Planet 22915 225966723 2008-07-16T06:36:14Z Serendipodous 414691 [[WP:UNDO|Undid]] revision 225966423 by [[Special:Contributions/Rursus|Rursus]] ([[User talk:Rursus|talk]]) Sorry; don't agree. The scientific community is the community of scientists {{Three other uses|the astronomical term|"planet" as defined by astrologers|Planets in astrology|the related but distinct class of objects|Dwarf planet}} {{sprotect|small=yes}} [[Image:Planetart.jpg|thumb|300 px|Artist's depiction of the [[extrasolar planet]] [[HD 209458 b]] orbiting its star]] A '''planet''', as [[2006 definition of planet|defined]] by the [[International Astronomical Union]] (IAU), is a celestial body [[orbit]]ing a [[star]] or [[Stellar evolution#Stellar remnants|stellar remnant]] that is massive enough to be rounded by its own [[gravity]], not massive enough to cause [[thermonuclear fusion]], and has [[cleared the neighbourhood|cleared its neighbouring region]] of [[planetesimal]]s.{{Ref_label|A|a|none}}<ref name=IAU>{{ cite web|title=IAU 2006 General Assembly: Result of the IAU Resolution votes|url=http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html|publisher=International Astronomical Union|year=2006|accessdate=2007-04-30}}</ref><ref name=WSGESP>{{cite web|year=2001|title=Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union| work=IAU|url=http://www.dtm.ciw.edu/boss/definition.html|accessdate=2006-05-25}}</ref> The term ''planet'' is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the [[astrology|movement of the planets affects their lives]], although such a causation is [[Astrology and astronomy#Historical divergence|rejected by the scientific community]]. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution [[2006 definition of planet|defining planets]] within the [[Solar System]]. This definition has been both praised and criticized, and remains disputed by some scientists. The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, [[astronomer]]s saw that, like Earth, the planets rotated around tilted axes and some share such features as ice-caps and seasons. Since the dawn of the [[Space Age]], close observation by probes has found that Earth and the other planets share characteristics such as [[volcano|volcanism]], [[hurricane]]s, [[tectonics]] and even [[hydrology]]. Since 1992, through the discovery of hundreds of [[extrasolar planet]]s (planets around other stars), scientists are beginning to observe similar features throughout the [[Milky Way|Milky Way Galaxy]]. Planets are generally divided into two main types: large, low-density [[gas giant]]s and smaller, rocky [[terrestrial planet|terrestrials]]. Under IAU definitions, there are eight planets in the Solar System; in order they are the four terrestrials: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]], with the four gas giants: [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. The Solar System also contains at least four [[dwarf planet]]s: [[Ceres (dwarf planet)|Ceres]], [[Pluto]], [[Makemake (dwarf planet)|Makemake]] and [[Eris (dwarf planet)|Eris]]. Many of these planets are orbited by one or more [[natural satellite|moons]], which can be larger than small planets. There are also 296 known extrasolar planets.<ref name="Encyclopaedia">{{cite web |title=Interactive Extra-solar Planets Catalog |work=The Extrasolar Planets Encyclopaedia |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=2008-06-13 |accessdate=2006-10-31}}</ref> ==History== {{main|History of astronomy|Definition of planet}}{{seealso|Timeline of solar system astronomy}} The idea of planets has evolved over its history, from the divine [[wandering stars]] of antiquity to the earthly objects of the scientific age. The concept has also now expanded to include worlds not only in our Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy. ===Antiquity=== {{seealso|Geocentric model}} [[Image:Ptolemaicsystem-small.png|thumb|right|Early printed rendition of a geocentric cosmological model]] In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights "πλάνητες ἀστέρες" (''planetes asteres'': wandering stars) or simply "πλανήτοι" (''planētoi'': wanderers),<ref>H. G. Liddell and R. Scott, ''A Greek&ndash;English Lexicon'', ninth edition, (Oxford: Clarendon Press, 1940).</ref> from which the today's word "planet" was derived.<ref>{{cite web|url=http://www.m-w.com/dictionary/planet|title=Definition of planet|publisher=Merriam-Webster OnLine|accessdate=2007-07-23}}</ref><ref name=oed/> In [[ancient Greece]] as well as in [[Chinese astronomy|ancient China]], ancient [[Babylon]] and indeed all pre-modern civilisations,<ref>{{cite journal|first=Otto E.|last=Neugebauer|year=1945|title=The History of Ancient Astronomy Problems and Methods|journal=Journal of Near Eastern Studies|volume=4|issue=1|pages=1&ndash;38|doi=10.1086/370729}}</ref><ref>{{citebook|first=Colin|last=Ronan|title=Astronomy in China, Korea and Japan|edition=Walker|chapter=Astronomy Before the Telescope|pages=264&ndash;5}}</ref> it was almost universally believed that Earth was in the centre of the [[Universe]] and that all the "planets" circled the Earth. The reasons for this perception was that stars and planets appeared to revolve around the Earth each day,<ref>{{cite book|first=Thomas S.|last=Kuhn|title=The Copernican Revolution|pages=5&ndash;20|publisher=Harvard University Press|year=1957}}</ref> and the apparently [[common sense]] perception that the Earth was solid and stable and that it is not moving but at rest. The Greek cosmological system was taken from that of the [[Babylonia]]ns,<ref name=ancientmes/> a contemporary [[Mesopotamia]]n civilisation from whom they began to acquire astronomical learning from around 600 BC, including the [[constellation]]s and the [[zodiac]].<ref name='burnet'>{{cite book | first=John | last=Burnet | title= Greek philosophy: Thales to Plato | year=1950 | publisher=Macmillan and Co. | pages=7&ndash;11 | url=http://books.google.com/books?id=7yUAmmqHHEgC&pg=PR4&sig=9lHUwUNTSf9HS8rTItG5CPVec7o#PPA7,M1 | accessdate=2008-02-07 }}<!--{{cite web|title=A Chronological History of Babylonian Astronomy|last=Thompson|first=Gary D.|url=http://members.optusnet.com.au/~gtosiris/page9k.html| publisher=members.optusnet.com.au/~gtosiris|year=2007|accessdate=2007-04-30}}{{Rs}}--></ref> In the 6th century BC, the Babylonians had a highly advanced level of astronomical knowledge, and had a theory of the planets centuries before the ancient Greeks. The oldest planetary astronomical text that we possess is the Babylonian [[Venus tablet of Ammisaduqa]], a [[7th century BC]] copy of a list of observations of the motions of the planet Venus that probably dates as early as the second millennium BC.<ref name= practice /> The Babylonians also laid the foundations of what would eventually become [[Western astrology]].<ref name=book>{{cite book |last=Holden |first=James Herschel |title=A History of Horoscopic Astrology |year=1996 |publisher=AFA |isbn=978-0866904636 |pages=1}}</ref> The ''[[Enuma anu enlil]]'', written during the [[Neo-Assyrian]] period in the 7th century BC,<ref>{{cite book| volume=8 | series=State Archives of Assyria | title=Astrological reports to Assyrian kings | editor=Hermann Hunger | year=1992 | publisher=Helsinki University Press | isbn=951-570-130-9}}</ref> comprises a list of [[omen]]s and their relationships with various celestial phenomena including the motions of the planets.<ref>{{cite journal|title=Babylonian Planetary Omens. Part One. Enuma Anu Enlil, Tablet 63: The Venus Tablet of Ammisaduqa|first=W. G.|last=Lambert|year=1987| url=http://links.jstor.org/sici?sici=0003-0279(198701%2F03)107%3A1%3C93%3ABAOATS%3E2.0.CO%3B2-0|journal=Journal of the American Oriental Society|accessdate=2008-02-04}}</ref> [[Sumer]]ians, predecessors of Babylonians which are credited as one of the [[Cradle of civilization|first civilizations]] and the inventors of [[writing]], had identified [[Inanna|at least Venus]] by 1500 BC.<ref name=ancientmes>{{cite journal| url=http://www.folklore.ee/Folklore/vol16/planets.pdf| author = Kasak, Enn; Veede, Raul | title=Understanding Planets in Ancient Mesopotamia (PDF)| journal = Electronic Journal of Folklore | accessdate=2008-02-06 | issn = 1406-0957| volume=16 | year = 2001 | pages = 7&ndash;35 | publisher = Estonian Literary Museum | editor=Mare Kõiva and Andres Kuperjanov}}</ref> Conversely, there is no evidence of a comparable knowledge of the planets in the earliest written Greek sources, such as the ''[[Iliad]]'' and the ''[[Odyssey]]''.<ref name= book/> By the first century BC, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness and account for much of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the ''[[Almagest]]'' written by [[Ptolemy]] in the 2nd century AD. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.<ref name=almagest /><ref name= practice /> To the Greeks and Romans there were seven known planets; each presumed to be [[Geocentric model|circling the Earth]] according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the [[Moon]], Mercury, Venus, the [[Sun]], Mars, Jupiter, and Saturn.<ref name=almagest>{{cite journal|first=Bernard R.|last=Goldstein|title=Saving the phenomena : the background to Ptolemy's planetary theory| journal=Journal for the History of Astronomy | volume=28 | issue=1 | year=1997 | pages=1&ndash;12 | location=Cambridge (UK) |url=http://adsabs.harvard.edu/abs/1997JHA....28....1G|accessdate=2008-02-06}}</ref><ref>{{cite book|title=Ptolemy's Almagest|author= Ptolemy|coauthors=Toomer, G. J.|publisher=Princeton University Press|year=1998|isbn=9780691002606}}</ref><ref name=oed>{{citeweb| url= http://dictionary.oed.com/cgi/entry/50180718?query_type=word&queryword=planet | publisher = Oxford English Dictionary| title = planet, n.| accessdate=2008-02-07 | date=December 2007}} ''Note: select the Etymology tab ''</ref> ===Modern times=== {{seealso|Heliocentrism}} The five [[naked-eye planet]]s have been known since ancient times, and have had a significant impact on [[mythology]], [[religious cosmology]], and ancient [[astronomy]]. As scientific knowledge progressed, however, understanding of the term "planet" changed from something that moved across the sky (in relation to the [[fixed stars|star field]]); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the [[heliocentric model]] of [[Nicolaus Copernicus|Copernicus]], [[Galileo Galilei|Galileo]] and [[Johannes Kepler|Kepler]] gained sway. [[Image:geoz wb en.jpg|thumb|300px|left|Heliocentrism (lower panel) in comparison to the geocentric model (upper panel)]] Thus the Earth became included in the list of planets,<ref name=galileo_project/> while the Sun and Moon were excluded. At first, when the first satellites of Saturn were discovered at the end of the 17th century, the terms "planet" and "satellite" were used interchangeably &ndash; although the latter would gradually become more prevalent in the following century.<ref>{{cite journal | last=Cassini | first=Signor | title=''A Discovery of two'' New Planets ''about'' Saturn, ''made in the Royal Parisian Observatory by Signor'' Cassini, ''Fellow of both the Royal Societys, of'' England ''and'' France; ''English't out of French.'' | journal=Philosophical Transactions (1665&ndash;1678) | year=1673 | volume=8 | pages=5178&ndash;85 | doi=10.1098/rstl.1673.0003}} ''Note: This journal became the Philosophical Transactions of the Royal Society of London in 1775. There may just be earlier publications within the ''[[Journal des sçavans]].''</ref> Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community. In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, [[2 Pallas|Pallas]], and [[4 Vesta|Vesta]]), were very different from the traditional one. These bodies shared the same region of space between Mars and Jupiter (the [[Asteroid belt]]), and had a much smaller mass; as a result they were reclassified as "[[asteroid]]s". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.<ref>{{cite web | last =Hilton | first =James L. | date = 2001-09-17 | url =http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html | title =When Did the Asteroids Become Minor Planets? | publisher =U. S. Naval Observatory | accessdate = 2007-04-08 }}</ref> However, in the 20th century, [[Pluto]] was discovered. After initial observations led to the belief it was larger than Earth,<ref>{{cite book | title = Planet Quest: The Epic Discovery of Alien Solar Systems | first = K.|last=Croswell | publisher = The Free Press | year = 1997 | pages = 57 | id = ISBN 978-0684832524}}</ref> the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, [[Raymond Lyttleton]] suggested that Pluto may be an escaped satellite of [[Neptune]],<ref>{{cite journal | last=Lyttleton | first=Raymond A. | year=1936 | journal=Monthly Notices of the Royal Astronomical Society | volume=97 | pages=108 | title= On the possible results of an encounter of Pluto with the Neptunian system | url=http://adsabs.harvard.edu/abs/1936MNRAS..97..108L }}</ref> and [[Fred Lawrence Whipple|Fred Whipple]] suggested in 1964 that Pluto may be a comet.<ref>{{cite journal| journal=Proceedings of the National Academy of Sciences of the United States of America | volume=52 | pages=565&ndash;594 | last=Whipple | first=Fred | year=1964 | url=http://adsabs.harvard.edu/abs/1964PNAS...52..565W | title= The History of the Solar System | doi= 10.1073/pnas.52.2.565}}</ref> However, as it was still larger than all known asteroids and seemingly did not exist within a larger population,<ref>{{cite journal | journal=Scientific American | year=1996 | month=May | pages=46&ndash;52 | last=Luu | first=Jane X. | coauthors=Jewitt, David C. | title=The Kuiper Belt | volume=274 | issue=5}}</ref> it kept its status until 2006. In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System (the [[Kuiper belt]]). Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of [[Eris (dwarf planet)|Eris]], a more massive object widely publicised as the "[[tenth planet]]", brought things to a head. The IAU set about creating the [[definition of planet]], and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had [[Cleared the neighbourhood|cleared their orbit]] (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of [[dwarf planet]]s was created, initially containing three objects (Ceres, Pluto and Eris).<ref>{{cite paper | last=Green | first=D. W. E. | version=Circular No. 8747 | publisher=Central Bureau for Astronomical Telegrams, International Astronomical Union | date=2006-09-13 | title=(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia) | url=http://www.cfa.harvard.edu/iau/special/08747.pdf | format=PDF | accessdate=2008-02-02 }}</ref> In 1992, astronomers [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of planets around a [[pulsar]], [[PSR B1257+12]].<ref name="Wolszczan">{{cite journal | author=Wolszczan, A.; Frail, D. A. | title=A planetary system around the millisecond pulsar PSR1257+12 | journal=Nature | year=1992 | volume=355 | pages=145&ndash;147|url=http://www.nature.com/nature/journal/v355/n6356/abs/355145a0.html | doi = 10.1038/355145a0 <!--Retrieved from CrossRef by DOI bot-->}}</ref> This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on [[October 6]] [[1995]], [[Michel Mayor]] and [[Didier Queloz]] of the [[University of Geneva]] announced the first definitive detection of an exoplanet orbiting an ordinary [[main sequence|main-sequence]] star ([[51 Pegasi]]).<ref name="Mayor">{{cite journal | last=Mayor|first=Michel|coauthors=Queloz, Didier| title=A Jupiter-mass companion to a solar-type star| journal=Nature| year=1995| volume=378| pages=355&ndash;359| doi= 10.1038/355145a0}}</ref> The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "[[brown dwarf]]s".<ref>{{cite web| year=2006 | title=IAU General Assembly: Definition of Planet debate| url=http://astro2006.meta.mediastream.cz/Astro2006-060822-01.asx | format=.wmv | publisher=MediaStream.cz | accessdate=2006-09-24}} </ref> Brown dwarfs are generally considered stars due to their ability to fuse [[deuterium]], a heavier isotope of [[hydrogen]]. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.<ref>{{cite journal | last=Basri | first=Gibor | title= Observations of Brown Dwarfs | journal=Annual Review of Astronomy and Astrophysics | year=2000 | volume=38 | pages=485 | doi=10.1146/annurev.astro.38.1.485}}</ref> As large [[Kuiper belt]] and scattered disc objects were discovered in the late 1990s and early years of the twenty-first century, a number including [[50000 Quaoar|Quaoar]], [[90377 Sedna|Sedna]] and [[Eris (dwarf planet)|Eris]] were heralded in the popular press as the 'tenth planet', however none of these received widespread scientific recognition as such, although [[Eris (dwarf planet)|Eris]] has now been classified as a [[Dwarf Planet]]. ===Former classifications=== The table below lists '''[[Solar System]] bodies formerly considered to be [[planet]]s''': {| class="wikitable" !width="320px"| Bodies ! Notes |- | [[Sun]], [[Moon]] |style="font-size:90%;"| Classified as planets in [[ancient history|antiquity]], in accordance with the definition then used. |- | [[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|Callisto]] |style="font-size:90%;"| The four largest moons of [[Jupiter]], known as the [[Galilean moons]] after their discoverer [[Galileo Galilei]]. He referred to them as the "Medicean Planets" in honor of his [[Patronage|patron]], the [[Medici|Medici family]]. |- | [[Titan (moon)|Titan]],{{Ref_label|B|b|none}} [[Iapetus (moon)|Iapetus]],{{Ref_label|C|c|none}} [[Rhea (moon)|Rhea]],{{Ref_label|C|c|none}} [[Tethys (moon)|Tethys]],{{Ref_label|D|d|none}} and [[Dione (moon)|Dione]]{{Ref_label|D|d|none}} |style="font-size:90%;"| Five of [[Saturn]]'s larger [[Saturn's natural satellites|moons]], discovered by [[Christiaan Huygens]] and [[Giovanni Domenico Cassini]]. |- | [[Ceres (dwarf planet)|Ceres]],{{Ref_label|E|e|none}} [[2 Pallas|Pallas]], [[3 Juno|Juno]], and [[4 Vesta|Vesta]] |style="font-size:90%;"| The first known [[asteroids]], from their discoveries between [[1801]] and [[1807]] until their reclassification as asteroids during the [[1850s]].<ref>{{cite web|title=The Planet Hygea|year=1849|work=spaceweather.com|url=http://spaceweather.com/swpod2006/13sep06/Pollock1.jpg|accessdate=2008-04-18}}</ref> Ceres has subsequently been classified as a [[dwarf planet]]. |- | [[5 Astraea|Astrea]], [[6 Hebe|Hebe]], [[7 Iris|Iris]], [[8 Flora|Flora]], [[9 Metis|Metis]], [[10 Hygiea|Hygeia]], [[11 Parthenope|Parthenope]], [[12 Victoria|Victoria]], [[13 Egeria|Egeria]], [[14 Irene|Irene]], [[15 Eunomia|Eunomia]] |style="font-size:90%;"| More [[Asteroids]], discovered between [[1845]] and [[1851]]. The rapidly expanding list of planets prompted their reclassification as [[asteroids]] by astronomers, and this was widely accepted by [[1854]].<ref>{{cite web | author=Hilton, James L. | title=When did asteroids become minor planets? | work=U.S. Naval Observatory | url=http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html | accessdate=2008-05-08}} </ref> |- | [[Pluto]]{{Ref_label|F|f|none}} |style="font-size:90%;"| [[Kuiper belt]] object [[Trans-Neptunian object|beyond the orbit]] of [[Neptune]]. In 2006, Pluto was reclassified as a [[dwarf planet]]. |} ===Modern definition===<!-- maybe: Rigurous definition --> {{main|2006 definition of planet}} With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a [[Asteroid belt|belt]], or if it was large enough to generate energy by the [[thermonuclear fusion]] of [[deuterium]]. In 2003, The [[International Astronomical Union]] (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:<ref name=WSGESP>{{cite web | year=2001 | work=IAU | title=Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union | url=http://www.dtm.ciw.edu/boss/definition.html | accessdate=2006-05-25}}</ref> <imagemap> Image:EightTNOs.png|thumb|275px|The largest Trans-Neptunian objects that prompted the IAU's 2006 decision #Earth rect 646 1714 2142 1994 [[Earth|The Earth]] #Eris and Dysnomia circle 226 412 16 [[Dysnomia (moon)|Dysnomia]] circle 350 626 197 [[Eris (dwarf planet)|(136199) Eris]] #Pluto and Charon circle 1252 684 86 [[Charon (moon)|Charon]] circle 1038 632 188 [[Pluto|(134340) Pluto]] #Makemake circle 1786 614 142 [[Makemake (dwarf planet)|(136472) Makemake]] #2003 EL61 circle 2438 616 155 [[(136108) 2003 EL61]] #Sedna circle 342 1305 137 [[90377 Sedna|(90377) Sedna]] #Orcus circle 1088 1305 114 [[90482 Orcus|(90482) Orcus]] #Quaoar circle 1784 1305 97 [[50000 Quaoar|(50000) Quaoar]] #Varuna circle 2420 1305 58 [[20000 Varuna|(20000) Varuna]] desc none # - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired #Notes: #Details on the new coding for clickable images is here: [[mw:Extension:ImageMap]] #While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top. #Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries. </imagemap> #Objects with [[true mass]]es below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same [[natural abundance|isotopic abundance]] as the Sun<ref>{{cite journal | last=Saumon|first=D.|coauthors=Hubbard, W. B.; Burrows, A.; Guillot, T.; Lunine, J. I.; Chabrier, G. | title=A Theory of Extrasolar Giant Planets | journal=Astrophysical Journal | year=1996 | volume=460 | pages=993&ndash;1018 | url=http://adsabs.harvard.edu/abs/1996ApJ...460..993S | doi=10.1086/177027}}</ref>) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System. #Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "[[brown dwarf]]s", no matter how they formed or where they are located. #Free-floating objects in young [[star cluster]]s with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate). This definition has since been widely used by astronomers when publishing discoveries in [[academic journal]]s.<ref>See for example the list of references for: {{cite web | author=Butler, R. P. ''et al'' | year=2006 | url=http://exoplanets.org/ | title=Catalog of Nearby Exoplanets | publisher =University of California and the Carnegie Institution | accessdate = 2007-07-23 }}</ref> Although temporary, it remains an effective, working definition until a more permanent one is formally adopted. Nevertheless, it did not address the dispute over the lower mass limit,<ref>{{citenews|url=http://www.spacedaily.com/news/outerplanets-04b.html|title=Gravity Rules: The Nature and Meaning of Planethood|last=Stern | first=S. Alan |date=2004-03-22|publisher=SpaceDaily|accessdate=2008-02-01}}</ref> and steered clear of the controversy regarding objects within the Solar System. This matter was finally addressed during the 2006 meeting of the [[International Astronomical Union#The XXVIth General Assembly and the definition of a planet|IAU's General Assembly]]. After much debate and one failed proposal, the assembly voted to pass a resolution that [[2006 definition of planet|defined planets within the Solar System]] as<ref name=IAU>{{cite web | author=Staff | year=2006 | url =http://www.iau.org/iau0603.414.0.html | title =IAU 2006 General Assembly: Result of the IAU resolution votes | publisher =IAU | accessdate = 2007-05-11 }}</ref>: {{quotation|A celestial body that is (a) 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 [[clearing the neighbourhood|cleared the neighbourhood]] around its orbit.}} Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake and Eris) are classified as [[dwarf planet]]s, providing they are not also [[natural satellite]]s of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.<ref>{{citenews|url=http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm|title=Planets plan boosts tally 12|publisher=[[BBC]]|date=2006-08-16|accessdate=2008-02-01|first=Paul|last=Rincon}}</ref> After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.<ref>{{citenews|url=http://news.bbc.co.uk/1/hi/world/5282440.stm|publisher=BBC|title=Pluto loses status as a planet|date=2006-08-24|accessdate=2008-02-01}}</ref> This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer [[Steven Soter]]:<ref>{{cite journal | last = Soter | first = Steven | title = What is a Planet | journal = Astronomical Journal | volume = 132 | issue = 6 | pages = 2513&ndash;19 | date = 2006 | doi=10.1086/508861 | id={{arxiv|astro-ph|0608359}}}}</ref> {{quotation|The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets.}} In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,<ref>{{citenews| publisher=BBC |url=http://news.bbc.co.uk/2/hi/science/nature/5283956.stm |title=Pluto vote 'hijacked' in revolt|first=Paul|last=Rincon|date=2006-08-25|accessdate=2008-02-01}}</ref><ref>{{cite web | date=2006-08-24 | first=Robert Roy | last=Britt | title=Pluto Demoted: No Longer a Planet in Highly Controversial Definition | work= Space.com | url=http://www.space.com/scienceastronomy/060824_planet_definition.html | accessdate=2006-08-24}}</ref> and some astronomers have even stated that they will not use it.<ref>{{cite web | date=2006-08-31 | first=Robert Roy | last=Britt | title=Pluto: Down But Maybe Not Out | work= Space.com | url=http://www.space.com/scienceastronomy/060831_planet_definition.html | accessdate=2006-08-24}}</ref> Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU [[Academic conference|conference]] is not until 2009, when modifications could be made to the definition, also possibly including extrasolar planets. Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status during most of the 20th century &ndash; similarly to Ceres and its kin in the 1800s. The discovery of Eris was widely reported in the [[mass media|media]] as the "[[tenth planet]]" and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention.<ref>{{cite news | first=Clara | last=Moskowitz | title=Scientist who found '10th planet' discusses downgrading of Pluto | publisher=Stanford news | date=2006-10-18 | url=http://news-service.stanford.edu/news/2006/october18/mbrown-101806.html | accessdate=2007-05-11 }}</ref> ==Mythology== {{seealso|Days of the week|Naked-eye planet}} [[Image:Olympians.jpg|thumb|left|220 px|The gods of [[Mount Olympus|Olympus]], after whom the Solar System's planets are named]] The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In [[ancient Greece]], the two great luminaries the Sun and the Moon were called ''[[Helios]]'' and ''[[Selene]]''; the farthest planet was called ''Phainon'', the shiner; followed by ''Phaethon'', "bright"; the red planet was known as ''Pyroeis'', the "fiery"; the brightest was known as ''Phosphoros'', the light bringer; and the fleeting final planet was called ''Stilbon'', the gleamer. The Greeks also made each planet sacred to one of their pantheon of gods, the [[Twelve Olympians|Olympians]]: Helios and Selene were the names of both planets and gods; Phainon was sacred to ''[[Cronus|Kronos]]'', the [[Titan (mythology)|Titan]] who fathered the Olympians; Phaethon was sacred to ''[[Zeus|Zeús]]'', Kronos's son who deposed him as king; Pyroeis was given to ''[[Ares]]'', son of Zeus and god of war; Phosphorus was ruled by ''[[Aphrodite]]'', the goddess of love; and ''[[Hermes]]'', messenger of the gods and god of learning and wit, ruled over Stilbon.<ref name=practice>{{cite web|title=The History and Practice of Ancient Astronomy|first=James|last=Evans|publisher=Oxford University Press|year=1998|pages=296&ndash;7|url=http://books.google.com/books?id=nS51_7qbEWsC&pg=PA17&lpg=PA17&dq=babylon+greek+astronomy&source=web&ots=c1afKhoAt6&sig=A4cDSCcvWmd6B9e9YPZ9T1I91GM#PPA15,M1 |accessdate=2008-02-04}}</ref> The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphorus after their goddess of love, ''[[Ishtar]]''; Pyroeis after their god of war, ''[[Nergal]]'', Stilbon after their god of wisdom [[Nabu]], and Phaethon after their chief god, ''[[Marduk]]''.<ref name= nergal>{{cite web|first=Kelley L.|last=Ross|year=2005|title=The Days of the Week|url=http://www.friesian.com/week.htm|publisher=The Friesian School|accessdate=2007-04-30}}</ref> There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.<ref name=practice /> The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.<ref>{{cite book|title=Martian Metamorphoses: The Planet Mars in Ancient Myth and Tradition|first=Ev|last=Cochrane|year=1997|publisher=Aeon Press|url=http://books.google.co.uk/books?hl=en&lr=&id=jz3eqRGuM0wC&oi=fnd&pg=PP9&dq=ares+nergal+planet+pestilence&ots=oFtmQnyNtC&sig=iXrJjw-K4YFAGxff5_WV7yCZp0E#PPP8,M1|accessdate=2008-02-07 | isbn=0965622908}}</ref> Today, most people in the western world know the planets by names derived from the [[Twelve Olympians|Olympian pantheon of gods]]. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the [[Roman Empire]] and, later, the [[Catholic Church]], use the Roman (or Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were [[Indo-European mythology|Indo-Europeans]], shared with them a [[Roman mythology|common pantheon]] under different names but lacked the rich narrative traditions that Greek poetic culture had given [[Greek mythology|their gods]]. During the later period of the [[Roman Republic]], Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.<ref>{{cite book|title=Greek Mythography in the Roman World|first=Alan|last=Cameron|year=2005|publisher=Oxford University Press | isbn=0195171217}}</ref> When the Romans studied Greek astronomy, they gave the planets their own gods' names: ''[[Mercury (mythology)|Mercurius]]'' (for Hermes), ''[[Venus (mythology)|Venus]]'' (Aphrodite), ''[[Mars (mythology)|Mars]]'' (Ares), ''[[Jupiter (mythology)|Iuppiter]]'' (Zeus) and ''[[Saturn (mythology)|Saturnus]]'' (Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: ''Uranus'' (''[[Uranus (mythology)|Ouranos]]'') and ''[[Neptune (mythology)|Neptūnus]]'' (''[[Poseidon]]''). <!-- <ref>{{cite web | title=Astra Planeta | publisher=Theoi Project | first=Aaron | last=Atsma | year=2007 | url=http://www.theoi.com/Titan/AstraPlaneta.html | accessdate=2007-02-25}}{{Rs|date=February 2008}}</ref> --> Some [[Ancient Rome|Romans]], following a belief possibly originating in [[Mesopotamia]] but developed in [[Hellenistic Egypt]],<!--<ref>{{cite web | first=Bill | last=Arnett | year=2006 | title=Appendix 5: Planetary Linguistics | url=http://www.nineplanets.org/days.html | publisher=www.nineplanets.org|accessdate=2008-02-02}}{{Rs|date=February 2008}}</ref>--> believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).<ref name=zerubavel>{{cite book | first=Eviatar | last=Zerubavel | year=1989 | publisher=University of Chicago Press | isbn=0226981657 | title= The Seven Day Circle: The History and Meaning of the Week | pages=14 | url=http://books.google.com/books?id=aGahKeojIUoC&pg=PA14&lpg=PA14&dq=seven+day+week+egypt+mesopotamia&source=web&ots=eDfcoCmjPi&sig=pzTzFgj6XBB3SjuqzxGwGZ2trH8#PPA14,M1 | accessdate=2008-02-07}}</ref> Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the [[days of the week]] in the [[Roman calendar]] &ndash; and still preserved many modern languages.<ref name="weekdays">{{cite journal | first=Michael | last=Falk | title=Astronomical Names for the Days of the Week | journal=Journal of the [[Royal Astronomical Society of Canada]] | year=1999 | volume=93 | pages=122&ndash;133 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1999JRASC..93..122F}}</ref> Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after ''[[Týr|Tiw]]'', (Tuesday) ''[[Woden|Wóden]]'' (Wednesday), ''[[Thor|Thunor]]'' (Thursday), and ''[[Frige|Fríge]]'' (Friday), the [[Anglo-Saxon gods]] considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively. Since Earth was only generally accepted as a planet in the 17th century,<ref name=galileo_project>{{cite web | last=Van Helden | first=Al | year=1995 | url=http://galileo.rice.edu/sci/theories/copernican_system.html | title=Copernican System | publisher=The Galileo Project | accessdate=2008-01-28 }}</ref> there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century [[Old English language|Anglo-Saxon]] word ''erda'', which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.<ref>{{citeweb| publisher= Oxford English Dictionary | url = http://dictionary.oed.com/cgi/entry/50071589?query_type=word&queryword=earth&first=1&max_to_show=10&sort_type=alpha&result_place=1&search_id=7aas-q054tm-4631&hilite=50071589| title = earth, n. | accessdate = 2008-02-06 | date = 1989}}</ref><ref name=etymearth>{{cite web | last = Harper| first = Douglas | date = 2001-09 | url = http://www.etymonline.com/index.php?term=earth | title = Earth | work= Online Etymology Dictionary | accessdate = 2007-08-07 }}</ref> It is the only planet whose name in English is not derived from [[Greek mythology|greco]]-[[roman mythology]]. Many of the [[Romance languages]] retain the old Roman word ''[[Terra (mythology)|terra]]'' (or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea").<ref>{{citeweb|last=Harper|first=Douglas|date=2001-09|url=http://www.etymonline.com/index.php?term=terrain|title=Etymology of "terrain"|work=Online Etymology Dictionary|accessdate=2008-01-30}}</ref> However, the non-Romance languages use their own respective native words. The Greeks retain their original name, ''[[Gaia (mythology)|Γή]]'' (''Ge'' or ''Yi''); the [[Germanic languages]], including English, use a variation of an ancient Germanic word ''ertho'', "ground,"<ref name=etymearth/> as can be seen in the English ''Earth'', the German ''Erde,'' the Dutch ''Aarde'', and the Scandinavian ''Jorde.'' Non-European cultures use other planetary naming systems. [[India]] uses a naming system based on the [[Navagraha]], which incorporates the seven traditional planets ([[Surya]] for the Sun, [[Chandra]] for the Moon, and [[Budha]], [[Shukra]], [[Mangala]], [[Bṛhaspati|{{IAST|Bṛhaspati}}]] and [[Shani]] for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending [[lunar node]]s [[Rahu]] and [[Ketu (mythology)|Ketu]]. [[China]] and the countries of [[eastern Asia]] influenced by it (such as [[Japan]], [[Korea]] and [[Vietnam]]) use a naming system based on the [[Wu Xing|five Chinese elements]]: ''[[Water (classical element)|water]]'' (Mercury), ''[[Metal (classical element)|metal]]'' (Venus), ''[[Fire (classical element)|fire]]'' (Mars), ''[[Wood (classical element)|wood]]'' (Jupiter) and ''[[Earth (classical element)|earth]]'' (Saturn).<ref name="weekdays"/> ==Formation== {{main|Nebular hypothesis}} It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a [[nebula]] into a thin disk of gas and dust. A [[protostar]] forms at the core, surrounded by a rotating [[protoplanetary disk]]. Through [[Accretion (astrophysics)|accretion]] (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as [[planetesimal]]s form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form [[protoplanet]]s.<ref>{{cite journal | first=G. W. | last=Wetherill | title=Formation of the Terrestrial Planets | journal=Annual Review of Astronomy and Astrophysics | year=1980 | volume=18 | pages=77&ndash;113 | url=http://adsabs.harvard.edu/abs/1980ARA&A..18...77W | accessdate=2007-07-23 | doi=10.1146/annurev.aa.18.090180.000453 }}</ref> After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of [[Drag (physics)|atmospheric drag]].<ref>{{cite journal | author=Inaba, S.; Ikoma, M. | title=Enhanced Collisional Growth of a Protoplanet that has an Atmosphere | journal=Astronomy and Astrophysics | year=2003 | volume=410 | pages=711&ndash;723 | url=http://adsabs.harvard.edu/abs/2003A&A...410..711I | accessdate = 2007-07-23 | doi = 10.1051/0004-6361:20031248 <!--Retrieved from CrossRef by DOI bot-->}}</ref> [[Image:Protoplanetary-disk.jpg|left|thumb|250px|An artist's impression of protoplanetary disk]] When the protostar has grown such that it ignites to form a [[star]], the surviving disk is removed from the inside outward by photoevaporation, the [[solar wind]], [[Poynting-Robertson effect|Poynting-Robertson drag]] and other effects.<ref>{{cite web | last = Dutkevitch | first = Diane | year =1995 | url =http://www.astro.umass.edu/theses/dianne/thesis.html | title =The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around Young Stars | publisher =Ph. D. thesis, University of Massachusetts Amherst | accessdate = 2007-05-13 }} ([[Astrophysics Data System]] [http://adsabs.harvard.edu/abs/1995PhDT..........D entry])</ref><ref>{{cite journal | author=Matsuyama, I.; Johnstone, D.; Murray, N. | title=Halting Planet Migration by Photoevaporation from the Central Source | journal=The Astrophysical Journal | year = 2005 | volume=585 | issue=2 | pages=L143&ndash;L146 | url=http://adsabs.harvard.edu/abs/2003astro.ph..2042M | accessdate=2007-05-13 | doi = 10.1086/374406 <!--Retrieved from CrossRef by DOI bot-->}}</ref> Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.<ref>{{cite journal | last=Kenyon | first=Scott J. | coauthors=Bromley, Benjamin C. | journal=Astronomical Journal | volume=131 | pages=1837| year=2006 | doi=10.1086/499807 | title= Terrestrial Planet Formation. I. The Transition from Oligarchic Growth to Chaotic Growth | laysummary = http://www.cfa.harvard.edu/~kenyon/pf/terra/index.html | laysource = Kenyon, Scott J. Personal web page }}</ref> Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become [[natural satellite]]s of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or [[Small solar system body|small Solar System bodies]]. The energetic impacts of the smaller planetesimals (as well as [[radioactive decay]]) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.<ref>{{cite journal | journal=Icarus | year=1987 | volume=69 | pages=239 | last=Ida | first=Shigeru | coauthors=Nakagawa, Yoshitsugu; Nakazawa, Kiyoshi | title= The Earth's core formation due to the Rayleigh-Taylor instability | doi=10.1016/0019-1035(87)90103-5 }}</ref> Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of [[comet]]s.<ref>{{cite journal | last=Kasting | first=James F. | title=Earth's early atmosphere | journal=Science | year=1993 | volume=259 | pages=920 | url=http://adsabs.harvard.edu/abs/1993Sci...259..920K | doi=10.1126/science.11536547 | pmid=11536547}}</ref> (Smaller planets will lose any atmosphere they gain through various [[Atmospheric escape|escape mechanisms]].) With the discovery and observation of [[planetary system]]s around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of [[metallicity]] &ndash; an astronomical term describing the abundance of [[chemical element]]s with an [[atomic number]] greater than 2 ([[helium]]) &ndash; is now believed to determine the likelihood that a star will have planets.<ref>{{cite news | author=Aguilar, D.; Pulliam, C. | date = 2004-01-06 | url = http://cfa-www.harvard.edu/press/pr0404.html | title = Lifeless Suns Dominated The Early Universe | publisher = Harvard-Smithsonian Center for Astrophysics (Press release) | accessdate = 2006-08-26 }}</ref> Hence it is thought less likely that a metal-poor, [[population II star]] will possess a more substantial planetary system than a metal-rich [[population I star]]. ==Solar System== [[Image:Terrestrial planet size comparisons.jpg|thumb|375 px|The terrestrial planets: Mercury, Venus, Earth, Mars ''(Sizes to scale)'']] [[Image:Gas giants and the Sun (1 px = 1000 km).jpg|thumb|375px|The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune ''(Sizes to scale)'']] {{main|Table of planets and dwarf planets in the Solar System}} According to the [[International Astronomical Union|IAU]]'s current definitions, there are eight planets in the [[Solar System]]. In increasing distance from the [[Sun]], they are: # [[Image:Mercury symbol.svg|14px|{{unicode|☿}}]] '''[[Mercury (planet)|Mercury]]''' # [[Image:Venus symbol.svg|14px|{{unicode|♀}}]] '''[[Venus]]''' # [[Image:Earth symbol.svg|14px|{{unicode|⊕}}]] '''[[Earth]]''' # [[Image:Mars symbol.svg|14px|{{unicode|♂}}]] '''[[Mars]]''' # [[Image:Jupiter symbol.svg|14px|{{unicode|♃}}]] '''[[Jupiter]]''' # [[Image:Saturn symbol.svg|14px|{{unicode|♄}}]] '''[[Saturn]]''' # [[Image:Uranus symbol.svg|14px|{{unicode|♅}}]] '''[[Uranus]]''' # [[Image:Neptune symbol.svg|14px|{{unicode|♆}}]] '''[[Neptune]]''' Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses. The planets of the Solar System can be divided into categories based on their composition: * '''[[Terrestrial planet|Terrestrial]]s''': Planets that are similar to Earth, with bodies largely composed of [[Rock (geology)|rock]]: Mercury, Venus, Earth and Mars. * '''[[Gas giant]]s''': Planets with a composition largely made up of [[gas]]eous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. [[Ice giant]]s, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass, and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice. {| class="wikitable" style="margin: 1em auto 1em auto" |+align=bottom style="text-align:left;"| |- bgcolor=#ccccff ! colspan=12 style="background:#dddddd;" | Planetary attributes |- ! !style="font-size: smaller;"|Name !style="font-size: smaller;"|Equatorial<br />diameter{{ref label|a|a|a}} !style="font-size: smaller;"|Mass{{ref label|a|a|a}} !style="font-size: smaller;"|Orbital<br />radius ([[Astronomical unit|AU]]) !style="font-size: smaller;"|[[Orbital period]]<br />(years) !style="font-size: smaller;"|[[Inclination|Inclination <br />to Sun's equator]] (°) !style="font-size: smaller;"|[[Orbital eccentricity|Orbital<br>eccentricity]] !style="font-size: smaller;"|[[Rotation period]]<br />(days) !style="font-size: smaller;"|Named<br>[[Natural satellite|moons]] !style="font-size: smaller;"|Rings !style="font-size: smaller;"|[[Atmosphere]] |- ! rowspan=4 style="background: #DDEEFF;" | [[Terrestrial planet|Terrestrials]] | [[Mercury (planet)|Mercury]] | align="center" | 0.382 | align="center" | 0.06 | align="center" | 0.39 | align="center" | 0.24 | align="center" | 3.38 | align="center" | 0.206 | align="center" | 58.64 | align="center" | &mdash; | align="center" | no | align="center" | minimal |- | [[Venus]] | align="center" | 0.949 | align="center" | 0.82 | align="center" | 0.72 | align="center" | 0.62 | align="center" | 3.86 | align="center" | 0.007 | align="center" | -243.02 | align="center" | &mdash; | align="center" | no | align="center" | [[Carbon dioxide|CO<sub >2</sub>]], [[Nitrogen|N<sub>2</sub>]] |- | [[Earth]]{{ref label|b|b|b}} | align="center" | 1.00 | align="center" | 1.00 | align="center" | 1.00 | align="center" | 1.00 | align="center" | 7.25 | align="center" | 0.017 | align="center" | 1.00 | align="center" | [[Moon|1]] | align="center" | no | align="center" | N<sub>2</sub>, [[Oxygen|O<sub>2</sub>]] |- | [[Mars]] | align="center" | 0.532 | align="center" | 0.11 | align="center" | 1.52 | align="center" | 1.88 | align="center" | 5.65 | align="center" | 0.093 | align="center" | 1.03 | align="center" | [[Moons of Mars|2]] | align="center" | no | align="center" | CO<sub >2</sub>, N<sub>2</sub> |- ! rowspan=4 style="background: #DDEEFF;" | [[Gas giant]]s | [[Jupiter]] | align="center" | 11.209 | align="center" | 317.8 | align="center" | 5.20 | align="center" | 11.86 | align="center" | 6.09 | align="center" | 0.048 | align="center" | 0.41 | align="center" | [[Moons of Jupiter|63]] | align="center" | [[Rings of Jupiter|yes]] | align="center" | [[Hydrogen|H<sub>2</sub>]], [[Helium|He]] |- | [[Saturn]] | align="center" | 9.449 | align="center" | 95.2 | align="center" | 9.54 | align="center" | 29.46 | align="center" | 5.51 | align="center" | 0.054 | align="center" | 0.43 | align="center" | [[Moons of Saturn|60]] | align="center" | [[Rings of Saturn|yes]] | align="center" | H<sub>2</sub>, He |- | [[Uranus]] | align="center" | 4.007 | align="center" | 14.6 | align="center" | 19.22 | align="center" | 84.01 | align="center" | 6.48 | align="center" | 0.047 | align="center" | -0.72 | align="center" | [[Moons of Uranus|27]] | align="center" | [[Rings of Uranus|yes]] | align="center" | H<sub>2</sub>, He |- | [[Neptune]] | align="center" | 3.883 | align="center" | 17.2 | align="center" | 30.06 | align="center" | 164.8 | align="center" | 6.43 | align="center" | 0.009 | align="center" | 0.67 | align="center" | [[Neptune's natural satellites|13]] | align="center" | [[Rings of Neptune|yes]] | align="center" | H<sub>2</sub>, He |- |colspan=12 style="background: #FFFFFF; border-right:1px solid white; border-bottom:1px solid white; border-left:1px solid white;"|<div class="references-small" style="margin-bottom: 0em;"> :{{note label|a|a|a}} Measured relative to the Earth. :{{note label|b|b|b}} See [[Earth]] article for absolute values. </div> |} ===Dwarf planets=== {{main|Dwarf planet}} Before the [[2006 definition of planet|August 2006 decision]], several objects were proposed by astronomers, including at one stage by the [[International Astronomical Union|IAU]], as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently four dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Makemake and Eris. Several other objects in both the [[Asteroid belt]] and the [[Kuiper belt]] are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain &ndash; namely that they are not [[clearing the neighbourhood|dominant in their orbits]]. Their attributes are: {| class="wikitable" style="margin: 1em auto 1em auto" |- bgcolor=#ccccff ! colspan=12 style="background:#dddddd;" | Dwarf planetary attributes |- style="font-size: smaller;" !Name !Equatorial<br />diameter{{ref label|c|c|c}} !Mass{{ref label|c|c|c}} !Orbital<br />radius ([[Astronomical Unit|AU]]) ![[Orbital period]]<br />(years) <!-- ecliptic is not the same thing as the Sun's equator! --> ![[Inclination|Inclination <br />to ecliptic]] (°) ![[Orbital eccentricity|Orbital<br>eccentricity]] ![[Rotation period]]<br />(days) ![[Natural satellite|Moons]] !Rings ![[Atmosphere]] |- align="center" ! align="left" | [[Ceres (dwarf planet)|Ceres]] | 0.08 | 0.0002 | 2.76 | 4.60 | 10.59 | 0.080 | 0.38 | 0 | no | none |- align="center" ! align=left| [[Pluto]] | 0.19 | 0.0022 | 39.48 | 248.09 | 17.14 | 0.249 | -6.39 | [[Moons of Pluto|3]] | no | temporary |- align="center" ! align=left| [[Makemake (dwarf planet)|Makemake]] | | | | | | | ? | ? | ? | ? |- align="center" ! align="left" | [[Eris (dwarf planet)|Eris]] | 0.19 | 0.0025 | 67.67 | ~557 | 44.19 | 0.442 | ~0.3 | [[Dysnomia (moon)|1]] | ? | temporary |- |colspan=12 style="background: #FFFFFF; border-right:1px solid white; border-bottom:1px solid white; border-left:1px solid white;"|<div class="references-small" style="margin-bottom: 0em;"> :{{note label|c|c|c}} Measured relative to the Earth. </div> |} By definition, all dwarf planets are members of larger [[population]]s. Ceres is the largest body in the [[asteroid belt]], while Pluto is a member of the Kuiper belt and Eris is a member of the [[scattered disc]]. Scientists such as [[Michael E. Brown|Mike Brown]] believe that there may soon be over forty [[trans-Neptunian objects]] that qualify as dwarf planets under the IAU's recent definition.<ref>{{cite web | first=Brad | last=Amburn | title=Behind the Pluto Mission: An Interview with Project Leader Alan Stern | work=Space.com | date=2006-02-28 | url=http://www.space.com/scienceastronomy/060228_stern_interview.html | accessdate=2006-11-01 }}</ref> ==Extrasolar planets== {{main|Extrasolar planet}} [[Image:Phot-14a-05-preview.jpg|right|250px|thumb|False-color infrared image of the brown dwarf [[2M1207]] (blue) and its planetary companion [[2M1207b]] (red), as viewed by the [[Very Large Telescope]] (obtained by [[Methods of detecting extrasolar planets#Direct imaging|direct imaging]])]] The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on [[6 October]] [[1995]], when [[Michel Mayor]] and [[Didier Queloz]] of the [[University of Geneva]] announced the detection of an exoplanet around [[51 Pegasi]]. Of the 270 extrasolar planets discovered by January 2008, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass.<ref name="Encyclopedia">{{cite web |title=Interactive Extra-solar Planets Catalog |work=The Extrasolar Planets Encyclopedia |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=2006-12-11|accessdate=2006-12-11}}</ref> The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called [[pulsar]]s, such as [[PSR B1257+12]].<ref>{{cite news | first=Barbara | last=Kennedy | title=Scientists reveal smallest extra-solar planet yet found | publisher=SpaceFlight Now | date=2005-02-11| url=http://www.spaceflightnow.com/news/n0502/11planet/ | accessdate=2006-07-28 }}</ref> There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,<ref name="Encyclopedia" /> such as those orbiting the stars [[Mu Arae]], [[55 Cancri]] and [[Gliese 436|GJ 436]].<ref>{{cite news | author=Santos, N.; Bouchy, F.; Vauclair, S.; Queloz, D.; Mayor, M. | title=Fourteen Times the Earth | publisher=European Southern Observatory (Press Release) | date=2004-08-25| url=http://www.eso.org/public/outreach/press-rel/pr-2004/pr-22-04.html | accessdate=2008-02-02 }}</ref> These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).<ref>{{cite news |url=http://www.astrobio.net/news/article1965.html |title=Trio of Neptunes |publisher=Astrobiology Magazine |date=May 21, 2006 |accessdate=2007-08-06}}</ref> Another new category are the so-called "[[super-Earth]]s", possibly [[terrestrial planet]]s far larger than Earth but smaller than Neptune or Uranus. To date, five possible super-Earths have been found: [[Gliese 876 d]], which is roughly six times Earth's mass,<ref>{{cite web|work=Extrasolar planet Encyclopedia|title=Star: Gliese 876|url=http://exoplanet.eu/star.php?st=Gliese+876|accessdate=2008-02-01}}</ref> [[OGLE-2005-BLG-390Lb]] and [[MOA-2007-BLG-192Lb]], frigid icy worlds discovered through [[gravitational microlensing]],<ref>{{cite web|title=Small Planet Discovered Orbiting Small Star|work=ScienceDaily|year=2008|url=http://www.sciencedaily.com/releases/2008/06/080602131105.htm|accessdate=2008-06-06}}</ref><ref>{{cite journal | first=J.-P.|last=Beaulieu | title=Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing | journal=Nature | date=2006-01-26 | volume=439 | pages=437&ndash;440 | url=http://www.nature.com/nature/journal/v439/n7075/full/nature04441.html | accessdate=2008-02-08 | doi = 10.1038/nature04441 | coauthors=D. P. Bennett; P. Fouqué; A. Williams; ''et al.''}}</ref> and two planets orbiting the nearby [[red dwarf]] [[Gliese 581]]. [[Gliese 581 d]] is roughly 7.7 times Earth's mass,<ref>{{cite web|title=Gliese 581 d|work=The Extrasolar Planets Encyclopedia|url=http://vo.obspm.fr/exoplanetes/encyclo/planet.php?p1=Gl+581&p2=d|year=2007|accessdate=2008-02-13}}</ref> while [[Gliese 581 c]] is five times Earth's mass and the first terrestrial planet found within a star's [[habitable zone]].<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/sci/tech/6589157.stm |title=New 'super-Earth' found in space |accessdate = 2007-04-25 |date=[[25 April]] [[2007]] |publisher=BBC News }}</ref> It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or [[carbon planet]]s. In particular, some of the newly-discovered planets, known as [[hot Jupiter]]s, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more [[solar radiation|stellar radiation]] than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called [[Chthonian planet]]s, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine cthonian planets have been discovered.<ref>{{cite journal | last=Lecavelier des Etangs|first=A.|coauthors=Vidal-Madjar, A.; McConnell, J. C.; Hébrard, G. | title=Atmospheric escape from hot Jupiters | journal=Astronomy and Astrophysics | year=2004 | volume=418 | pages=L1&ndash;L4 | url=http://adsabs.harvard.edu/abs/2004A&A...418L...1L | accessdate=2007-09-10 | doi=10.1051/0004-6361:20040106}}</ref> More detailed observation of extrasolar planets will require a new generation of instruments, including [[space telescope]]s. Currently the [[COROT]] spacecraft is searching for stellar luminosity variations due to [[Astronomical transit|transiting planets]]. Several projects have also been proposed to create an array of [[space telescope]]s to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's [[Kepler Mission]], [[Terrestrial Planet Finder]], and [[Space Interferometry Mission]] programs, the [[European Space Agency|ESA]]'s [[Darwin Mission|Darwin]], and the CNES' [[PEGASE]].<ref>{{cite web | url=http://www.spacetoday.org/DeepSpace/Stars/Planets/PlanetFindingMissions.html | title =Future American and European Planet Finding Missions | publisher = Space Today Online | editor = Anthony R. Curtis | accessdate = 2008-02-06}}</ref> The [[New Worlds Mission]] is an occulting device that may work in conjunction with the [[James Webb Space Telescope]]. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 ([[HD 209458 b]] and [[HD 189733 b]]).<ref>{{citenews|url=http://www.spitzer.caltech.edu/Media/releases/ssc2007-04/release.shtml|title=NASA's Spitzer First To Crack Open Light of Faraway Worlds|date=2007-02-21|first=Tabatha|last=Thompson|coauthors=Clavin, Whitney|publisher=Jet Propulsion Laboratory, California Institute of Technology (Press Release)|accessdate=2008-02-01}}</ref><ref>{{cite journal | last=Richardson | first=L. Jeremy | coauthors=Deming, Drake; Horning, Karen; Seager, Sara; Harrington, Joseph | journal=Nature | year=2007 | volume=445 | pages=892 | title=A spectrum of an extrasolar planet | url=http://www.nature.com/nature/journal/v445/n7130/abs/nature05636.html | doi=10.1038/nature05636 }}</ref> The frequency of occurrence of such terrestrial planets is one of the variables in the [[Drake equation]] which estimates the number of [[Extraterrestrial life|intelligent, communicating civilizations]] that exist in our galaxy.<ref>{{cite news | last=Drake | first=Frank | title=The Drake Equation Revisited | publisher=Astrobiology Magazine | date=2003-09-29 | url=http://www.astrobio.net/news/article610.html | accessdate=2007-07-23 }}</ref> ==Interstellar "planets"== {{Main|Rogue planet}} Several [[computer simulation]]s of stellar and planetary system formation have suggested that some [[planemo|objects of planetary mass]] would be ejected into interstellar [[space]].<ref>{{cite journal| last=Lissauer| first= J. J.| title= Timescales for Planetary Accretion and the Structure of the Protoplanetary disk| journal= Icarus| volume= 69| pages=249&ndash;265| year=1987| doi=10.1016/0019-1035(87)90104-7}}</ref> Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, others have suggested that they could be low-mass stars.<ref name="clavin">{{cite journal| journal=Astrophysical Journal | last=Luhman | first=K. L. | coauthors=Adame, Lucía; D'Alessio, Paola; Calvet, Nuria | title= Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk | volume=635 | pages=L93 | doi=10.1086/498868 | year= 2005 | laysummary=http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html | laysource=NASA Press Release | laydate=2005-11-29 }}</ref> The [[International Astronomical Union|IAU's]] working definition on extrasolar planets takes no position on the issue. In 2005, astronomers announced the discovery of [[Cha 110913-773444]], the smallest brown dwarf found to date, at only seven times Jupiter's mass. Since it was not found in orbit around a fusing star, it is a [[sub-brown dwarf]] according to the IAU's working definition. However, some astronomers believe it should be referred to as a planet.<ref name="clavin" /> For a brief time in 2006, astronomers believed they had found a binary system of such objects, [[Oph 162225-240515]], which the discoverers described as "[[planemo]]s", or "planetary mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair [[brown dwarf]]s.<ref>{{cite journal| title=The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries? | journal= Astrophysical Journal |author=Close, Laird M. ''et al'' | volume=660 | pages=1492 | doi=10.1086/513417 | date=2007 | id={{arxiv|astro-ph|0608574}}}}</ref><ref>{{cite journal|last=Luhman | first=K. L. | coauthrs=Allers, K. N.; Jaffe, D. T.; Cushing, M. C.|year=2007|month=April | journal=The Astrophysical Journal | title=Ophiuchus 1622-2405: Not a Planetary-Mass Binary | volume=659 | issue=2 | pages=1629&ndash;36 | doi=10.1086/512539}}</ref><ref>{{cite web | url=http://www.space.com/scienceastronomy/planet_photo_040910.html | title=Likely First Photo of Planet Beyond the Solar System | first=Robert Roy | last=Britt| work=Space.com | date=2004-09-10 | accessdate=2008-02-02}}</ref> ==Attributes==<!-- This section is linked from [[Earth radius]] --> Although each planet has unique physical characteristics, a number of broad commonalities do exist between them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets. ===Dynamic characteristics=== {{seealso|Kepler's laws of planetary motion}} ====Orbit==== [[Image:TheKuiperBelt Orbits Pluto Ecliptic.svg|thumb|250 px|right|The orbit of the planet Neptune compared to that of [[Pluto]]. Note the elongation of Pluto's orbit in relation to Neptune's ([[orbital eccentricity|eccentricity]]), as well as its large angle to the ecliptic ([[inclination]]).]] All planets revolve around stars. In the Solar System, all the planets orbit in the same direction as the Sun rotates. It is not yet known whether all extrasolar planets follow this pattern. The period of one revolution of a planet's orbit is known as its [[sidereal period]] or ''[[year]]''.<ref name="young">{{cite book | first=Charles Augustus | last=Young | year=1902 | title=Manual of Astronomy: A Text Book | publisher=Ginn & company | pages=324&ndash;7 }}</ref> A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's [[gravity]]. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its [[periastron]] ([[perihelion]] in the Solar System), while its farthest separation from the star is called its [[apastron]] ([[aphelion]]). As a planet approaches periastron, its speed increases as the pull of its star's gravity strengthens; as it reaches apastron, its speed decreases.<ref>{{cite book | author=Dvorak, R.; Kurths, J.; Freistetter, F. | year=2005 | title=Chaos And Stability in Planetary Systems | publisher=Springer | location=New York | isbn=3540282084 }}</ref> Each planet's orbit is delineated by a set of [[orbital elements|elements]]: *The ''[[Orbital eccentricity|eccentricity]]'' of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in our Solar System have very low eccentricities, and thus nearly circular orbits.<ref name="young"/> Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.<ref>{{cite journal|title=Eccentricity evolution of giant planet orbits due to circumstellar disk torques|author=Moorhead, Althea V.; Adams, Fred C. | journal=Icarus | year=2008 | volume=193 | pages=475 | doi=10.1016/j.icarus.2007.07.009 | id={{arxiv|0708.0335}}}}</ref><ref>{{cite web|title=Planets - Kuiper Belt Objects|work=The Astrophysics Spectator|date=2004-12-15| url=http://www.astrophysicsspectator.com/topics/planets/KuiperBelt.html|accessdate=2008-02-01}}</ref> [[Image:Semimajoraxis.png|thumb|200 px|Illustration of the semi-major axis]] *The ''[[semi-major axis]]'' is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apasteron, as no planet's orbit has its star at its exact centre.<ref name="young"/> *The ''[[inclination]]'' of a planet tells how far above or below an established reference plane its orbit lies. In our Solar System, the reference plane is the plane of Earth's orbit, called the [[ecliptic]]. For extrasolar planets, the plane, known as the ''sky plane'' or ''plane of the sky'', is the plane of the observer's line of sight from Earth.<ref>{{cite book | url=http://astrowww.phys.uvic.ca/~tatum/celmechs.html|title=Celestial Mechanics | year=2007 | chapter=17. Visual binary stars | first=J. B. | last=Tatum|accessdate=2008-02-02 | publisher=Personal web page}}</ref> The eight planets of our Solar System all lie very close to the ecliptic; comets and [[Kuiper belt object]]s like [[Pluto]] are at far more extreme angles to it.<ref>{{cite journal|title=A Correlation between Inclination and Color in the Classical Kuiper Belt| last=Trujillo | first=Chadwick A. | coauthors= Brown, Michael E. | journal=Astrophysical Journal | year=2002 | url=http://adsabs.harvard.edu/abs/2002ApJ...566L.125T| volume=566| pages=L125 | doi=10.1086/339437}}</ref> The points at which a planet crosses above and below its reference plane are called its [[ascending node|ascending]] and [[descending node]]s.<ref name="young"/> The [[longitude of the ascending node]] is the angle between the reference plane's 0 longitude and the planet's ascending node. The [[argument of periapsis]] (or perihelion in our Solar System) is the angle between a planet's ascending node and its closest approach to its star.<ref name="young"/> [[Image:AxialTiltObliquity.png|thumb|left|190px|Earth's axial tilt is about 23°.]] ====Axial tilt==== Planets also have varying degrees of [[axial tilt]]; they lie at an angle to the [[reference plane|plane]] of their [[inclination|stars' equators]]. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses [[season]]s; changes to the climate over the course of its year. The point at which each hemisphere is farthest or nearest from its star is known as its [[solstice]]. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.<ref name=Weather>{{cite web | last=Harvey | first=Samantha | date=2006-05-01 | url=http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725 | title=Weather, Weather, Everywhere? | publisher=NASA | accessdate=2007-09-09 }}</ref> Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.<ref>{{cite journal|title=Obliquity Tides on Hot Jupiters|author=Winn, Joshua N.; Holman, Matthew J.|journal=The Astrophysical Journal|year=2005| doi=10.1086/432834| volume=628|pages=L159}}</ref> ====Rotation==== The planets also rotate around invisible axes through their centres. A planet's [[rotation period]] is known as its [[day]]. All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which [[Retrograde and direct motion|rotates clockwise]]<ref>{{cite journal |title=Rotation of Venus: Period Estimated from Radar Measurements|author=Goldstein, R. M.; Carpenter, R. L.|year=1963|journal =Science|volume=139|pages=910|doi=10.1126/science.139.3558.910 |pmid=17743054}}</ref> (Uranus is generally said to be rotating clockwise as well<ref>{{cite web|contribution=Rotational properties of Uranus and Neptune|first=M. J. S. | last=Belton | coauthors=Terrile R. J. | title=Uranus and Neptune | year=1984 | url=http://adsabs.harvard.edu/abs/1984urnp.nasa..327B | pages=327 | editor=Bergstralh, J. T. | accessdate=2008-02-02}}</ref> though because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction).<ref>{{cite book|title=The Outer Worlds; Uranus, Neptune, Pluto, and Beyond|pages=195&ndash;206|year=2006|first=Michael P.|last=Borgia|publisher=Springer New York}}</ref> There is great variation in the length of day between the planets, with Venus taking 243 [[day|Earth days]] to rotate, and the gas giants only a few hours.<ref>{{cite web|title=Planet tables|url=http://www.astronomynotes.com/tables/tablesb.htm|first=Nick|last=Strobel|publisher=astronomynotes.com|accessdate=2008-02-01}}</ref> The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are [[Tidal locking|tidaly locked]] (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.<ref>{{cite journal|title=Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets| last=Zarka | first=Philippe | coauthors=Treumann, Rudolf A.; Ryabov, Boris P.; Ryabov, Vladimir B. |year=2001|journal=Astrophysics & Space Science|volume=277|pages=293|doi = 10.1023/A:1012221527425}}</ref> ====Orbital clearance==== The defining dynamic characteristic of a planet is that it has [[Cleared the neighbourhood|cleared its neighborhood]]. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the [[planetesimal]]s in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the [[International Astronomical Union|IAU]]'s official [[2006 definition of planet|definition of a planet]] in August, 2006. This criterion excludes such planetary bodies as [[Pluto]], [[Eris (dwarf planet)|Eris]] and [[Ceres (dwarf planet)|Ceres]] from full-fledged planethood, making them instead [[dwarf planet]]s.<ref name=IAU /> Although to date this criterion only applies to our Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.<ref>{{cite web|title=The Total Number of Giant Planets in Debris Disks with Central Clearings|date=2007-07-12|author=Faber, Peter; Quillen, Alice C. |work=Department of Physics and Astronomy, University of Rochester|year=2007|url=http://arxiv.org/abs/0706.1684|accessdate=2008-02-02}}</ref> ===Physical characteristics=== ====Mass==== A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the [[electromagnetic]] forces binding its physical structure, leading to a state of [[hydrostatic equilibrium]]. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.<ref>{{cite web|title=The Dwarf Planets|url=http://www.gps.caltech.edu/~mbrown/dwarfplanets/|authorlink=Michael E. Brown|last=Brown|first=Michael E.|work=California Institute of Technology|year=2006|accessdate=2008-02-01}}</ref> Mass is also the prime attribute by which planets are distinguished from [[star]]s. The upper mass limit for planethood is roughly 13 times Jupiter's mass, beyond which it achieves conditions suitable for [[nuclear fusion]]. Other than the Sun, no objects of such mass exist in our Solar System; however a number of extrasolar planets lie at that threshold. The ''Extrasolar Planets Encyclopedia'' lists several planets that are close to this limit: [[HD 38529]]c, [[AB Pictoris]]b, [[HD 162020]]b, and [[HD 13189]]b. A number of objects of higher mass are also listed, but since they lie above the fusion threshold, they would be better described as [[brown dwarf]]s.<ref name="Encyclopedia" /> The smallest known planet, excluding dwarf planets and satellites, is [[PSR B1257+12]] a, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a [[pulsar]]. Its mass is roughly half that of the planet Mercury.<ref name="Encyclopedia" /> [[Image:Jupiter interior.png|150px|right|thumb|Illustration the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen]] ====Internal differentiation==== Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a [[Planetary differentiation|differentiated]] interior consisting of a dense [[planetary core]] surrounded by a [[Mantle (geology)|mantle]] which either is or was a [[fluid]]. The terrestrial planets are sealed within hard [[Crust (geology)|crusts]],<ref name=terrestrial>{{cite web|title=Planetary Interiors|work=Department of Physics, University of Oregon|url=http://abyss.uoregon.edu/~js/ast121/lectures/lec16.html|accessdate=2007-09-10}}</ref> but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as [[iron]] and [[nickel]], and mantles of [[silicate]]s. [[Jupiter]] and [[Saturn]] are believed to possess cores of rock and metal surrounded by mantles of [[metallic hydrogen]].<ref>{{cite book | first=Linda T. | last=Elkins-Tanton | year=2006 | title=Jupiter and Saturn | publisher=Chelsea House | location=New York | id=ISBN 0-8160-5196-8 }}</ref> [[Uranus]] and [[Neptune]], which are smaller, possess rocky cores surrounded by mantles of [[water]], [[ammonia]], [[methane]] and other [[Ice#In planetary science|ices]].<ref>{{cite journal|last=Podolak|first=M.|coauthors=Weizman, A.; Marley, M.|title=Comparative model of Uranus and Neptune|journal=Planet. Space Sci.|volume=43|issue=12|pages=1517&ndash;1522|year=1995| url=http://adsabs.harvard.edu/abs/1995P%26SS...43.1517P|doi=10.1016/0032-0633(95)00061-5}}</ref> The fluid action within these planets' cores creates a [[geodynamo]] that generates a [[magnetic field]].<ref name=terrestrial /> ====Atmosphere==== {{seealso|Extraterrestrial atmospheres}} [[Image:Top of Atmosphere.jpg|thumb|left|150 px|Earth's atmosphere]] All of the Solar System planets have [[atmosphere]]s as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases [[hydrogen]] and [[helium]] close by, while the smaller planets lose these gases into [[space]].<ref>{{cite journal|last=Sheppard|first=Scott S.|coauthors=Jewitt, David; Kleyna, Jan|title=An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness|journal=The Astronomical Journal| volume=129|pages=518&ndash;525 | year=2005| doi=10.1086/426329 | id={{arxiv|astro-ph|0410059v1}}}}</ref> The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular [[oxygen]].<ref name=zeilik>{{cite book | last=Zeilik | first=Michael A. | coauthors=Gregory, Stephan A. | title=Introductory Astronomy & Astrophysics | edition=4th ed. | year=1998 | publisher=Saunders College Publishing | isbn=0030062284 | pages=67 }}</ref> The only solar planet without a true atmosphere is Mercury which had it mostly, although not entirely, blasted away by the [[solar wind]].<ref>Hunten D. M., Shemansky D. E., Morgan T. H. (1988), ''The Mercury atmosphere'', In: Mercury (A89-43751 19-91). University of Arizona Press, pp. 562&ndash;612</ref> Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic [[weather system]]s such as [[hurricane]]s, (on Earth), planet-wide [[dust storm]]s (on Mars), an Earth-sized [[Anticyclonic storm|anticyclone]] on Jupiter (called the [[Great Red Spot]]), and [[Great Dark Spot|holes in the atmosphere]] (on Neptune).<ref name=Weather /> At least one extrasolar planet, [[HD 189733 b]], has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.<ref name="knutson">{{cite journal | last=Knutson | first=Heather A. | coauthors=Charbonneau, David; Allen, Lori E.; Fortney, Jonathan J. | title=A map of the day-night contrast of the extrasolar planet HD 189733b | journal=Nature | year=2007 | volume=447 | pages=183 | doi=10.1038/nature05782 | laysummary=http://www.cfa.harvard.edu/press/2007/pr200713.html | laysource=Center for Astrophysics press release | laydate=2007-05-09}}</ref> Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.<ref>{{cite web | author=Weaver, D.; Villard, R. | url=http://hubblesite.org/newscenter/archive/releases/2007/07/full/ | title=Hubble Probes Layer-cake Structure of Alien World's Atmosphere | work=University of Arizona, Lunar and Planetary Laboratory (Press Release) | date=2007-01-31 | accessdate=2007-08-15}}</ref><ref>{{cite journal | journal=Nature | last=Ballester | first=Gilda E. | coauthors=Sing, David K.; Herbert, Floyd | title=The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b | volume=445 | pages=511 | year=2007 | doi=10.1038/nature05525}}</ref> These planets may have vast differences in temperature between their day and night sides which produce supersonic winds,<ref>{{cite journal | last=Harrington | first=Jason | coauthors=Hansen, Brad M.; Luszcz, Statia H.; Seager, Sara | title=The phase-dependent infrared brightness of the extrasolar planet Andromeda b | journal=Science | volume=314 | pages=623 | year=2006 | doi=10.1126/science.1133904 | laysummary=http://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html | laysource=NASA press release | laydate=2006-10-12 | pmid=17038587}}</ref> although the day and night sides of HD 189733b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.<ref name="knutson" /> ====Magnetosphere==== [[Image:Structure of the magnetosphere.svg|thumb|right|300px|Schematic of Earth's magnetosphere]] One important characteristic of the planets is their intrinsic [[magnetic moment]]s which in turn give rise to [[magnetosphere]]s. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of [[electrical conductivity|electrically conducting]] material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity around itself called [[magnetosphere]], which the solar wind can not penetrate. The size of the magnetosphere can be much larger than that of the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the [[ionosphere]] with the solar wind, which can not effectively protect the planet.<ref name=Kivelson2007/> Of the eight planets in our Solar System, only Venus and Mars lack such a magnetic field.<ref name=Kivelson2007/> In addition, the moon of Jupiter [[Ganymede (moon)|Ganymede]] also has one. Of the magnetized planets Mercury has the weakest magnetic field, and is barely enough to deflect the [[solar wind]]. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System. The magnetic fields of other giant planets are roughly similar in strength of that of Earth but their magnetic moments are significantly larger than the Earth's magnetic moment. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational [[Axis of rotation|axis]] and displaced from the centre of the planet.<ref name=Kivelson2007>{{cite book | last=Kivelson|first=Margaret Galland | coauthors=Bagenal, Fran | chapter=Planetary Magnetospheres|title=Encyclopedia of the Solar System | year=2007|publisher=Academic Press|editor= Lucyann Mcfadden, Paul Weissman, Torrence Johnson|isbn=9780120885893 | page=519}}</ref> In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star [[HD 179949]], which appeared to be creating a sunspot on the surface of its parent star. The team hypothesised that the planet's magnetosphere was transferring energy onto the star's surface increasing its already high 14,000 degree surface temperature by an additional 750 degrees.<ref>{{cite web | title=Magnetic planet|first=Amanda| last=Gefter| work=Astronomy | date=[[2004-01-17]] |url=http://www.astronomy.com/asy/default.aspx?c=a&id=2090|accessdate=2008-01-29}}</ref> [[Image:Voyager ring spokes.jpg|thumb|left|150px|The rings of Saturn]] ===Secondary characteristics=== Planets in our Solar System possess orbital resonances in their own right. All except Mercury and Venus have [[natural satellite]]s, often called "moons." Earth has one, and Mars has two, and the [[gas giant]]s have numerous moons in complex planetary systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life (especially [[Europa (moon)|Europa]]).<ref name=Grasset2000>{{cite journal | last= Grasset | first=O. | coauthors=Sotin C.; Deschamps F. | title = On the internal structure and dynamic of Titan|year = 2000 | journal = Planetary and Space Science | volume = 48| pages = 617&ndash;636|doi=10.1016/S0032-0633(00)00039-8}}</ref><ref name=Fortes2000>{{cite journal | journal = Icarus | volume= 146 | issue = 2 | pages = 444&ndash;452 | year= 2000 | doi = 10.1006/icar.2000.6400 | title = Exobiological implications of a possible ammonia-water ocean inside Titan | author = Fortes, A. D.}}</ref><ref>{{citenews|first=Nicola | last=Jones |date=2001-12-11|work=New Scientist Print Edition|url=http://www.newscientist.com/article.ns?id=dn1647|title=Bacterial explanation for Europa's rosy glow|accessdate=2008-02-01}}</ref> The four gas giants are also orbited by [[planetary ring]]s of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny '[[moonlet]]s' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's [[Roche limit]] and were torn apart by [[tidal force]]s.<ref>{{cite journal | author=Molnar, L. A.; Dunn, D. E. | title=On the Formation of Planetary Rings | journal=Bulletin of the American Astronomical Society | year=1996 | volume=28 | pages=77&ndash;115 | url=http://adsabs.harvard.edu/abs/1996DPS....28.1815M }}</ref><ref>{{cite book | first=Encrenaz | last=Thérèse | year=2004 | title=The Solar System | edition=Third edition | pages=388&ndash;390 | publisher=Springer | isbn=3540002413 }}</ref> No secondary characteristics have been observed around extrasolar planets. However the [[sub-brown dwarf]] [[Cha 110913-773444]], which has been described as a [[rogue planet]], is believed to be orbited by a tiny [[protoplanetary disc]].<ref name="clavin" /> == See also == {{portal|Astronomy|Crab Nebula.jpg}} {{portal|Solar System|Solar system.jpg}} {{portal|Space|Q space.svg}} <div style="-moz-column-count:3; column-count:3;"> * [[Extraterrestrial skies]] * [[Hypothetical planetary object]] * [[Landings on other planets]] * [[Minor planet]] – celestial body smaller than a planet * [[Planetary habitability]] * [[Planetary science]] * [[Planets in astrology]] * [[Planets in science fiction]] </div> ==Notes== <div class="references-small"> <ol type="a"> <li>{{Note_label|A|a|none}}This [[Definition of planet|definition]] is drawn from two separate [[International Astronomical Union|IAU]] declarations; a formal definition agreed by the Union in 2006, and an informal working definition established by the Union in 2003. The [[2006 definition of planet|2006 definition]], while official, applies only to our Solar System, while the 2003 definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference.</li> <li>{{Note_label|B|b|none}}Referred to by Huygens as a ''Planetes novus'' ("new planet") in his [http://www.sil.si.edu/DigitalCollections/HST/Huygens/huygens-text.htm ''Systema Saturnium'']</li> <li>{{Note_label|C|c|none}}Both labelled ''nouvelles planètes'' (new planets) by Cassini in his [http://wwwnt.unifi.it/biblio/libri_studio/indice.asp?flag=2&tipo=lib&id=8 ''Découverte de deux nouvelles planetes autour de Saturne'']</li> <li>{{Note_label|D|d|none}}Both once referred to as "planets" by Cassini in his [http://links.jstor.org/sici?sici=0260-7085%281686%2F1692%2916%3C79%3AAEOTJD%3E2.0.CO%3B2-J ''An Extract of the Journal Des Scavans...'']. The term "satellite", however, had already begun to be used to distinguish such bodies from those around which they orbited ("primary planets").</li> <li>{{Note_label|E|e|none}}[[2006 definition of planet|Recently reclassified]] as a [[dwarf planet]] in 2006.</li> <li>{{Note_label|F|f|none}}Regarded as a planet from its discovery in 1930 until [[2006 definition of planet|redesignated]] as a [[Trans-Neptunian Object|trans-Neptunian]] [[dwarf planet]] in August 2006.</li> </ol> </div> ==References== {{reflist|3}} ==External links== {{commonscat|Planets}} {{wiktionary|planet}} * [http://www.iau.org International Astronomical Union website] * [http://www.sky-pics.net/ Pictures of the Solar System] * [http://planetquest.jpl.nasa.gov/ NASA Planet Quest - Exoplanet Exploration] *[http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets with each other, the Sun, and other stars] * [http://www.iau.org/STATUS_OF_PLUTO.238.0.html IAU Press Releases since 1999 "The status of Pluto: A Clarification"] * [http://www.boulder.swri.edu/~hal/planet_def.html "Regarding the criteria for planethood and proposed planetary classification schemes."] article by Stern and Levinson {{Solar System}} {{featured article}} [[Category:Planets| ]] [[Category:Planetary science]] [[af:Planeet]] [[als:Planet]] [[ar:كوكب]] [[frp:Planèta]] [[ast:Planeta]] [[bn:গ্রহ]] [[zh-min-nan:He̍k-chheⁿ]] [[map-bms:Planet]] [[be:Планета]] [[be-x-old:Плянэта]] [[bs:Planeta]] [[br:Planedenn]] [[bg:Планета]] [[ca:Planeta]] [[cv:Планета]] [[cs:Planeta]] [[cy:Planed]] [[da:Planet]] [[de:Planet]] [[et:Planeet]] [[el:Πλανήτης]] [[es:Planeta]] [[eo:Planedo]] [[eu:Planeta]] [[fa:سیاره]] [[fr:Planète]] [[fy:Planeet]] [[fur:Planet]] [[ga:Pláinéad]] [[gd:Planaid]] [[gl:Planeta]] [[zh-classical:行星]] [[ko:행성]] [[hi:ग्रह]] [[hr:Planet]] [[io:Planeto]] [[id:Planet]] [[ia:Planeta]] [[is:Reikistjarna]] [[it:Pianeta]] [[he:כוכב לכת]] [[jv:Planet]] [[pam:Planeta]] [[kn:ಗ್ರಹ]] [[ka:პლანეტა]] [[kk:Ғаламшар]] [[kw:Planet]] [[sw:Sayari]] [[kg:Mweta]] [[ht:Planèt]] [[ku:Exter]] [[la:Planeta]] [[lv:Planēta]] [[lb:Planéit]] [[lt:Planeta]] [[ln:Monzɔ́tɔ mwa malíli]] [[jbo:plini]] [[hu:Bolygó]] [[mk:Планета]] [[ml:ഗ്രഹം]] [[mt:Pjaneta]] [[mr:ग्रह]] [[ms:Planet]] [[nah:Nehnencācītlalli]] [[nl:Planeet]] [[nds-nl:Planeet]] [[ja:惑星]] [[nap:Chianeta]] [[no:Planet]] [[nn:Planet]] [[nrm:Plianète]] [[nov:Planete]] [[oc:Planeta]] [[uz:Sayyora]] [[nds:Planet]] [[pl:Planeta]] [[pt:Planeta]] [[ksh:Planet (Aßtronomie)]] [[ro:Planetă]] [[qu:Puriq quyllur]] [[ru:Планета]] [[se:Planehtta]] [[sq:Planeti]] [[scn:Pianeta]] [[simple:Planet]] [[sk:Planéta]] [[sl:Planet]] [[sr:Планета]] [[sh:Planeta]] [[su:Planét]] [[fi:Planeetta]] [[sv:Planet]] [[tl:Planeta]] [[ta:கோள்]] [[te:గ్రహం]] [[th:ดาวเคราะห์]] [[vi:Hành tinh]] [[tg:Сайёра]] [[tpi:Planet]] [[tr:Gezegen]] [[uk:Планета]] [[vec:Pianeta]] [[wa:Bole (astronomeye)]] [[yi:פלאנעט]] [[yo:Plánẹ́tì]] [[zh-yue:行星]] [[zh:行星]]