sections|Conics are of three types: parabolas , ellipses, including circles, and hyperbolas. Credit: .]]
[[Image:Ellipse parameters 2.svg|left|thumb|300px|Conic parameters are shown in the case of an ellipse. Credit: .]]
'''Def.''' "[a]ny of the four distinct shapes that are the intersections of a cone with a plane, namely the circle, ellipse, parabola and hyperbola"<ref name=ConicSectionWikt>{{ cite book
|title=conic section
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=January 15, 2013
|url=http://en.wiktionary.org/wiki/conic_section
|accessdate=2013-01-31 }}</ref> is called a '''conic section'''.

In [[mathematics]], a '''conic section''' (or just '''conic''') is a [[w:curve|curve]] obtained as the intersection of a [[w:cone (geometry)|cone]] (more precisely, a right circular [[w:conical surface|conical surface]]) with a [[w:plane (mathematics)|plane]].

Various parameters are associated with a conic section, as shown in the following table. (For the ellipse, the table gives the case of ''a''>''b'', for which the major axis is horizontal; for the reverse case, interchange the symbols ''a'' and ''b''. For the hyperbola the east-west opening case is given. In all cases, ''a'' and ''b'' are positive.)

{| class="wikitable"
! conic section
! equation
! eccentricity (''e'')
! linear eccentricity (''c'')
! semi-latus rectum (''ℓ'')
! focal parameter (''p'')
|-
|  [[w:circle|circle]]
|| <math>x^2+y^2=a^2 \,</math>
|| <math> 0 \,</math>
|| <math> 0 \,</math>
|| <math> a \,</math>
|| <math> \infty</math>
|-
|  [[w:ellipse|ellipse]]
|| <math>\frac{x^2}{a^2}+\frac{y^2}{b^2}=1</math>
|| <math>\sqrt{1-\frac{b^2}{a^2}}</math>
|| <math>\sqrt{a^2-b^2}</math>
|| <math>\frac{b^2}{a}</math>
|| <math>\frac{b^2}{\sqrt{a^2-b^2}}</math>
|-
|  [[w:parabola|parabola]]
|| <math>y^2=4ax \,</math>
|| <math> 1 \,</math>
|| <math> a \, </math>
|| <math> 2a \,</math>
|| <math> 2a \, </math>
|-
|  [[hyperbola]]
|| <math>\frac{x^2}{a^2}-\frac{y^2}{b^2}=1</math>
|| <math>\sqrt{1+\frac{b^2}{a^2}}</math>
|| <math>\sqrt{a^2+b^2}</math>
|| <math>\frac{b^2}{a}</math>
|| <math>\frac{b^2}{\sqrt{a^2+b^2}}</math>
|}
{{clear}}

==Antapex==
{{main|Stars/Sun/Antapexes|Solar antapexes}}
'''Def.''' "[t]he point to which the Sun appears to be moving with respect to the local stars"<ref name=SolarApexWikt>{{ cite book
|title=solar apex
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 10, 2012
|url=http://en.wiktionary.org/wiki/solar_apex
|accessdate=2013-02-01 }}</ref> is called the '''solar apex'''.

An antapex is a point that an astronomical object's total motion is directed away from. It is opposite to the apex.

The '''local standard of rest''' or '''LSR'''  follows the mean motion of material in the [[w:Milky Way|Milky Way]] in the neighborhood of the [[Stars/Sun|Sun]].<ref name=Shu>{{cite book
|title=The Physical Universe
|author= Frank H Shu
|page= 261
|url=http://books.google.com/?id=v_6PbAfapSAC&pg=PA261
|isbn=0935702059
|date=1982
|publisher=University Science Books }}</ref> The path of this material is not precisely circular.<ref name=Binney>{{cite book
|title=Galactic Astronomy
|author=James Binney, Michael Merrifield
|url=http://books.google.com/?id=arYYRoYjKacC&pg=PA536
|page= 536
|date=1998
|isbn=0691025657
|publisher=Princeton University Press }}</ref> The Sun follows the '''solar circle''' ([[w:Ellipse#Eccentricity|eccentricity]] ''e'' < 0.1 ) at a speed of about 220&nbsp;km/s in a clockwise direction when viewed from the [[w:galactic coordinates|galactic north pole]] at a radius of ≈ 8 [[w:kiloparsec|kpc]] about the center of the galaxy near [[w:Sgr A*|Sgr A*]], and has only a slight  motion, towards the [[w:Solar apex|solar apex]], relative to the LSR.<ref name=Reid>{{ cite book
|url=http://books.google.com/?id=bP9hZqoIfhMC&pg=PA19
|title=Mapping the Galaxy and Nearby Galaxies
|editor=F. Combes
|editor2=Keiichi Wada
|date=2008
|publisher=Springer
|isbn=0387727671
|author= Mark Reid
|display-authors=etal
|chapter=Mapping the Milky Way and the Local Group
|pages=19–20 }}</ref> [The Sun's [[w:peculiar motion|peculiar motion]] relative to the LSR is 13.4 km/s.<ref name=Binney1>{{ cite book
|author=Binney J.
|author2=Merrifield M.
|title=op. cit.
|isbn=0691025657
|chapter=§10.6
}}</ref><ref name=Mamajek>{{cite journal
|title=On the distance to the Ophiuchus star-forming region
|journal=Astron. Nachr.
|volume=AN 329
|doi= 10.1002/asna.200710827
|arxiv=0709.0505
|year=2008
|page=12
|author=E.E. Mamajek
|bibcode = 2008AN....329...10M }}</ref>] The LSR velocity is  anywhere from 202–241&nbsp;km/s.<ref name=Majewski>{{ cite journal
|title=Precision Astrometry, Galactic Mergers, Halo Substructure and Local Dark Matter
|author=Steven R. Majewski
|journal=Proceedings of IAU Symposium 248
|arxiv=0801.4927
|year=2008
|bibcode = 2008IAUS..248..450M
|doi = 10.1017/S1743921308019790
|volume=3 }}</ref>

==Eccentricity==

'''Def.''' "[t]he ratio, constant for any particular conic section, of the distance of a point from the focus to its distance from the [[wikt:directrix|directrix]]"<ref name=EccentricityWikt>{{ cite book
|title=eccentricity
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=December 1, 2012
|url=http://en.wiktionary.org/wiki/eccentricity
|accessdate=2013-02-01 }}</ref> is called the '''eccentricity'''.

"For an ellipse, the eccentricity is the ratio of the distance from the center to a focus divided by the length of the semi-major axis."<ref name=EccentricityWikt/>

"Mercury's orbit eccentricity [''e''] varies between about 0.11 and 0.24 with the shortest time lapse between the extremes being about 4 x 10<sup>5</sup> yr".<ref name=Peale>{{ cite journal
|author=Peale, S. J.
|title=Possible histories of the obliquity of Mercury
|journal=Astronomical Journal
|month=June
|year=1974
|volume=79
|issue=6
|pages=722-44
|bibcode=1974AJ.....79..722P
|doi=10.1086/111604
|pmid= }}</ref> "Smaller amplitude variations occur with about a 10<sup>5</sup> yr period."<ref name=Peale/>

==Inclination==
[[Image:Orbit1.svg|thumb|right|200px|The diagram describes the parameters associated with orbital inclination (''i''). Credit: [[w:User:Lasunncty|Lasunncty]].]]
'''Def.''' "[t]he angle of intersection of a reference plane"<ref name=InclinationWikt>{{ cite book
|title=inclination
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=December 14, 2012
|url=http://en.wiktionary.org/wiki/inclination
|accessdate=2013-02-01 }}</ref> is called an '''inclination'''.

"The orbital inclination [''i''] [of Mercury] varies between 5° and 10° with a 10<sup>6</sup> yr period with smaller amplitude variations with a period of about 10<sup>5</sup> yr."<ref name=Peale/>
{{clear}}

==Obliquity==

'''Def.''' "[t]he quality of being oblique in direction, deviating from the horizontal or vertical; or the angle created by such a deviation"<ref name=ObliquityWikt>{{ cite book
|title=obliquity
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 28, 2012
|url=http://en.wiktionary.org/wiki/obliquity
|accessdate=2013-02-01 }}</ref> is called '''obliquity'''.

'''Axial tilt''' (also called '''obliquity''') is the angle between an object's [[w:Axis of rotation|rotational axis]], and a line [[w:Perpendicular|perpendicular]] to its [[w:Orbital plane (astronomy)|orbital plane]]. The planet [[w:Venus|Venus]] has an axial tilt of 177.3° because it is rotating in retrograde direction, opposite to other planets like [[Keynote lectures/Earth|Earth]]. The planet [[w:Uranus|Uranus]] is rotating on its side in such a way that its rotational axis, and hence its north pole, is pointed almost in the direction of its orbit around the [[Stars/Sun|Sun]]. Hence the axial tilt of Uranus is 97°.<ref name=Williams>{{ cite book
|author=David R. Williams
|title=Planetary Fact Sheet Notes
|url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/planetfact_notes.html }}</ref>

The obliquity of the Earth's axis has a period of about 41,000 years.<ref name=Hays>{{ cite journal
|author=J. D. Hays
|author2=John Imbrie
|author3=N. J. Shackleton
|title=Variations in the Earth's Orbit: Pacemaker of the Ice Ages
|journal=Science
|month=December
|year=1976
|volume=194
|issue=4270
|pages=
|url=http://www.whoi.edu/science/GG/paleoseminar/ps/hays76.ps
|arxiv=
|bibcode=
|doi=
|pmid=
|accessdate=2011-11-08 }}</ref>

==Precession==

'''Def.''' "[a]ny of several slow changes in an astronomical body's rotational or orbital parameters ... [such as] [t]he slow gyration of the [[Keynote lectures/Earth|Earth]]’s axis around the pole of the ecliptic"<ref name=PrecessionWikt>{{ cite book
|title=precession
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=December 29, 2012
|url=http://en.wiktionary.org/wiki/precession
|accessdate=2013-02-01 }}</ref> is called a '''precession'''.

'''Def.''' "[t]he slow westward shift of the equinoxes along the plane of the ecliptic, resulting from precession of [an object's] axis of rotation, and causing the equinoxes to occur earlier each year"<ref name=PrecessionoftheEquinoxesWikt>{{ cite book
|title=precession of the equinoxes
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=December 26, 2011
|url=http://en.wiktionary.org/wiki/precession_of_the_equinoxes
|accessdate=2013-02-01 }}</ref> is called the '''precession of the equinoxes'''.

The equinoxes of Earth precess with a period of about 21,000 years.<ref name=Hays/>

==Resonances==
[[Image:Galilean moon Laplace resonance animation.gif|right|thumb|365px|The [[w:Laplace resonance|Laplace resonance]]s of Ganymede, [[Europa]], and [[Draft:Io|Io]] is illustrated. Credit: User:Matma Rex.]]
An orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. The physics principle behind orbital resonance is similar in concept to pushing a child on a swing, where the orbit and the swing both have a natural frequency, and the other body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i.e., their ability to alter or constrain each other's orbits. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of [[Jupiter]]'s moons [[Ganymede]], [[Europa]] and [[Draft:Io|Io]], and the 2:3 resonance between [[Draft:Pluto|Pluto]] and [[Neptune]]. Unstable resonances with [[Saturn]]'s inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance (between bodies with similar orbital radii) causes large [[Solar System]] bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.
{{clear}}

==Orbital poles==
[[Image:Snapshot of the planetary orbital poles.png|thumb|right|250px|This is a snapshot of the planetary orbital poles. Credit: [[w:User:Urhixidur|Urhixidur]].]]
An '''orbital pole''' is either end of an imaginary line running through the center of an [[w:orbit|orbit]] perpendicular to the orbital plane, projected onto the [[w:celestial sphere|celestial sphere]]. It is similar in concept to a [[w:celestial pole|celestial pole]] but based on the planet's orbit instead of the planet's rotation.

The north orbital pole of a celestial body is defined by the [[w:right-hand rule|right-hand rule]]: If you curve the fingers of your right hand along the direction of orbital motion, with your thumb extended parallel to the orbital [[w:Axis of rotation|axis]], the direction your thumb points is defined to be north.

At right is a snapshot of the planetary orbital poles.<ref name=Herschel>{{ cite journal
|author=J. Herschel
|title=The poles of planetary orbits
|journal=The Observatory
|month=June
|year=1918
|volume=41
|issue=
|pages=255-7
|url=http://adsabs.harvard.edu/full/1918Obs....41..255H
|arxiv=
|bibcode=1918Obs....41..255H
|doi=
|pmid=
|accessdate=2013-07-10 }}</ref> The field of view is about 30°. The yellow dot in the centre is the Sun's North pole. Off to the side, the orange dot is Jupiter's orbital pole. Clustered around it are the other planets: Mercury in pale blue (closer to the Sun than to Jupiter), Venus in green, the Earth in blue, Mars in red, Saturn in violet, Uranus in grey partly underneath Earth and Neptune in lavender. Dwarf planet Pluto is the dotless cross off in Cepheus.
{{clear}}

==Orbital decay==

Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit. This can be due to drag produced by an atmosphere [frequent collisions between the satellite and surrounding air molecules]. The drag experienced by the object is larger in the case of increased solar activity, because it heats and expands the upper atmosphere.

==Planetary astronomy==
[[Image:Ganymede g1 true 2.jpg|thumb|right|250px|A true color image of Ganymede is acquired by the Galileo spacecraft on June 26, 1996. Credit: NASA/JPL.]]
"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."<ref name=Madey>{{ cite journal
|author=Theodore E. Madey
|author2=Robert E. Johnson
|author3=Thom M. Orlando
|title=Far-out surface science: radiation-induced surface processes in the solar system
|journal=Surface Science
|month=March
|year=2002
|volume=500
|issue=1-3
|pages=838-58
|url=http://www.physics.rutgers.edu/~madey/Publications/Full_Publications/PDF/madey_SS_2002.pdf
|arxiv=
|bibcode=
|doi=10.1016/S0039-6028(01)01556-4
|pmid=
|accessdate=2012-02-09 }}</ref> "[I]nterplanetary space ... is a stormy and sometimes very violent environment permeated by energetic particles and radation constantly emanating from the Sun."<ref name=Madey/>

Each of the astronomical objects that constitute planetary science emits, reflects, or fluoresces radiation that is observed and analyzed.

"The spectrum of gaseous methane at 77 K in the 1.1-2.6 µm region [is] a benchmark for planetary astronomy".<ref name=McKellar>{{ cite journal
|author=A. R. W. McKellar
|title=The spectrum of gaseous methane at 77 K in the 1.1-2.6 μm region: a benchmark for planetary astronomy
|journal=Canadian Journal of Physics
|month=November
|year=1989
|volume=67
|issue=11
|pages=1027-35
|url=http://www.nrcresearchpress.com/doi/abs/10.1139/p89-180
|arxiv=
|bibcode=
|doi=10.1139/p89-180
|pmid=
|accessdate=2012-02-09 }}</ref>
{{clear}}

==Planetary science==
[[Image:Venuspioneeruv.jpg|thumb|right|250px|An ultraviolet image of the planet Venus is taken on February 26, 1979, by the [[w:Pioneer Venus Orbiter|Pioneer Venus Orbiter]]. Credit: NASA.]]
Planetary science (rarely planetology) is the scientific study of [[w:planet|planet]]s (including [[Keynote lectures/Earth|Earth]]), [[w:Natural satellite|moons]], and [[w:planetary system|planetary system]]s, in particular those of the [[Solar System|solar system]] and the processes that form them. It studies objects ranging in size from [[w:micrometeoroid|micrometeoroid]]s to [[w:gas giant|gas giant]]s, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly [[w:Interdisciplinarity|interdisciplinary]] field, originally growing from astronomy and [[w:earth science|earth science]],<ref name=Taylor/> but which now incorporates many disciplines, including [[w:planetary astronomy|planetary astronomy]], [[w:planetary geology|planetary geology]] (together with [[w:geochemistry|geochemistry]] and [[w:geophysics|geophysics]]), [[w:atmospheric science|atmospheric science]], [[Draft:oceanography|oceanography]], [[hydrology]], [[w:theoretical planetary science|theoretical planetary science]], [[glaciology]], and the study of [[w:extrasolar planets|extrasolar planets]].<ref name=Taylor>{{ cite journal
|author=Stuart Ross Taylor
|date=29 July 2004
|title=Why can't planets be like stars?
|journal=Nature
|pmid=15282586
|volume=430
|issue=6999
|pages=509
|doi=10.1038/430509a
|url=http://www.nature.com/nature/journal/v430/n6999/full/430509a.html
|bibcode = 2004Natur.430..509T }}</ref> Allied disciplines include [[w:space physics|space physics]], when concerned with the effects of the [[Sun]] on the bodies of the Solar System, and astrobiology.
{{clear}}

==Planetary geology==
[[Image:Moon-apollo17-schmitt boulder.jpg|thumb|right|250px|Planetary [[w:geologist|geologist]] and [[w:NASA|NASA]] [[w:astronaut|astronaut]] [[w:Harrison Schmitt|Harrison "Jack" Schmitt]] collects [[w:lunar sample|lunar sample]]s during the Apollo 17 mission. Credit: NASA.]]
Planetary geology, astrogeology or exogeology, is a planetary science concerned with the [[geology]] of the [[w:celestial bodies|celestial bodies]] such as the [[w:planet|planet]]s and their [[Draft:moon|moon]]s, [[w:asteroid|asteroid]]s, [[w:comet|comet]]s, and [[w:meteorite|meteorite]]s. It includes determining the internal structure of the [[w:terrestrial planets|terrestrial planets]], planetary [[w:volcanism|volcanism]] and surface processes such as impact [[w:Impact crater|crater]]s, [[w:fluvial|fluvial]] and [[w:Aeolian processes|aeolian]] processes.
{{clear}}

==Planetary physics==

'''Def.''' a process in which fragments of material (spall) are ejected from a body due to impact or stress is called '''spallation'''.

In planetary physics, spallation describes meteoritic impacts on a planetary surface and the effects of a stellar wind on a planetary atmosphere.

From a theoretical planetary physics perspective: "The shape of objects with mass above 5 x 10<sup>20</sup> kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation."<ref name=Christensen1>{{ cite book
|author=Lars Lindberg Christensen
|title=The IAU draft definition of "planet" and "plutons"
|publisher=International Astronomical Union
|date=August 16, 2006
|url=http://www.iau.org/public_press/news/detail/iau0601/
|accessdate=2011-10-30 }}</ref>

'''Def.''' a celestial body "formed by accumulation of a rocky core, on a much longer timescale, ≳ 10<sup>7</sup> yr, with subsequent acquisition of a gaseous envelope if the circumstances allow this, and with an initially fractionated elemental composition" is called a '''planet'''.<ref name=Whitworth>{{ cite journal
|author=Anthony Whitworth
|author2=Dimitri Stamatellos
|author3=Steffi Walch
|author4=Murat Kaplan
|author5=Simon Goodwin
|author6=David Hubber
|author7=Richard Parker
|title=The formation of brown dwarfs, In: ''Star clusters: basic galactic building blocks'', Proceedings IAU Symposium No. 266
|publisher=International Astronomical Union
|location=
|month=
|year=2009
|editor=R. de Grijs & J. R. D. Lépine
|pages=264-71
|url=http://journals.cambridge.org/download.php?file=%2FIAU%2FIAU5_S266%2FS174392130999113Xa.pdf&code=410ba8317302679d01f7ea49a2a202a6
|bibcode=
|doi=10.1017/S174392130999113X
|pmid=
|isbn=
|pdf=http://arxiv.org/pdf/astro-ph/0602367
|accessdate=2011-10-30 }}</ref>

==Mercury==
{{main|Keynote lectures/Mercury}}
[[Image:Mercury in color - Prockter07-edit1.jpg|thumb|right|250px|The color image shown here at right was generated by combining the mosaics taken through the MESSENGER WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.]]
"MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters. As the spacecraft receded from Mercury after making its closest approach on January 14, 2008, the WAC recorded a 3x3 mosaic covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through the WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here. The human eye is sensitive only across the wavelength range from about 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in black-and-white (single-color) images."<ref name=JHUAPL>{{ cite book
|author=JHU/APL
|title=Mercury Shows Its True Colors
|publisher=JHU/APL
|location=Baltimore, Maryland USA
|date=January 30, 2008
|url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=1&gallery_id=2&image_id=143
|accessdate=2013-04-01 }}</ref>

"Color differences on Mercury are subtle, but they reveal important information about the nature of the planet's surface material. A number of bright spots with a bluish tinge are visible in this image. These are relatively recent impact craters. Some of the bright craters have bright streaks (called "rays" by planetary scientists) emanating from them. Bright features such as these are caused by the presence of freshly crushed rock material that was excavated and deposited during the highly energetic collision of a meteoroid with Mercury to form an impact crater. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin. Mariner 10 viewed only the eastern (right) portion of this enormous impact basin, under lighting conditions that emphasized shadows and elevation differences rather than brightness and color differences. MESSENGER has revealed that Caloris is filled with smooth plains that are brighter than the surrounding terrain, hinting at a compositional contrast between these geologic units. The interior of Caloris also harbors several unusual dark-rimmed craters, which are visible in this image. The MESSENGER science team is working with the 11-color images in order to gain a better understanding of what minerals are present in these rocks of Mercury's crust."<ref name=JHUAPL/>
{{clear}}

==Venus==
[[Image:Venus-real color.jpg|thumb|right|250px|This real-color image of Venus is processed from imaging through the clear and blue filters on Mariner 10. Credit: NASA or Ricardo Nunes.]]
In the image at right the whiter cloud areas have a bluish tint. This is a real-color image of Venus processed through the clear and blue filters onboard Mariner 10.

"The side of [[Draft:Venus|Venus]]' ionosphere that faces away from the sun can billow outward like the tail of a comet, while the side facing the star remains tightly compacted, researchers said. ... "As this significantly reduced solar wind hit Venus, Venus Express saw the planet’s ionosphere balloon outwards on the planet’s ‘downwind’ nightside, much like the shape of the ion tail seen streaming from a comet under similar conditions," ESA officials said in a statement today (Jan. 29). It only takes 30 to 60 minutes for the planet's comet-like tail to form after the solar wind dies down. Researchers observed the ionosphere stretch to at least 7,521 miles (12,104 kilometers) from the planet, said Yong Wei, a scientist at the Max Planck Institute in Katlenburg, Germany who worked on this research."<ref name=Kramer>{{ cite book
|author=Miriam Kramer
|title=Venus Can Have 'Comet-Like' Atmosphere
|publisher=Yahoo! News
|location=
|date=January 31, 2013
|url=http://news.yahoo.com/venus-havecomet-atmosphere-120238337.html
|accessdate=2013-01-31 }}</ref>

Because of the lack of a [[w:Magnetosphere|planetary magnetic field]], the free hydrogen has been [[w:Atmospheric escape|swept into interplanetary space]] by the [[w:solar wind|solar wind]].<ref name="solarwind">{{cite book
 |date=28 November 2007
 |title=Caught in the wind from the Sun
 |publisher=ESA (Venus Express)
 |url= http://www.esa.int/SPECIALS/Venus_Express/SEM0G373R8F_0.html
 |accessdate=2008-07-12 }}</ref>

The clouds of Venus are capable of producing [[w:lightning|lightning]] much like the clouds on Earth.<ref name="Russell_2007" /> The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet [[w:Venera|Venera probe]]s. In 2006–07 [[w:Venus Express|Venus Express]] clearly detected [[w:Electromagnetic electron wave|whistler mode wave]]s, the signatures of lightning. Their [[w:intermittent|intermittent]] appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.<ref name="Russell_2007">{{ cite journal
| doi = 10.1038/nature05930
| author = S. T. Russell
|author2=T. L. Zhang
|author3=M. Delva
|author4=W. Magnes
|author5=R. J. Strangeway
|author6=H. Y. Wei
| year = 2007
| title = Lightning on Venus inferred from whistler-mode waves in the ionosphere
|journal=Nature
| volume = 450
| issue = 7170
| pages = 661–662
| pmid = 18046401
| bibcode = 2007Natur.450..661R }}</ref> In 2007 the Venus Express probe discovered that a huge double [[w:Polar vortex|atmospheric vortex]] exists at the south pole of the planet.<ref>{{cite journal
|first=Eric|last=Hand |year=2007
|title=European mission reports from Venus
|month=November|journal=Nature|issue=450
|pages=633–660|doi=10.1038/news.2007.297
}}</ref><ref>{{cite book|author=Staff|url=http://news.bbc.co.uk/1/hi/sci/tech/7117303.stm
|title=Venus offers Earth climate clues
|accessdate=2007-11-29|work=BBC News|date=28 November 2007}}</ref>

Another discovery made by the Venus Express probe in 2011 is that an [[w:ozone|ozone]] layer exists high in the atmosphere of Venus.<ref name="esaozone">{{cite book
 |url = http://www.esa.int/esaCP/SEMU3N9U7TG_Life_0.html
 |title=ESA finds that Venus has an ozone layer too|publisher=ESA
 |date=6 October 2011|accessdate=2011-12-25}}</ref>

Venus has an extremely dense [[w:celestial body atmosphere|atmosphere]], which consists mainly of [[w:carbon dioxide|carbon dioxide]] and a small amount of [[w:nitrogen|nitrogen]]. The atmospheric mass is 93 times that of Earth's atmosphere, while the pressure at the planet's surface is about 92 times that at Earth's surface—a pressure equivalent to that at a depth of nearly 1&nbsp;kilometer under Earth's oceans. The density at the surface is 65&nbsp;kg/m³ (6.5% that of water).
{{clear}}

==Earth==
{{main|Keynote lectures/Earth}}
[[Image:Top of Atmosphere.jpg|thumb|250px|right|The Earth has a blue halo when seen from [[w:space|space]]. Credit: NASA Earth Observatory.]]
[[Image:Holmengrå2.JPG|thumb|right|250px|The Earth can have a blue sky and a blue ocean. Credit: [[commons:User:Frokor|Frokor]].]]
[[Image:The Blue Marble 4463x4163.jpg|thumb|left|250px|This is the famous '''Blue Marble''' image of Earth taken by Apollo 17. Credit: NASA. Photo taken by either Harrison Schmitt or Ron Evans (of the Apollo 17 crew).]]
"Earth is a blue planet"<ref name=Young>{{ cite journal
|author=AT Young
|title=What color is the solar system?
|journal=Sky and Telescope
|month=May
|year=1985
|volume=69
|issue=5
|pages=399-402
|url=http://adsabs.harvard.edu/abs/1985S&T....69..399Y
|arxiv=
|bibcode=1985S&T....69..399Y
|doi=
|pmid=
|accessdate=2012-06-19 }}</ref>.

Atmospheric [[w:gas|gas]]es scatter blue light more than other wavelengths, giving the [[Earth]] a blue halo when seen from space. This is shown in the image at right.

When light passes through our atmosphere, [[w:photon|photon]]s interact with it through ''scattering''. If the light does not interact with the atmosphere, it is called ''direct radiation'' and is what you see if you were to look directly at the Sun. ''Indirect radiation'' is light that has been scattered in the atmosphere. For example, on an [[w:overcast|overcast]] day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called [[w:Rayleigh scattering|Rayleigh scattering]], shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why [[w:sunset|sunset]]s are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

The bluish [[w:color of water|color of water]] is a composite of several contributing agents. Prominent contributors include dissolved organic matter and [[w:chlorophyll|chlorophyll]].<ref name=Coble>{{ cite journal
|author=Paula G. Coble
|title=Marine Optical Biogeochemistry:  The Chemistry of Ocean Color
|journal=Chemical Reviews
|year=2007
|volume=107
|pages=402–18
|doi=10.1021/cr050350 }}</ref>

The image on the left is the famous '''Blue Marble''' image of Earth taken by Apollo 17. The image shows the eastern Southern Atlantic Ocean, the South African portion of the Southern Ocean, and the Western Indian Ocean. The land consists of most of Africa, Madagascar, Saudi Arabia, and portions of Iran, Irag, Turkey, and southern Greece. The gaseous portion consists of water vapor clouds over the southern portion of this hemisphere. Antarctica is completely covered in snow (a water-ice rocky substance).

If this third image is the one chosen to decide whether the Earth is a dwarf gaseous object, a dwarf liquid object, or a dwarf rocky object, the decision becomes difficult. Here, the Earth is primarily a liquid body.
{{clear}}

==Mars==
{{main|Keynote lectures/Mars}}
[[Image:2005-1103mars-full.jpg|thumb|right|250px|Mars is imaged from [[w:Hubble Space Telescope|Hubble Space Telescope]] on October 28, 2005, with dust storm visible. Credit: NASA, ESA, The Hubble Heritage Team (STScI/AURA), J. Bell (Cornell University) and M. Wolff (Space Science Institute).]]
La couleur rouge sang caractéristique de Mars lui valut dans l’Antiquité le rapprochement avec le dieu grec de la guerre ''Arès'' puis avec son équivalent romain ''Mars'', le rouge évoquant le sang des champs de bataille. Les Babyloniens la nommaient ''Nirgal'' ou ''Nergal'', le dieu de la mort, des destructions et du feu. Les Égyptiens la nommaient « Horus rouge » (''ḥr Dšr, Hor-desher'') et connaissaient son « déplacement à reculons » (actuellement connu sous le nom de mouvement rétrograde). Dans la mythologie hindoue, Mars est nommée ''Mangala'' मंगल) du nom du dieu de la guerre. Mangala Vallis est nommé en son honneur.
{{clear}}

==Pallas==
{{main|Rocks/Rocky objects/Pallas|Pallas}}
[[Image:PallasHST2007.jpg|thumb|right|250px|This is an [[w:Ultraviolet photography|ultraviolet image]] of Pallas showing its flattened shape taken by the [[w:Hubble Telescope|Hubble Space Telescope]]. Credit: NASA.]]
'''Pallas''', [[w:minor-planet designation|minor-planet designation]] '''2 Pallas''', is the second [[w:asteroid|asteroid]] to have been discovered (after [[Draft:Ceres]]), and one of the largest in the [[Solar System]]. It is estimated to comprise 7% of the mass of the [[w:asteroid belt|asteroid belt]],<ref name="Pitjeva05">{{ cite journal
|author=Elena V. Pitjeva
|title=High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants
|journal=Solar System Research
|year=2005
|volume=39
|issue=3
|page=176
|url=http://iau-comm4.jpl.nasa.gov/EPM2004.pdf
|doi=10.1007/s11208-005-0033-2
|bibcode = 2005SoSyR..39..176P }}</ref> and its diameter of {{convert|544|km}} is slightly larger than that of [[Vesta|4 Vesta]]. It is however 10–30% less massive than Vesta,<ref name="Baer2007">{{ cite journal
  |author=James Baer
|author2=Steven R. Chesley
  |title=Astrometric masses of 21 asteroids, and an integrated asteroid ephemeris
  |journal=Celestial Mechanics and Dynamical Astronomy
  |volume=100
|issue=2008
|pages=27–42
  |url=http://www.springerlink.com/content/h747307j43863228/fulltext.pdf
  |doi=10.1007/s10569-007-9103-8
  |accessdate=2008-11-11
  |year=2008 
|bibcode = 2008CeMDA.100...27B }}</ref> placing it third among the asteroids.

"Spectrally blue (B-type) asteroids are rare, with the second discovered asteroid, Pallas, being the largest and most famous example."<ref name=Yang>{{ cite journal
|author=Bin Yang
|author2=David Jewitt
|title=Identification of Magnetite in B-type Asteroids
|journal=The Astronomical Journal
|month=September
|year=2010
|volume=140
|issue=3
|pages=692
|url=http://iopscience.iop.org/1538-3881/140/3/692
|arxiv=
|bibcode=
|doi=10.1088/0004-6256/140/3/692
|pmid=
|accessdate=2013-06-01 }}</ref>

"[T]he negative optical spectral slope of some B-type asteroids is due to the presence of a broad absorption band centered near 1.0 μm. The 1 μm band can be matched in position and shape using magnetite (Fe<sub>3</sub>O<sub>4</sub>), which is an important indicator of past aqueous alteration in the parent body. ... Observations of B-type asteroid (335) Roberta in the 3 μm region reveal an absorption feature centered at 2.9 μm, which is consistent with the absorption due to phyllosilicates (another hydration product) observed in CI chondrites. ... at least some B-type asteroids are likely to have incorporated significant amounts of water ice and to have experienced intensive aqueous alteration."<ref name=Yang/>
{{clear}}

==Jupiter==
{{main|Jupiter}}
[[File:PIA02863 - Jupiter surface motion animation.gif|thumb|300px|center|Zones, belts and vortices on Jupiter are shown. Credit: NASA/JPL/University of Arizona.]]
The wide equatorial zone is visible in the center surrounded by two dark equatorial belts (SEB and NEB).

"The large grayish-blue [irregular] "hot spots" at the northern edge of the white Equatorial Zone change over the course of time as they march eastward across the planet."<ref name=Lavoie00>{{ cite book
|author=Sue Lavoie
|title=PIA02863: Planetwide Color Movie
|publisher=NASA/JPL/University of Arizona
|location=Tucson, Arizona USA
|date=December 28, 2000
|url=http://photojournal.jpl.nasa.gov/catalog/PIA02863
|accessdate=2013-05-30 }}</ref>

"The Great Red Spot shows its counterclockwise rotation, and the uneven distribution of its high haze is obvious. To the east (right) of the Red Spot, oval storms, like ball bearings, roll over and pass each other. Horizontal bands adjacent to each other move at different rates. Strings of small storms rotate around northern-hemisphere ovals."<ref name=Lavoie00/>

"Small, very bright features appear quickly and randomly in turbulent regions, candidates for lightning storms."<ref name=Lavoie00/>

"The smallest visible features at the equator are about 600 kilometers (about 370 miles) across."<ref name=Lavoie00/>

"The clip consists of 14 unevenly spaced timesteps, each a true color cylindrical projection of the complete circumference of Jupiter, from 60 degrees south to 60 degrees north. The maps are made by first assembling mosaics of six images taken by Cassini's narrow-angle camera in the same spectral filter over the course of one Jupiter rotation and, consequently, covering the whole planet. Three such global maps -- in red, green and blue filters -- are combined to make one color map showing Jupiter during one Jovian rotation. Fourteen such maps, spanning 24 Jovian rotations at uneven time intervals comprise the movie."<ref name=Lavoie00/>

The passage of time is accelerated by a factor of 600,000.
{{clear}}

==Saturn==
{{main|Saturn}}
[[Image:Blue Saturn.jpg|thumb|right|250px|The image shows Saturn's northern hemisphere from the Cassini spacecraft with Mimas in front. Credit: NASA/JPL/Space Science Institute.]]
In the image at right, "Mimas drifts along in its orbit against the azure backdrop of Saturn's northern latitudes in this true color view from NASA's Cassini spacecraft. The long, dark lines on the atmosphere are shadows cast by the planet's rings."<ref name=Wilson>{{ cite book
|author=Jim Wilson
|title=Saturn's Blues
|publisher=NASA/JPL
|location=Pasadena, California USA
|date=March 23, 2008
|url=http://www.nasa.gov/multimedia/imagegallery/image_feature_264.html
|accessdate=2013-03-27 }}</ref>

"Saturn's northern hemisphere is presently relatively cloud-free, and rays of sunlight take a long path through the atmosphere. This results in sunlight being scattered at shorter (bluer) wavelengths, thus giving the northernmost latitudes their bluish appearance at visible wavelengths."<ref name=Wilson/>
{{clear}}

==Uranus==
{{main|Uranus}}
[[Image:Uranuscolour.png|thumb|right|250px|Uranus's southern hemisphere in approximate natural colour (left) and in shorter wavelengths (right), shows its faint cloud bands and atmospheric "hood" as seen by ''Voyager 2''. Credit: NASA.]]
In 1986 ''Voyager 2'' found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).<ref name="Smith Soderblom et al. 1986">Smith, B. A.; Soderblom, L. A.; Beebe, A.; Bliss, D.; Boyce, J. M.; Brahic, A.; Briggs, G. A.; Brown, R. H. et al (4 July 1986). "Voyager 2 in the Uranian System: Imaging Science Results". Science 233 (4759): 43–64. Bibcode 1986Sci...233...43S. doi:10.1126/science.233.4759.43. {{PMID|17812889}}</ref> Their boundary is located at about -45 degrees of [[w:latitude|latitude]]. A narrow band straddling the latitudinal range from -45 to -50 degrees is the brightest large feature on the visible surface of the planet.<ref name="Smith Soderblom et al. 1986" /><ref name="Hammel de Pater et al. Uranus in 2003, 2005">Hammel, H. B.; de Pater, I.; Gibbard, S. G.; Lockwood, G. W.; Rages, K. (June 2005). "Uranus in 2003: Zonal winds, banded structure, and discrete features" (PDF). Icarus 175 (2): 534–545. Bibcode 2005Icar..175..534H. doi:10.1016/j.icarus.2004.11.012</ref> It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2&nbsp;bar (see above).<ref name="Rages Hammel et al. 2004">Rages, K. A.; Hammel, H. B.; Friedson, A. J. (11 September 2004). "Evidence for temporal change at Uranus' south pole". Icarus 172 (2): 548–554. Bibcode 2004Icar..172..548R. doi:10.1016/j.icarus.2004.07.009</ref> Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.<ref name="Smith Soderblom et al. 1986" /> In all other respects Uranus looked like a dynamically dead planet in 1986. Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and [[w:Keck telescopes|Keck]] telescope initially observed neither a collar nor a polar cap in the northern hemisphere.<ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.<ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> In 2007, when Uranus passed its equinox, the southern collar almost disappeared, while a faint northern collar emerged near 45 degrees of [[w:latitude|latitude]].<ref name="Sromovsky Fry et al. 2009">Sromovsky, L. A.; Fry, P. M.; Hammel, H. B.; Ahue, W. M.; de Pater, I.; Rages, K. A.; Showalter, M. R.; van Dam, M. A. (September 2009). "Uranus at equinox: Cloud morphology and dynamics". Icarus 203 (1): 265–286. Bibcode 2009Icar..203..265S. doi:10.1016/j.icarus.2009.04.015.</ref>

There are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.<ref name=Hammel2007>{{ cite journal
|last=Hammel|first=H.B.|coauthors=Lockwood, G.W.
|title=Long-term atmospheric variability on Uranus and Neptune
|journal=Icarus
|year=2007
|volume=186
|pages=291–301
|doi=10.1016/j.icarus.2006.08.027
| bibcode=2007Icar..186..291H }}</ref> During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.<ref name="Lockwood & Jerzykiewicz 2006">Lockwood, G. W.; Jerzykiewicz, Mikołaj A. (February 2006). "Photometric variability of Uranus and Neptune, 1950–2004". Icarus 180 (2): 442–452. Bibcode 2006Icar..180..442L. doi:10.1016/j.icarus.2005.09.009.</ref> This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.<ref name=Hammel2007/> Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the [[w:meridional|meridional]] [[w:albedo|albedo]] patterns.<ref name=Hammel2007/> Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),<ref name="Rages Hammel et al. 2004" /> while the northern hemisphere demonstrated increasing activity,<ref name=planetary>{{ cite book
|title=No Longer Boring: 'Fireworks' and Other Surprises at Uranus Spotted Through Adaptive Optics
|author=Emily Lakdawalla
|date=2004
|accessdate=June 13, 2007 |url=http://web.archive.org/web/20060525015410/http://www.planetary.org/news/2004/1111_No_Longer_Boring_Fireworks_and_Other.html }}</ref> such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.<ref name="Hammel de Pater et al. Uranus in 2004, 2005" >Hammel, H. B.; de Pater, I.; Gibbard, S. G.; Lockwood, G. W.; Rages, K. (May 2005). "New cloud activity on Uranus in 2004: First detection of a southern feature at 2.2 µm" (PDF). Icarus 175 (1): 284–288. Bibcode 2005Icar..175..284H. doi:10.1016/j.icarus.2004.11.016.</ref> This indeed happened in 2007 when the planet passed an equinox: a faint northern polar collar arose, while the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.<ref name="Sromovsky Fry et al. 2009" />
{{clear}}

==Neptune==
{{main|Neptune}}
[[Image:Neptune.jpg|thumb|right|250px|This picture from the [[w:Voyager 2|Voyager 2]] sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. Credit: NASA.]]
Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia, and methane. Traces of methane in the outermost regions in part account for the planet's blue appearance.<ref name=bluecolour>{{ cite book
|author=Kirk Munsell
|author2=Harman Smith
|author3=Samantha Harvey
|date=13 November 2007
|url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=OverviewLong
|title=Neptune overview
|publisher=NASA
|accessdate=20 February 2008
}}</ref>
{{clear}}

==Interplanetary medium==
{{main|Interplanetaries|Interplanetary medium}}
[[Image:EZL-Faulkes.jpg|thumb|right|250px|The Zodiacal Light is over the Faulkes Telescope, Haleakala, Maui. Credit: 808caver.]]
'''Def.''' that part of outer space between the planets of a solar system and its star"<ref name=InterplanetarySpaceWikt>{{ cite book
|title=interplanetary space
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 10, 2012
|url=http://en.wiktionary.org/wiki/interplanetary_space
|accessdate=2013-05-13 }}</ref> is called '''interplanetary space'''.

'''Def.''' the material which fills the [[w:Solar system|solar system]] and through which all the larger solar system bodies such as planets, asteroids and comets move is called an '''interplanetary medium'''.

Chemical ions above the Earth's atmosphere, moving at very high speeds and at concentrations up to 100 particles per cm<sup>3</sup> ([[w:Centimetre|centimeter]] cubed, a unit of [[w:Volume|volume]]) constitute the [[w:interplanetary medium|interplanetary medium]].

The '''Zodiacal light''' is a faint, roughly triangular, diffuse white glow seen in the night sky that appears to extend up from the vicinity of the [[Sun (star)|Sun]] along the [[w:ecliptic|ecliptic]] or [[w:zodiac|zodiac]].<ref name=Darling>[http://www.daviddarling.info/encyclopedia/Z/zodiacal_cloud.html Internet Encyclopedia of Science] Accessed April 2010</ref> It is best seen just after sunset and before sunrise in spring and autumn when the zodiac is at a steep angle to the horizon. Caused by sunlight scattered by [[w:space dust|space dust]] in the zodiacal cloud, it is so faint that either [[w:moonlight|moonlight]] or [[w:light pollution|light pollution]] renders it invisible. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed in a band completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible for major part<ref name=Reach>Reach, W. T. (1997). [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997ASPC..124...33R&amp;db_key=AST "The structured zodiacal light: IRAS, COBE, and ISO observations", page 1 (in Introduction)]</ref> of the total skylight on a moonless night. There is also a very faint, but still slightly increased, oval glow directly opposite the Sun which is known as the [[w:gegenschein|gegenschein]]. The dust forms a thick pancake-shaped cloud in the [[w:Solar System|Solar System]] collectively known as the '''zodiacal cloud''', which occupies the same plane as the ecliptic. The dust particles are between 10 and 300 [[w:micrometre|micrometre]]s in diameter, with most mass around 150 micrometres.<ref>{{cite book
 | author=Peucker-Ehrenbrink, Bernhard
|author2=Schmitz, Birger
 | title=Accretion of extraterrestrial matter throughout earth's history | pages=66&ndash;67
 | publisher=Springer | date=2001 | isbn=0-306-46689-9 }}</ref>

'''Interplanetary scintillation''' refers to random fluctuations in the intensity of [[w:radio wave|radio wave]]s of [[w:Sky|celestial]] origin, on the timescale of a few seconds. It is analogous to the [[w:Scintillation (astronomy)|twinkling]] one sees looking at [[w:star|star]]s in the [[w:sky|sky]] at night, but in the radio part of the [[w:electromagnetic spectrum|electromagnetic spectrum]] rather than the [[w:visible spectrum|visible one]]. Interplanetary scintillation is the result of radio waves traveling through fluctuations in the density of the [[w:electron|electron]] and [[w:proton|proton]]s that make up the [[w:solar wind|solar wind]].

Scintillation occurs as a result of variations in the [[w:refractive index|refractive index]] of the medium through which waves are traveling. The solar wind is a [[w:plasma (physics)|plasma]], composed primarily of electrons and lone protons, and the variations in the index of refraction are caused by variations in the [[w:density|density]] of the plasma.<ref>Jokipii (1973), pp. 11–12.</ref> Different indices of refraction result in [[w:phase (waves)|phase]] changes between waves traveling through different locations, which results in [[w:interference (physics)|interference]]. As the waves interfere, both the [[w:frequency|frequency]] of the wave and its [[w:angular size|angular size]] are broadened, and the intensity varies.<ref>Alurkar (1997), p. 11.</ref>

"The Raman spectra of some [interplanetary dust particle] IDPs also show red photoluminescence that is similar to the excess red emission seen in some astronomical objects and that has also been attributed to [polycyclic aromatic hydrocarbons] PAH s and hydrogenated amorphous carbon. Moreover, a part of the carbonaceous phase in IDPs and meteorites contains deuterium to hydrogen ratios that are greater than those for terrestrial samples."<ref name=Allamandola>{{ cite journal
|author=L. J. Allamandola
|author2=S. A. Sandford
|author3=B. Wopenka
|title=Interstellar Polycyclic Aromatic Hydrocarbons and Carbon in Interplanetary Dust Particles and Meteorites
|journal=Science
|month=July 3,
|year=1987
|volume=237
|issue=4810
|pages=56-9
|url=http://www.sciencemag.org/content/237/4810/56.short
|arxiv=
|bibcode=
|doi=10.1126/science.237.4810.56
|pmid=
|accessdate=2013-08-02 }}</ref>
{{clear}}

==Interplanetary magnetic field==

'''Notation''': let the symbol '''IMF''' stand for '''interplanetary magnetic field'''.

"[T]he IMF switched from northward to southward while Mariner [10] was in the Mercurian magnetosphere."<ref name=Russell>{{ cite book
|author=C. T. Russell
|author2=D. N. Baker
|author3=J. A. Slavin
|title=The Magnetosphere of Mercury, In: ''Mercury''
|publisher=University of Arizona Press
|location=Tucson, Arizona, United States of America
|date=January 1, 1988
|editor=Faith Vilas
|editor2=Clark R. Chapman
|editor3=Mildred Shapley Matthews
|pages=514-61
|url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/magMercury.pdf
|arxiv=
|bibcode=1988merc.book..514R
|doi=
|isbn=0816510857
|pmid=
|accessdate=2012-08-23 }}</ref>

==HL Tauri==
[[Image:Eso1436a.tif|right|thumb|300px|ALMA image shows the protoplanetary disc around HL Tauri. Credit: ALMA (ESO/NAOJ/NRAO).{{tlx|free media}}]]
In September 2014, "the Atacama Large Millimeter/submillimeter Array (ALMA), [...] features radio antennas separated by distances of up to 15 kilometres. With such long spans between them, the antennas work as a high-resolution receiver that can discern cool objects less than a millimetre across."<ref name=Boyle>{{ cite web
|author=Rebecca Boyle
|title=These dusty young stars are changing the rules of planet-building
|publisher=Nature
|location=
|date=4 December 2018
|url=https://www.nature.com/articles/d41586-018-07591-8?utm_source=Nature+Briefing&utm_campaign=560158b08c-briefing-dy-20190808&utm_medium=email&utm_term=0_c9dfd39373-560158b08c-43855389
|accessdate=10 August 2019 }}</ref>

"When the telescope [was] trained [...] on the young star, named HL Tauri, [...] a bright smear of dust and gas [appeared]. Instead, when ALMA’s supercomputer stitched together those photons, the image resolved into a disk with a well-defined ring structure [image on the right], with gaps seemingly etched by small, infant planets orbiting a central star."<ref name=Boyle/>

"What the [telescope] had captured was a picture of a planetary nursery — where baby planets were forming in a disk of gas and dust around HL Tauri. This observation marked the start of a revolution in the burgeoning field of planetary-disk imaging."<ref name=Boyle/>

"These planet-forming regions exhibit a wide variety of patterns. Some are neat ovals, with lanes as clearly defined as those of a race track. Others look like galaxies in miniature, with swirling arms that branch off into open arcs."<ref name=Boyle/>

"The latest observations, including results announced in April and July, have revealed planets in the process of being sculpted, with dust and gas flowing onto bulbous, red-hot infant worlds<sup>2,3</sup>."<ref name=Boyle/>

"The Solar System has rocky planets near the Sun and giant gas balls farther out, but the panoply of exoplanets obeys no tidy patterns. And the rule book for world-building is getting more complicated as [telescopes] find evidence of planets in the process of being born."<ref name=Boyle/>

"We see all kinds of structure in these disks, even at very young ages. Even younger than we classically thought planets should form."<ref name=Follette>{{ cite web
|author=Kate Follette
|title=These dusty young stars are changing the rules of planet-building
|publisher=Nature
|location=
|date=4 December 2018
|url=https://www.nature.com/articles/d41586-018-07591-8?utm_source=Nature+Briefing&utm_campaign=560158b08c-briefing-dy-20190808&utm_medium=email&utm_term=0_c9dfd39373-560158b08c-43855389
|accessdate=10 August 2019 }}</ref>
{{clear}}

==HD 1353445B==
[[Image:Protoplanetary disk of HD 135344B (eso1640d).jpg|right|thumb|300px|Using the ESO's SPHERE instrument at the Very Large Telescope, the planetary disc surrounding the star HD 135344B, about 450 light-years away, has been observed. Credit: ESO, T. Stolker et al.{{tlx|free media}}]]
"Using ESO’s SPHERE instrument at the Very Large Telescope, a team of astronomers observed the planetary disc surrounding the star HD 135344B, about 450 light-years away. The disc shows prominent spiral-arm-like structures. These are thought to have been created by one or multiple massive protoplanets, destined to become Jupiter-like worlds."<ref name=Stolker>{{ cite web
|author=T. Stolker
|title=Disc around the star HD 135344B
|publisher=European Southern Observatory
|location=
|date=9 November 2016
|url=https://www.eso.org/public/images/eso1640d/
|accessdate=10 August 2019 }}</ref>

"The central part of the image appears dark because SPHERE blocks out the light from the brilliant central star to reveal the much fainter structures surrounding it."<ref name=Stolker/>
{{clear}}

==IM Lupi==
[[Image:Dusty disk around IM Lupi2.jpg|right|thumb|300px|SPHERE captures a dusty disk around IM Lupi. Credit: ESO/H. Avenhaus et al./DARTT-S collaboration.{{tlx|fairuse}}]]
"This spectacular image from the SPHERE instrument on ESO's Very Large Telescope shows the dusty disc around the young star IM Lupi in finer detail than ever before."<ref name=Avenhaus>{{ cite web
|author=H. Avenhaus
|title=SPHERE image of the dusty disc around IM Lupi
|publisher=European Southern Observatory
|location=
|date=11 April 2018
|url=https://www.eso.org/public/images/eso1811c/
|accessdate=10 August 2019 }}</ref>
{{clear}}

==Classical planets==
{{main|Classical planets}}
In antiquity the '''classical planets''' were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and [[Draft:Moon|Moon]] and the five non-earth planets of our [[solar system]] closest to the sun (and closest to the Earth); all easily visible without a telescope. They are [[Keynote lectures/Mercury|Mercury]], [[Draft:Venus|Venus]], [[Keynote lectures/Mars|Mars]], [[Jupiter]], and [[Saturn]].

Apparently 5102 b2k (before the year [[w:Epoch astronomy|2000.0]]), -3102 or 3102 BC, is the historical year assigned to a Hindu table of planets that does not include the classical planet [[Draft:Venus|Venus]].<ref name=Delambre>{{ cite book
|author=Jean Baptiste Joseph Delambre
|title=Histoire de l'astronomie ancienne
|publisher=Courcier
|location=Paris
|date=1817
|editor=
|pages=639
|url=http://books.google.com/books?id=2lVUjJSxjhQC&pg=PR3&cad=3#v=onepage&f=false
|arxiv=
|bibcode=
|doi=
|pmid=
|isbn=
|accessdate=2012-01-13 }}</ref> "Vénus seule ne s'y trouvait pas."<ref name=Delambre/> "Venus alone is not found there."<ref name=Velikovsky>{{ cite book
|author=Immanuel Velikovsky
|title=Worlds in Collision
|publisher=Dell Publishing Co., Inc.
|location=New York
|date=January 1965
|editor=
|pages=401
|url=http://books.google.com/books?id=FJst27kSVBgC&pg=PA13&hl=en
|arxiv=
|bibcode=
|doi=
|pmid=
|isbn=
|accessdate=2012-01-13 }}</ref>

==History==

"Aristarchus also figured out how to measure the distances to the Sun and the Moon and their sizes."<ref name=Koutsoyiannis>{{ cite book
|author=D. Koutsoyiannis
|author2=A. N. Angelakis
|title=Hydrologic and Hydraulic Science and Technology in Ancient Greece, In: ''Encyclopedia of Water Science''
|publisher=Marcel Dekker, Inc.
|location=New York
|date=2003
|editor=
|pages=415-7
|url=http://www.a-angelakis.gr/files/books/2002Encycl2WatResTechAncGreEntry2.pdf
|bibcode=
|doi=10.1081/E-EWS-120016393
|pmid=
|isbn=
|accessdate=2011-10-26 }}</ref>

Ancient astronomers were able to differentiate between stars and [[w:planets|planets]], as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.

==Technology==
[[Image:Transiting Exoplanet Survey Satellite artist concept (transparent background).png|right|thumb|300px|Artist concept is of TESS. Credit: NASA.{{tlx|free media}}]]
The '''Transiting Exoplanet Survey Satellite''' ('''TESS''') is a space telescope for NASA's Explorers program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the ''Kepler'' mission.<ref name="tess-nasa-faq">{{ cite book
|url=http://tess.gsfc.nasa.gov/documents/TESS_FactSheet_Oct2014.pdf
|title=TESS: Transiting Exoplanet Survey Satellite
|publisher=NASA
|date=October 2014
|accessdate=December 17, 2014
}}</ref> It was launched on April 18, 2018 atop a Falcon 9 rocket.<ref name="NYT-20180326">{{ cite book
|author=Dennis Overbye
|title=Meet Tess, Seeker of Alien Worlds, In: ''The New York Times''
|url=https://www.nytimes.com/2018/03/26/science/tess-nasa-exoplanets.html
|date=March 26, 2018
|accessdate=March 26, 2018
}}</ref><ref name="sfn-schedule">{{ cite book
|url=https://spaceflightnow.com/launch-schedule/
|title=Launch Schedule
|date=February 27, 2018
|accessdate=February 28, 2018
}}</ref><ref name=bbc-amos>{{ cite journal
|url=https://www.bbc.com/news/science-environment-43790557
|title=Planet-hunter launches from Florida
|first=Jonathan
|last=Amos
|journal=BBC News
|date=April 19, 2018
}}</ref><ref>{{ cite book
|url=https://www.nasa.gov/press-release/nasa-planet-hunter-on-its-way-to-orbit
|title=NASA Planet Hunter on Its Way to Orbit
|date=April 19, 2018
|accessdate=April 19, 2018
}}</ref> During its primary mission, it is expected to find more than 20,000 exoplanets,<ref name="astrobio.net">{{ cite book
|url=https://www.astrobio.net/news-exclusive/the-astrobiology-magazine-guide-to-tess/
|title=The Astrobiology Magazine Guide to TESS
|first=Keith
|last=Cooper
|date=April 13, 2018
|accessdate=April 14, 2018
}}</ref> compared to about 3,800 exoplanets known when it launched.<ref>{{ cite book
|last1=Schneider
|first1=J.
|title=Interactive Extra-solar Planets Catalog
|url=http://exoplanet.eu/catalog.php
|accessdate=April 14, 2018
}}</ref> The first light image from TESS was taken on August 7, 2018, and released publicly on September 17, 2018.<ref name="NYT-20180920">{{ cite book
|author=Dennis Overbye
|title=NASA’s TESS Starts Collecting Planets - The satellite, launched in April, has already identified at least 73 stars that may harbor exoplanets, most of them new to astronomers.
|url=https://www.nytimes.com/2018/09/20/science/nasa-tess-planets.html
|date=September 20, 2018
|accessdate=September 23, 2018
}}</ref><ref name="NASA-20180917a">{{ cite book
|last1=Kazmierczak
|first1=Jeanette
|last2=Garner
|first2=Rob
|title=NASA’s TESS Shares First Science Image in Hunt to Find New Worlds
|url=https://www.nasa.gov/feature/goddard/2018/nasa-s-tess-shares-first-science-image-in-hunt-to-find-new-worlds
|date=September 17, 2018
|accessdate=September 23, 2018
}}</ref><ref name="NASA-20180917b">{{ cite book |last1=Wiessinger |first1=Scott |last2=Lepsch |first2=Aaron E. |last3=Kazmierczak |first3=Jeanette |last4=Reddy |first4=Francis |last5=Boyd |first5=Padi |title=NASA’s TESS Releases First Science Image |url=https://svs.gsfc.nasa.gov/13069 
|date=September 17, 2018 |accessdate=September 23, 2018 }}</ref>

TESS records the nearest and brightest main sequence stars hosting transiting exoplanets, which are the most favorable targets for detailed investigations.<ref name="NASA-2014">{{ cite book
|url=http://www.nasa.gov/sites/default/files/files/NASA_2015_Budget_Estimates.pdf
|title=NASA FY 2015 President's Budget Request Summary
|publisher=NASA
|date=March 10, 2014
}}</ref>

TESS orbits Earth twice during the time the Moon orbits once, a 2:1 resonance with the Moon.<ref name="McGiffin">{{cite book
|last1=McGiffin
|first1=Daniel A.
|last2=Mathews
|first2=Michael
|last3=Cooley
|first3=Steven
|title=HIGH EARTH ORBIT DESIGN FOR LUNAR-ASSISTED MEDIUM CLASS EXPLORER MISSIONS
|date=June 1, 2001
|publisher=NASA Goddard Space Flight Center
|location=Greenbelt, MD
|url=https://ntrs.nasa.gov/search.jsp?R=20010084964
}}</ref>

TESS is designed to carry out the first spaceborne astronomical all-sky transiting exoplanet survey.<ref name="selection">{{ cite book
|url=http://www.nasa.gov/home/hqnews/2013/apr/HQ_13-088_Astro_Explorer_Mission_.html
|title=NASA Selects Explorer Investigations for Formulation
|publisher=NASA
|first=J. D.
|last=Harrington
|date=April 5, 2013
}}</ref><ref name="Ricker2014">{{ cite book
|title=Discovering New Earths and Super-Earths in the Solar Neighborhood, In: ''SPIE Astronomical Telescopes + Instrumentation''
|location=Montréal, Quebec, Canada
|first=George R.
|last=Ricker
|date=June 26, 2014
|doi=10.1117/2.3201407.18
}}</ref>

Every 13.7 days at its perigee of {{cvt|67000|mi|km|order=flip}}, TESS will downlink the data it has collected during the orbit to Earth over a period of approximately three hours.<ref>{{ cite book
|url=http://www.nasa.gov/content/goddard/new-explorer-mission-chooses-the-just-right-orbit/
|title=New Explorer Mission Chooses the 'Just-Right' Orbit
|publisher=NASA
|first=Lori
|last=Keesey
|date=July 31, 2013
}}</ref>

==Hypotheses==
{{main|Hypotheses}}
# For a spheroidal rogue astronomical object to assume a planetary orbit around a star may require a third astronomical object of comparable or larger size.

==See also==
{{div col|colwidth=20em}}
* [[Draft:Astrogeology]] (52 kB) (11 August 2019)
* [[Draft:Astrognosy]] (43 kB) (13 August 2019)
* [[Draft:Astrohistory]] (118 kB) (18 May 2019)
* [[Draft:Atmospheric sciences]] (37 kB) (16 May 2019)
* [[Draft:Callisto]] (45 kB) (2 July 2019)
* [[Draft:Ceres]] (31 kB) (23 June 2019)
* [[Draft:Charge ontology|Charge ontology]] (37 kB) (25 June 2019)
* [[Draft:Charges|Charges]] (25 kB) (9 April 2019)
* [[Draft:Chemicals]], (189 kB) 1 April 2019 - 13 September 2019 (last update)
* [[Draft:Classical mechanics]] (11 kB) (23 July 2019)
* [[Classical planets]] (Hasselo stadial) (109 kB)
* [[Draft:Comets]] (36 kB) (23 June 2019)
* [[Draft:Dione]] (17 kB) (22 May 2019)
* [[Draft:Distances]] (12 kB) (25 May 2019)
* [[Keynote lectures/Earth|Earth]]
* [[Draft:Electrospheres|Electrospheres]] (14 kB) (26 May 2019)
* [[Draft:Enceladus]] (39 kB) (22 May 2019)
* [[Draft:Fringe sciences]] (107 kB) (13 September 2019)
* [[Draft:Gases]] (23 kB) (22 May 2019)
* [[Introduction to the Planets]]
* [[Draft:Io]] (36 kB) (24 June 2019)
* [[Jupiter]]
* [[Draft:Liquids]] (37 kB) (12 August 2019)
* [[Keynote lectures/Mars|Mars]]
* [[Draft:Measurements]] (3 kB) (25 June 2019)
* [[Keynote lectures/Mercury|Mercury]]
* [[Meteor astronomy]]
* [[Draft:Moon]] (105 kB) (17 May 2019)
* [[Draft:Natural electric field of the Earth|Natural electric field of the Earth]] (27 kB) (23 June 2019)
* [[Neptune]]
* [[Draft:Planets]] (127 kB) (10 August 2019)
* [[Planets around other stars]]
* [[Draft:Plasmas]] (13 kB) (2 July 2019)
* [[Draft:Pluto|Pluto]] (18 kB) (25 July 2019)
* [[Draft:Keynote lectures/Radiation]] (154 kB) (25 June 2019)
* [[Draft:Keynote lectures/Radiation astronomy]] (169 kB) (23 June 2019)
* [[Draft:Regions]] (20 kB) (25 June 2019)
* [[Saturn]]
* [[Draft:Sedimentary rocks]] (48 kB) (31 August 2019)
* [[Draft:Sediments]] (57 kB) (19 September 2019)
* [[Solar binary]]
* [[Solar systems]]
* [[Draft:Solids]] (13 kB) (19 September 2019)
* [[Star fission]]
* [[Draft:Structural geology]] (60 kB) (9 September 2019)
* [[Draft:Titan]] (48 kB) (18 May 2019)
* [[Uranus]]
* [[Draft:Venus]] (87 kB) (17 May 2019)
* [[Draft:Vesta]] (35 kB) (20 June 2019)
* [[Draft:Volcanoes]] (95 kB) (19 September 2019)
{{Div col end}}

==References==
{{reflist|2}}

==External links==
* [http://www.bing.com/search?q=&go=&qs=n&sk=&sc=8-15&qb=1&FORM=AXRE Bing Advanced search]
* [http://books.google.com/ Google Books]
* [http://scholar.google.com/advanced_scholar_search?hl=en&lr= Google scholar Advanced Scholar Search]
* [http://www.iau.org/ International Astronomical Union]
* [http://www.jstor.org/ JSTOR]
* [http://www.lycos.com/ Lycos search]
* [http://nedwww.ipac.caltech.edu/ NASA/IPAC Extragalactic Database - NED]
* [http://nssdc.gsfc.nasa.gov/ NASA's National Space Science Data Center]
* [http://www.osti.gov/ Office of Scientific & Technical Information]
* [http://www.questia.com/ Questia - The Online Library of Books and Journals]
* [http://online.sagepub.com/ SAGE journals online]
* [http://www.adsabs.harvard.edu/ The SAO/NASA Astrophysics Data System]
* [http://www.scirus.com/srsapp/advanced/index.jsp?q1= Scirus for scientific information only advanced search]
* [http://simbad.u-strasbg.fr/simbad/ SIMBAD Astronomical Database]
* [http://simbad.harvard.edu/simbad/ SIMBAD Web interface, Harvard alternate]
* [http://nssdc.gsfc.nasa.gov/nmc/SpacecraftQuery.jsp Spacecraft Query at NASA]
* [http://www.springerlink.com/ SpringerLink]
* [http://www.tandfonline.com/ Taylor & Francis Online]
* [http://onlinelibrary.wiley.com/advanced/search Wiley Online Library Advanced Search]
* [http://search.yahoo.com/web/advanced Yahoo Advanced Web Search]

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