Definition of planet
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[[Image:Voyager 2 Neptune and Triton.jpg|thumb|300 px|Photograph of the planet [[Neptune]] (large, background) and its moon [[Triton (moon)|Triton]] (small, foreground), taken by [[Voyager 2]] as it entered the [[outer Solar System]].]]
From its beginnings denoting the "wandering stars" of the classical world, the '''definition of "[[planet]]"''' has been fraught with ambiguity. In its long life, the word has meant many different things, often simultaneously. Over the millennia, use of the term was never strict and its meaning has twisted and blurred to include or exclude a variety of different objects, from the Sun and the Moon to satellites and asteroids. As knowledge of the universe grew, the word "planet" grew and changed with it, casting off old meanings and adopting new ones, though never arriving at a single, concrete definition.
By the end of the 19th century, the word "planet" had, without being defined, settled into a comfortable working term. It only applied to objects in the [[Solar System]]; a number small enough that any differences could be dealt with on an individual basis. After 1992 however, astronomers began to discover many additional objects beyond the orbit of [[Neptune]], as well as hundreds of objects [[extrasolar planet|orbiting other stars]]. These discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some were nearly large enough to be [[star]]s, while others were smaller than [[moon|Earth's moon]]. Several were nearly as large as Pluto. These discoveries challenged long perceived notions of what a planet could be.
The issue of a clear definition for "planet" came to a head in 2005 with the discovery of the [[trans-Neptunian object]] [[Eris (dwarf planet)|Eris]], a body larger than the smallest then-accepted planet, [[Pluto]]. In its 2006 response, the [[International Astronomical Union]] (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its [[2006 definition of planet|decision]] on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the [[Sun]], is large enough for its own gravity to make it round, and has "[[Cleared the neighbourhood|cleared its neighbourhood]]" of smaller objects. Under this new definition, Pluto does not qualify as a planet. The IAU's decision has not resolved all controversies, and while many scientists have accepted the definition, some in the astronomical community have rejected it outright.
== History ==
{{seealso|Geocentric model|Heliocentrism|Celestial spheres|Naked eye planet}}
=== Planets in antiquity ===
[[Image:Platon-2.jpg|thumb|100 px|The philosopher [[Plato]]]]
While knowledge of the planets likely predates history and is common to most civilisations, the word "planet" itself dates to [[ancient Greece]]. The Greeks believed the Earth to be stationary and at the centre of the universe in accordance with the [[geocentric model]], and that the objects in the sky, and indeed the sky itself, revolved around it. Greek astronomers employed the term ''asteres planetai'', "wandering stars",<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>{{cite web|url=http://www.wordsources.info/words-mod-planets.html|title=Words For Our Modern Age: Especially words derived from Latin and Greek sources|publisher=Wordsources.info|accessdate=2007-07-23}}</ref> to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the ''asteres aplanis'', the "fixed stars", which stayed motionless relative to one another. The five bodies currently called "planets" that were known to the Greeks are [[Mercury (planet)|Mercury]], Venus, Mars, Jupiter, and Saturn.
Graeco-Roman cosmology commonly considered seven planets, with the Sun and the Moon counted among them (as is the case in modern [[Planets in astrology|astrology]]); however, there is some ambiguity on that point, as many ancient astronomers distinguished the five starlike planets from the Sun and Moon. As the 19th century German naturalist [[Alexander von Humboldt]] noted in his work ''Cosmos,''
<blockquote>Of the seven cosmical bodies which, by their continually varying relative positions and distances apart, have ever since the remotest antiquity been distinguished from the "unwandering orbs" of the heaven of the "fixed stars", which to all sensible appearance preserve their relative positions and distances unchanged, five only -Mercury, Venus, Mars, Jupiter and Saturn- wear the appearance of stars- "''cinque stellas errantes''"- while the Sun and Moon, from the size of their disks, their importance to man, and the place assigned to them in mythological systems, were classed apart.<ref>{{cite web|title=Cosmos: A Sketch of a Physical Description of the Universe|author=Alexander von Humboldt|year=1849|work=digitised 2006|url=http://books.google.com/books?id=3CEJAAAAIAAJ|accessdate=2007-07-23}}</ref></blockquote>
[[Image:Cellarius ptolemaic system c2.jpg|thumb|left|The planets as understood before the acceptance of the [[heliocentric model]].]]
In his ''[[Timaeus (dialogue)|Timaeus]]'', written in roughly 360 BC, [[Plato]] mentions, "the Sun and Moon and five other stars, which are called the planets".<ref>{{cite web|title=Timaeus by Plato|work=The Internet Classics|url=http://classics.mit.edu/Plato/timaeus.html|accessdate= 2007-02-22}}</ref> His student [[Aristotle]] makes a similar distinction in his ''[[On the Heavens]]'': "The movements of the sun and moon are fewer than those of some of the planets".<ref>{{cite web|title=On the Heavens by Aristotle, Translated by J. L. Stocks|work=University of Adelaide Library|url=http://etext.library.adelaide.edu.au/a/aristotle/heavens/|year=2004|accessdate=2007-02-24}}</ref> In his ''Phaenomena'', which set to verse an astronomical treatise written by the philosopher [[Eudoxus of Cnidus|Eudoxus]] in roughly 350 BC,<ref>{{cite web|title=Phaenomena Book I - ARATUS of SOLI|url=http://www.geocities.com/astrologysources/classicalgreece/phaenomena/index.htm|accessdate=2007-06-16}}</ref> the poet [[Aratus]] describes "those five other orbs, that intermingle with [the constellations] and wheel wandering on every side of the twelve figures of the Zodiac."<ref>{{cite web|title=ARATUS, PHAENOMENA|author=A. W. & G. R. Mair (translators)|work=theoi.com|url=http://www.theoi.com/Text/AratusPhaenomena.html|accessdate=2007-06-16}}</ref>
In his ''[[Almagest]]'' written in the 2nd century, [[Claudius Ptolemy|Ptolemy]] refers to "the Sun, Moon and five planets."<ref>{{cite book|title=The Almagest by Ptolemy|author=R. Gatesby Taliaterro (trans.)|publisher= University of Chicago Press|year= 1952|pages=270}}</ref> [[Gaius Julius Hyginus|Hyginus]] explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta."<ref name=planeta>{{cite web|title=Astra Planeta|author=theoi.com|url=http://www.theoi.com/Titan/AstraPlaneta.html|accessdate=2007-02-25}}</ref> [[Marcus Manilius]], a Latin writer who lived during the time of [[Caesar Augustus]] and whose poem ''Astronomica'' is considered one of the principal texts for modern [[astrology]], says, "Now the [[dodecatemory]] is divided into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven."<ref>{{cite book|author=GP Goold (trans.)|title=Marcus Manilius: Astronomica|publisher=Harvard University Press|year=1977|pages=141}}</ref>
The single view of the seven planets is found in [[Cicero]]'s ''[[Dream of Scipio]]'', written sometime around 53 BC, where the spirit of [[Scipio Africanus]] proclaims, "Seven of these spheres contain the planets, one planet in each sphere, which all move contrary to the movement of heaven."<ref>{{cite web|title=Roman Philospohy: Cicero: The Dream of Scipio|author=Richard Hooker (translator)|url=http://www.wsu.edu/~dee/ROME/SCIPIO.HTM|year=1996|accessdate=2007-06-16}}</ref> In his ''[[Natural History (Pliny)|Natural History]]'', written in 77 AD, [[Pliny the Elder]] refers to "the seven stars, which owing to their motion we call planets, though no stars wander less than they do."<ref>{{cite book|title=Natural History vol 1|author=IH Rackham|publisher=William Heinemann Ltd.|year=1938|pages= 177, viii}}</ref> [[Nonnus]], the 5th century Greek poet, says in his ''[[Dionysiaca]]'', "I have oracles of history on seven tablets, and the tablets bear the names of the seven planets."<ref name=planeta />
=== Planets in the Middle Ages ===
[[Image:John Gower world Vox Clamantis.jpg|thumb|100px|John Gower]]
Medieval and Renaissance writers generally accepted the idea of seven planets. The standard medieval introduction to astronomy, [[Sacrobosco|Sacrobosco's]] ''[[De sphaera mundi|De Sphaera]]'', includes the Sun and Moon among the planets,<ref>Sacrobosco, "On the Sphere", in Edward Grant, ed. ''A Source Book in Medieval Science,'' (Cambridge: Harvard University Press, 1974), p. 450. "every planet except the sun has an epicycle."</ref> the more advanced ''Theorica planetarum'' presents the "theory of the seven planets,"<ref>Anonymous, "The Theory of the Planets," in Edward Grant, ed. ''A Source Book in Medieval Science,'' (Cambridge: Harvard University Press, 1974), p. 452.</ref> while the instructions to the ''[[Alfonsine Tables]]'' show how "to find by means of tables the mean ''motuses'' of the sun, moon, and the rest of the planets."<ref>[[John of Saxony (astronomer)|John of Saxony]], "Extracts from the Alfonsine Tables and Rules for their use", in Edward Grant, ed. ''A Source Book in Medieval Science,'' (Cambridge: Harvard University Press, 1974), p. 466.</ref> In his ''[[Confessio Amantis]]'', 14th century poet [[John Gower]], referring to the planets' connection with the craft of [[Classical planets in Western alchemy|alchemy]], writes, "Of the planetes ben begonne/The gold is tilted to the Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon were planets.<ref>{{cite journal|author=P. Heather|title=The Seven Planets|journal=Folklore|year=1943|pages=338–361}}</ref> Even [[Nicolaus Copernicus]], who rejected the geocentric model, was ambivalent concerning whether the Sun and Moon were planets. In his ''[[De Revolutionibus]]'', Copernicus clearly separates "the sun, moon, planets and stars";<ref name=koper>{{cite web|title=The text of Nicholas Copernicus' De Revolutionibus (On the Revolutions), 1543 C.E.|author=Edward Rosen (trans.)|work=Calendars Through the Ages|url=http://webexhibits.org/calendars/year-text-Copernicus.html|accessdate=2007-02-28}}</ref> however, in his Dedication of the work to Pope Paul III, Copernicus refers to, "the motion of the sun and the moon... and of the five other planets."<ref>{{cite web|title=Dedication of the Revolutions of the Heavenly Bodies to Pope Paul III|author=Nicholas Copernicus|work=The Harvard Classics. 1909–14|url=http://www.bartleby.com/39/12.html|accessdate=2007-02-23}}</ref>
=== Modern planets ===
[[Image:Nikolaus Kopernikus.jpg|thumb|100px|Copernicus]]
Eventually, when Copernicus's [[heliocentric model]] was accepted over the [[geocentric]], [[Earth]] was placed among the planets and the Sun and Moon were demoted, necessitating a conceptual revolution in the understanding of planets. As the [[historian of science]], [[Thomas Kuhn]], noted in his book, ''[[The Structure of Scientific Revolutions]]'':<ref>Thomas S. Kuhn, (1962) ''The Structure of Scientific Revolutions'', 1st. ed., (Chicago: University of Chicago Press), pp. 115, 128-9. </ref>
<blockquote>The Copernicans who denied its traditional title 'planet' to the sun ... were changing the meaning of 'planet' so that it would continue to make useful distinctions in a world where all celestial bodies ... were seen differently from the way they had been seen before... Looking at the moon, the convert to Copernicanism ... says, 'I once took the moon to be (or saw the moon as) a planet, but I was mistaken.'</blockquote>
It could be said, therefore, that Earth was the first planet of the modern era. Copernicus obliquely refers to Earth as a planet in ''De Revolutionibus'' when he says,
<blockquote>Having thus assumed the motions which I ascribe to the Earth later on in the volume, by long and intense study I finally found that if the motions of the other planets are correlated with the orbiting of the earth, and are computed for the revolution of each planet, not only do their phenomena follow therefrom but also the order and size of all the planets and spheres, and heaven itself is so linked together that in no portion of it can anything be shifted without disrupting the remaining parts and the universe as a whole.<ref name=koper/></blockquote>
Galileo also indirectly asserts that Earth is a planet in the ''[[Dialogue Concerning the Two Chief World Systems]]''; "[T]he Earth, no less than the moon or any other planet, is to be numbered among the natural bodies that move circularly."<ref name=gal>{{cite web|title=Dialogue Concerning the Two Chief World Systems|work=Calendars Through the Ages|url=http://www.webexhibits.org/calendars/year-text-Galileo.html|accessdate=2008-06-14}}</ref>
[[Image:William Herschel01.jpg|thumb|100px|William Herschel, discoverer of Uranus]]
In 1781, the astronomer [[William Herschel]] was searching the sky for elusive [[stellar parallax]]es, when he observed what he termed a [[comet]] in the constellation of [[Taurus (constellation)|Taurus]]. Unlike stars, which remained mere points of light even under high magnification, this object's size increased in proportion to the power used. That this strange object might have been a planet simply did not occur to Herschel; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. However, unlike a comet, this object's orbit was nearly circular and within the ecliptic plane. Before Herschel announced his discovery of his "comet", his colleague, British [[Astronomer Royal]] [[Nevil Maskelyne]], wrote to him, saying, "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it."<ref>{{cite book|title=William Herschel: Astronomer and Musician of 19 New King Street, Bath|author=Patrick Moore|publisher=PME Erwood|year=1981|pages=8}}</ref> The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognised as the seventh planet and named [[Uranus]] after the father of Saturn.
Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of [[Neptune]] in 1846, and presumed irregularities in Neptune's orbit subsequently led to the search which ultimately located [[Pluto]] in 1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all.<ref>
{{cite book | author=Croswell, Ken | year=1999 | title=Planet Quest: The Epic Discovery of Alien Solar Systems| publisher= Oxford University Press pp. 48, 66 (ISBN 0-19-288083-7)}}</ref> In 1989, [[Voyager 2]] determined the irregularities to be due to an overestimation of Neptune's mass.<ref>{{cite web | author=Paul Schlyter | title=Appendix 7: Hypothetical Planets| work=nineplanets.org| url=http://www.nineplanets.org/hypo.html| accessdate=2006-10-04}} </ref>
=== Satellites ===
[[Image:Galileo.arp.300pix.jpg|thumb|100px|Galileo Galilei]]
When Copernicus placed the Earth among the planets, he also placed the Moon in orbit around the Earth, making the Moon the first [[natural satellite]] to be discovered. When [[Galileo Galilei|Galileo]] discovered his four [[Galilean moons|satellites]] of Jupiter in 1610, they lent weight to Copernicus's argument, since if other planets could have satellites, then the Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo initially intended to name them the "Medicean stars", in honour of his patrons, the [[Medicis]], but also referred to them as planets in his ''Dialogue'': "Just as we see in the planets (and most palpably in the satellites of Jupiter) ...".<ref name=gal/> Similarly, [[Christiaan Huygens]], upon discovering Saturn's largest moon [[Titan (moon)|Titan]] in 1655, employed many terms to describe it, including "planeta", (planet) "stella" (star) "Luna" (moon), and the more modern "satellite".<ref>{{cite book|title=Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo|author= Christiani Hugenii (Christiaan Huygens)|publisher=Adriani Vlacq|year= 1659|pages= 1–50}}</ref> [[Giovanni Cassini]], in announcing his discovery of Saturn's moons [[Iapetus (moon)|Iapetus]] and [[Rhea (moon)|Rhea]] in 1671 and 1672, described them as ''Nouvelles Planetes autour de Saturne'' ("New planets around Saturn").<ref>{{cite book|title=Decouverte de deux Nouvelles Planetes autour de Saturne|author=Giovanni Cassini|Publisher=Sabastien Mabre-Craniusy|year= 1673| pages=6–14}}</ref> However, when the "Journal de Scavans" reported Cassini's discovery of two new Saturnian moons in 1686, it referred to them strictly as "satellites".<ref>{{cite journal|title=An Extract of the Journal Des Scavans. of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris|author=Cassini|journal=Philosophical Transactions|volume=16|pages=79–85|year=1686|url=http://links.jstor.org/sici?sici=0260-7085%281686%2F1692%2916%3C79%3AAEOTJD%3E2.0.CO%3B2-J|accessdate =2007-02-24|doi=10.1098/rstl.1686.0013}}</ref> When William Herschel announced his discovery of two objects in orbit around Uranus in 1787, he referred to them as "satellites" and "secondary planets".<ref>{{cite book|title=An Account of the Discovery of Two Satellites Around the Georgian Planet. Read at the Royal Society|author=William Herschel|publisher=J. Nichols|year=1787|pages=1–4}}</ref> All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,<ref>See primary citations in [[Timeline of discovery of Solar System planets and their moons#References|Timeline of discovery of Solar System planets and their moons]]</ref> though the 1868 book, "Smith's Illustrated Astronomy"<ref name="smith">{{cite book
|author=Asa Smith
|year=1868
|title=Smith's Illustrated Astronomy
|publisher=Nichols & Hall
|url=http://books.google.com/books?id=ZLgXAAAAIAAJ&pg=PA23&lpg=PA23&dq=%22secondary+planet%22+Herschel&source=web&ots=4hmJstEMT1&sig=yNU_6U4h_1GwfWg4gfFeGkqDnm0&hl=en&sa=X&oi=book_result&resnum=10&ct=result }}</ref> referred to satellites as "secondary planets". Even in the late 1800's astronomers calculated that the [[Galilean moons]] were larger than our own, with one being larger than [[Mercury (planet)|Mercury]].<ref>Recreations in Astronomy by Henry White Warren D.D. 1886</ref>
=== Minor planets ===
[[Image:Giuseppe Piazzi.jpg|thumb|100px|Giuseppe Piazzi, discoverer of Ceres]]
One of the unexpected results of [[William Herschel|William Herschel's]] discovery of Uranus was that it appeared to validate [[Bode's law]], a mathematical function which generates the size of the [[semimajor axis]] of planetary [[orbit]]s. Astronomers had considered the "law" a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, astronomer [[Giuseppe Piazzi]] found a miniature new world, [[1 Ceres|Ceres]], lying at just the correct point in space. The object was hailed as a new planet.<ref name=Hilton>{{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=2006-05-25}} </ref>
Then in 1802, [[Heinrich Wilhelm Matthäus Olbers|Heinrich Olbers]] discovered [[2 Pallas|Pallas]], a second "planet" at roughly the same distance from the Sun as Ceres. That two planets could occupy the same orbit was an affront to centuries of thinking; even [[Shakespeare]] had ridiculed the idea ("Two stars keep not their motion in one sphere").<ref>{{cite book|title=King Henry the Fourth Part One in The Globe Illustrated Shakespeare: The Complete Works Annotated|author=William Shakespeare|publisher=Granercy Books|year= 1979|page= 559}}</ref> Even so, in 1804, another world, [[3 Juno|Juno]], was discovered in a similar orbit.<ref name=Hilton/> In 1807, Olbers discovered a fourth object, [[4 Vesta|Vesta]], at a similar orbital distance.{{Ref_label|A|a|none}}
Herschel suggested that these four worlds be given their own separate classification, [[asteroid]]s (meaning "starlike" since they were too small for their disks to resolve and thus resembled [[star]]s), though most astronomers preferred to refer to them as planets.<ref name=Hilton/> This conception was entrenched by the fact that, due to the difficulty of distinguishing asteroids from yet-uncharted stars, those four remained the only asteroids known until 1845.<ref name=18planets> {{cite web
|title=The Planet Hygea
|year=1849
|work=spaceweather.com
|url=http://spaceweather.com/swpod2006/13sep06/Pollock1.jpg
|accessdate=2008-06-24 }}</ref><ref name=police>{{cite journal|title=Call the Police! The story behind the discovery of the asteroids|journal=Astronomy Now|first=Keith|last= Cooper|pages=60–61|volume=21|issue=6|date=June 2007}}</ref> Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets.<ref name=Hilton/> With the arrival of more refined star charts, the search for asteroids resumed, and a fifth and sixth were discovered by [[Karl Ludwig Hencke]] in 1845 and 1847.<ref name=police /> By 1851 the number of asteroids had increased to 15, and a new method of classifying them, by affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own distinct category. Ceres became "(1) Ceres", Pallas became "(2) Pallas", and so on. By the 1860s, the number of known asteroids had increased to over a hundred, and observatories in Europe and the United States began referring to them collectively as "[[minor planet]]s", or "small planets", though it took the first four asteroids longer to be grouped as such.<ref name=Hilton/> To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun (whether asteroid or not), and each new discovery is numbered accordingly in the IAU's [[Minor Planet Catalogue]].<ref>{{cite web|title=The MPC Orbit (MPCORB) Database|url=http://www.cfa.harvard.edu/iau/MPCORB.html|accessdate=2007-10-15}}</ref>
=== Pluto ===
[[Image:Clyde Tombaugh image.jpg|thumb|100px|Clyde Tombaugh, discoverer of Pluto]]
The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of [[Pluto]], which was named a planet soon after its discovery by [[Clyde Tombaugh]] in 1930. Uranus and Neptune had been declared planets based on their circular orbits, large masses and proximity to the ecliptic plane. None of these applied to Pluto; a tiny, icy world in a region of [[gas giant]]s with an orbit that carried it high above the [[ecliptic]] and even inside that of Neptune. In 1978, astronomers discovered its largest moon, [[Charon (moon)|Charon]]. This allowed astronomers to determine Pluto's mass. It was found to be much tinier than expected, smaller even than the Earth's Moon. However, it was, as far as anyone could tell, unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-hypothesised [[Kuiper belt]] (sometimes called the Edgeworth-Kuiper belt), a band of icy debris that is the source for "short-period" comets—those with orbital periods of up to 200 years.<ref>{{cite web | author=Weissman, Paul R. | title=The Kuiper Belt| work=Annual Review of Astronomy and Astrophysics| url=http://adsabs.harvard.edu/abs/1995ARA&A..33..327W
| accessdate=2006-10-04}}</ref>
Pluto's orbit lay within this band and thus its planetary status was thrown into question; the precedent set by Ceres in downgrading an object from planet status because of a shared orbit led many to conclude that Pluto must be reclassified as a minor planet as well. [[Michael E. Brown|Mike Brown]] of the [[California Institute of Technology]] suggested that a "planet" should be redefined as "any body in the Solar System that is more massive than the total mass of all of the other bodies in a similar orbit."<ref>{{cite web | author=Brown, Mike. | title=A World on the Edge | work=[[NASA]] Solar System Exploration | url=http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=105 | accessdate=2006-05-25}}</ref> Those objects under that mass limit would become minor planets. In 1999, [[Brian Marsden]] of [[Harvard University]]'s [[Minor Planet Center]] suggested that Pluto be given the [[minor planet number]] 10000 while still retaining its official position as a planet.<ref name=comet>{{cite web
|title=Is Pluto a giant comet?
|publisher=Harvard-Smithsonian Center for Astrophysics
|url=http://cfa-www.harvard.edu/icq/ICQPluto.html
|accessdate=2008-06-25}}</ref><ref>{{cite web|title=Xena becomes Eris -- Pluto reduced to a number|author=Kenneth Chang|work= New York Times|year=2006|url=http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2006/09/15/MNGS8L67LJ1.DTL|accessdate=2008-06-18}}</ref> The prospect of Pluto's "demotion" created a public outcry, and in response the [[International Astronomical Union]] clarified that it was not at that time proposing to remove Pluto from the planet list.<ref>{{cite web | year=1999 | title=The Status of Pluto:A clarification | work=[[International Astronomical Union]], Press release | url=http://www.iau.org/STATUS_OF_PLUTO.238.0.html | accessdate=2006-05-25}}</ref><ref> {{cite web | year=1999 | author=Witzgall, Bonnie B. | title=Saving Planet Pluto | work= Amateur Astronomer article| url=http://www.asterism.org/newsletter/l9904-3.htm | accessdate=2006-05-25}} </ref>
[[Image:Michael E Brown 1.jpg|thumb|100px|Michael E Brown, discoverer of Eris]]
The discovery of several other [[trans-Neptunian object]]s approaching the size of Pluto, such as [[50000 Quaoar|Quaoar]] and [[90377 Sedna|Sedna]], continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On [[July 29]], [[2005]], Mike Brown and his team announced the discovery of a trans-Neptunian object confirmed to be larger than Pluto,<ref>{{cite web | year=2006 | author=Brown, Mike | title=The discovery of 2003 UB313, the 10th planet.| work= California Institute of Technology| url=http://www.gps.caltech.edu/~mbrown/planetlila/ | accessdate=2006-05-25}} </ref> named [[136199 Eris|Eris]].<ref> {{cite web | year=2005 | author=M. E. Brown, C. A. Trujillo, and D. L. Rabinowitz | title=DISCOVERY OF A PLANETARY-SIZED OBJECT IN THE SCATTERED KUIPER BELT| work= The American Astronomical Society.| url=http://www.gps.caltech.edu/%7Embrown/papers/ps/xena.pdf | accessdate=2006-08-15}} </ref>
In the immediate aftermath of the object's discovery, there was much discussion as to whether it could be termed a "[[tenth planet]]". NASA even put out a press release describing it as such.<ref>{{cite web|title=NASA-Funded Scientists Discover Tenth Planet|work=Jet Propulsion Laboratory|url=http://www.jpl.nasa.gov/news/news-print.cfm?release=2005-126| year=2005|accessdate=2007-02-22}}</ref> However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum size. Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognise Pluto's historical identity as a planet by "[[grandfathering]]" it into the planet list.<ref>{{cite web|title=Topic - First Mission to Pluto and the Kuiper Belt; "From Darkness to Light: The Exploration of the Planet Pluto"|author=Dr. Bonnie Buratti|work=Jet Propulsion Laboratory|url=http://www.jpl.nasa.gov/events/lectures/nov05.cfm|year=2005|accessdate=2007-02-22}}</ref>
== IAU definition ==
{{main|2006 definition of planet}}
<imagemap>
Image:EightTNOs.png|thumb|300 px|The relative sizes of the largest trans-Neptunian objects as compared to Earth.
#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 is 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>
The discovery of [[Eris (dwarf planet)|Eris]] forced the [[International Astronomical Union|IAU]] to act on a definition. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of [[90377 Sedna|Sedna]] in 2003, narrowed their choices to a shortlist of three, using [[approval voting]]. The definitions were:
* A planet is any object in [[orbit]] around the Sun with a diameter greater than 2000 km. ''(eleven votes in favour)''
* A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. ''(eight votes in favour)''
* A planet is any object in orbit around the Sun that is dominant in its immediate neighbourhood. ''(six votes in favour)''<ref>{{cite web | author=McKee, Maggie| year=2006 | title=Xena reignites a planet-sized debate | work=NewScientistSpace | url=http://www.newscientistspace.com/article.ns?id=dn8681| accessdate=2006-05-25}} </ref><ref>{{cite web | author=Croswell, Ken| year=2006 | title=The Tenth Planet's First Anniversary| url=http://kencroswell.com/TenthPlanetFirstAnniversary.html| accessdate=2006-05-25}} </ref>
Since no overall consensus could be reached, the committee decided to put these three definitions to a wider vote at the IAU General Assembly meeting in [[Prague]] in August 2006,<ref>{{cite web| year=2006 | title=Planet Definition| work=IAU | url=http://www.astronomy2006.com/planet-definition.php| accessdate=2006-08-14}} </ref> and on [[August 24]], the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between "planet" and "rock" (or, in the new parlance, "[[small Solar System body]]"), called "[[dwarf planet]]" and placed [[Pluto]] in it, along with Ceres and Eris.<ref>{{cite web| date=2006-08-24 | title=IAU General Assembly Newspaper| format=PDF | url=http://astro.cas.cz/nuncius/nsiii_09.pdf| accessdate=2007-03-03}}</ref><ref name=IAU0602>{{cite web | date = 2006-08-24 | url = http://www.iau.org/public_press/news/release/iau0602/ | title = The Final IAU Resolution on the Definition of "Planet" Ready for Voting | publisher = IAU (News Release - IAU0602) | accessdate = 2007-03-02 }}</ref> The vote was passed, with 424 astronomers taking part in the ballot.<ref name=britt/><ref>{{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|publisher=IAU|date=2006-08-24|publisher=IAU}}</ref><ref name=IAU0603>{{cite press release
|date=2006-08-24
|publisher=International Astronomical Union (News Release - IAU0603)
|title=IAU 2006 General Assembly: Result of the IAU Resolution votes
|url=http://www.iau.org/public_press/news/release/iau0603/
|accessdate=2007-12-31}} ([http://www.iau.org/iau0603.414.0.html orig link])</ref>
{{cquote|The IAU therefore resolves that planets and other bodies in our [[Solar System]], except [[natural satellite|satellites]], be defined into three distinct categories in the following way:''
(1) A "[[planet]]"<sup>1</sup> is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a [[hydrostatic equilibrium]] (nearly round) shape, and (c) has [[cleared the neighbourhood]] around its orbit.
(2) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape<sup>2</sup>, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects<sup>3</sup> except satellites orbiting the Sun shall be referred to collectively as "[[Small Solar System Body|Small Solar System Bodies]]".
Footnotes:
<sup>1</sup> The [[Planets#Within the Solar System|eight planets]] are: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], [[Mars]], [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]].<br />
<sup>2</sup> An IAU process will be established to assign borderline objects into either "dwarf planet" and other categories.<br />
<sup>3</sup> These currently include most of the Solar System [[asteroid]]s, most [[Trans-Neptunian object|Trans-Neptunian Objects (TNOs)]], [[comet]]s, and other small bodies.
----
The IAU further resolves:
[[Pluto]] is a "dwarf planet" by the above definition and is recognised as the prototype of a new category of trans-Neptunian objects.}}
The IAU also resolved that "''planets'' and ''dwarf planets'' are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets.<ref name=IAU0603/>
On [[September 13]], [[2006]], the IAU placed Eris, its moon Dysnomia, and Pluto into their [[Minor Planet Catalogue]], giving them the official minor planet designations [[(134340) Pluto]], [[(136199) Eris]], and [[(136199) Eris I Dysnomia]].<ref>{{cite web|title=Circular No. 8747|author=Central Bureau for Astronomical Telegrams, International Astronomical Union|url=http://www.cfa.harvard.edu/iau/special/08747.pdf|year=2006|accessdate=2007-02-23}}</ref> Other [[dwarf planet candidates]], such as [[2003 EL61|2003 EL<sub>61</sub>]], [[Makemake (dwarf planet)|2005 FY<sub>9</sub>]], [[90377 Sedna|Sedna]] and [[50000 Quaoar|Quaoar]], were left in temporary limbo until a formal decision could be reached regarding their status.
On [[June 11]], [[2008]], the IAU executive committee announced the establishment of a subclass of dwarf planets comprising the aforementioned "new category of trans-Neptunian objects" to which Pluto is a prototype. This new class of objects, termed [[plutoid]]s, would include Pluto, Eris and any other future trans-Neptunian dwarf planets, but excluded Ceres. The IAU also determined that, for naming purposes, only those TNOs with an [[absolute magnitude]] brighter than H = +1 would be allowed into the category. To date, only two other TNOs, 2003 EL<sub>61</sub> and (136472) Makemake (formerly {{mp|2005 FY|9}}), meet the absolute magnitude requirement, while other potential dwarf planets, such as Sedna, Orcus and Quaoar, do not.<ref>{{cite web
|date=2008-06-11, Paris
|title=Plutoid chosen as name for Solar System objects like Pluto
|publisher=[[International Astronomical Union]] (News Release - IAU0804)
|url=http://www.iau.org/public_press/news/release/iau0804
|accessdate=2008-06-11}}</ref> On [[July 11]], [[2008]], the Working Group on Planetary Nomenclature included FY9 in the Plutoid class, naming it [[Makemake (dwarf planet)|Makemake]].<ref name=name>{{cite web
|date=07/11/2008 11:42:58
|title=Dwarf Planets and their Systems
|publisher= Working Group for Planetary System Nomenclature (WGPSN)
|url=http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets
|accessdate=2008-07-13}}</ref>
== Acceptance of the definition ==
[[Image:Outersolarsystem objectpositions labels comp.png|thumb|400 px|Plot of the current positions of all known Kuiper belt objects, set against the outer planets]]
Among the most vocal proponents of the IAU's decided definition are [[Michael E. Brown|Mike Brown]], the discoverer of Eris, and [[Steven Soter]], professor of astrophysics at the [[American Museum of Natural History]].
In an article in the [[January 2007]] issue of ''[[Scientific American]]'', Soter cited the definition's incorporation of current theories of the [[formation and evolution of the Solar System]]; that as the earliest [[protoplanet]]s emerged from the swirling dust of the [[protoplanetary disc]], some bodies "won" the initial competition for limited material and, as they grew, their increased gravity meant that they accumulated more material, and thus grew larger, eventually outstripping the other bodies in the Solar System by a very wide margin. The asteroid belt, disturbed by the gravitational tug of nearby Jupiter, and the Kuiper belt, too widely spaced for its constituent objects to collect together before the end of the initial formation period, both failed to win the accretion competition.
When the numbers for the winning objects are compared to those of the losers, the contrast is quite striking; if we accept Soter's concept that each planet occupies an "orbital zone,"{{Ref_label|B|b|none}} then the least orbitally dominant planet, Mars, is larger than all other collected material in its orbital zone by a factor of 5100. Ceres, the largest asteroid, is only larger by a factor of 0.33; Pluto's ratio is even lower, at 0.07.<ref name=Soter>{{cite web|title=What is a Planet?|author= Steven Soter|work=Department of Astrophysics, American Museum of Natural History|url=http://www.sciam.com/article.cfm?chanID=sa006&articleID=93385350-E7F2-99DF-3FD6272BB4959038&pageNumber=2&catID=2|year=2007|accessdate=2007-02-21}}</ref> Mike Brown asserts that this massive difference in orbital dominance leaves "absolutely no room for doubt about which objects do and do not belong."<ref name=Brown>{{cite web|title=The Eight Planets|author=Michael E. Brown|work=Caltech|url=http://www.gps.caltech.edu/~mbrown/eightplanets/|year=2006| accessdate=2007-02-21}}</ref>
== Ongoing controversies ==
Despite the IAU's declaration, a number of critics remain unconvinced. The definition is seen by many as arbitrary and confusing, and a number of [[Pluto]]-as-planet proponents, in particular [[Alan Stern]], head of [[NASA|NASA's]] [[New Horizons]] mission to Pluto, have circulated a petition among astronomers to alter the definition. His claim is that, since less than 5 percent of astronomers voted for it, the decision was not representative of the entire astronomical community.<ref name=britt>{{cite web | year=2006 | author=Robert Roy Britt| title=Pluto demoted in highly controversial definition| work= Space.com| url=http://www.space.com/scienceastronomy/060824_planet_definition.html| accessdate=2006-08-24}}</ref><ref>{{cite web | year=2006 | author=Robert Roy 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> The issue of what constitutes a planet will likely remain contentious at least until 2009, when the IAU holds its next Congress in [[Rio de Janeiro]].<ref>
{{cite web| year=2006 | title= IAU Colloquia and Symposia sponsored by Division VIII | url=http://www.star.bris.ac.uk/iau/meetings/colloquia_symposia.html | accessdate=2006-10-04}}</ref> Even with this controversy excluded however, there remain several ambiguities in the definition.
=== Clearing the neighborhood ===
{{main|Cleared the neighbourhood}}
One of the main points at issue is the precise meaning of "cleared the neighbourhood around its [[orbit]]". [[Alan Stern]] recently objected that "it is impossible and contrived to put a dividing line between dwarf planets and planets,"<ref>{{cite web|author=Paul Rincon|url=http://news.bbc.co.uk/2/hi/science/nature/5283956.stm| title=Pluto vote 'hijacked' in revolt|work=BBC News|year=2006|accessdate=2007-02-28}}</ref> and that since neither Earth, Mars, Jupiter, nor Neptune have entirely cleared their regions of debris, none could properly be considered planets under the [[International Astronomical Union|IAU]] definition.{{Ref_label|C|c|none}}
[[Image:InnerSolarSystem-en.png|thumb|The asteroids of the inner Solar System; note the [[Trojan asteroid]]s (green), trapped into Jupiter's orbit by its gravity]]
Mike Brown counters these claims by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Jupiter may coexist with a large number of small bodies in its orbit (the [[Trojan asteroid]]s), but these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called [[plutino]]s, into a 3:2 resonance, i.e., they orbit the Sun twice for every three Neptune orbits. The orbits of these objects are entirely dictated by Neptune's gravity, and thus, Neptune is gravitationally dominant.<ref name=Brown />
Whatever definition of "clearing the neighborhood" is ultimately accepted by the IAU, it is still an ambiguous concept. Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona and organiser of the petition, explained the ambiguity to [[National Public Radio]]. Since the definition does not categorise a planet by composition or formation, but, effectively, by its location, a Mars-sized or larger object beyond the orbit of Pluto would be considered a dwarf planet, since it would not have time to clear its orbit and would therefore be surrounded by objects of similar mass, whereas an object smaller than Pluto orbiting in isolation would be considered a planet.<ref>{{cite web| url=http://www.npr.org/templates/story/story.php?storyId=5788798| format=RealPlayer| title= Astronomers Prepare to Fight Pluto Demotion| first=Sykes| last=Mark| date=2006-09-08| accessdate=2006-10-04}} </ref>
[[Image:Proteus (Voyager 2).jpg|thumb|left|[[Proteus (moon)|Proteus]], the moon of [[Neptune]], is irregular, despite being larger than many spherical objects.]]
=== Hydrostatic equilibrium ===
The [[International Astronomical Union|IAU's]] definition mandates that planets be large enough for their own [[gravity]] to form them into a state of [[hydrostatic equilibrium]]; this means that they will reach a shape that is, if not spherical, then [[spheroid]]al. Up to a certain mass, an object can be irregular in shape, but beyond that point gravity begins to pull an object towards its own [[centre of mass]] until the object collapses into a sphere. Relaxing the demand for strict sphericity was mandated by the fact that many large objects in the [[Solar System]], such as the planets Jupiter and Saturn, the moons [[Mimas (moon)|Mimas]], [[Enceladus (moon)|Enceladus]] and [[Miranda (moon)|Miranda]], and the [[Kuiper belt object]] [[2003 EL61]],<ref> {{cite web| author= Brown, Michael E.| title=2003EL61| work= California Institute of Technology| url=http://www.gps.caltech.edu/~mbrown/2003EL61| accessdate=2006-05-25}} </ref> have been distorted into oblate or prolate spheroids by rapid rotation or [[tidal force]]s.
However, there is no one point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his article,"How are we to quantify the degree of roundness that distinguishes a planet? Does gravity dominate such a body if its shape deviates from a spheroid by 10 percent or by 1 percent? Nature provides no unoccupied gap between round and nonround shapes, so any boundary would be an arbitrary choice."<ref name=Soter /> Furthermore, the point at which an object's mass compresses it into a sphere varies depending on the chemical makeup of the object. Objects made of ices,{{Ref_label|D|d|none}} such as Enceladus and Miranda, assume that state more easily than those made of rock, such as Vesta and Pallas.<ref>{{cite web|title=The Dwarf Planets|author=Mike Brown|url=http://www.gps.caltech.edu/~mbrown/dwarfplanets/|accessdate=2007-08-04}}</ref> Heat energy, from [[gravitational collapse]], [[impact event|impacts]], tidal forces, or [[radioactive decay]] also factors into whether an object will be spherical or not; Saturn's icy moon Mimas is spheroidal, but Neptune's larger moon Proteus, which is similarly composed but colder because of its greater distance from the Sun, is irregular.
=== Double planets and moons===
{{main|Double planet}}
[[Image:pluto and charon.jpg|thumb|A [[telescope|telescopic]] image of [[Pluto]] and [[Charon (moon)|Charon]].]]
The definition specifically excludes [[natural satellite|satellites]] from the category of dwarf planet, though it does not directly define the term "satellite". In the original draft proposal, an exception was made for [[Pluto]] and its largest satellite, [[Charon (moon)|Charon]], which possess a [[barycenter]] outside the volume of either body. The initial proposal classified Pluto/Charon as a double planet, with the two objects orbiting the Sun in tandem. However, the final draft made clear that, even though they are similar in relative size, only Pluto would currently be classified as a dwarf planet.<ref name=IAU0603/>
Under the same definition, the Earth-[[Moon]] system is not formally recognised as a double planet, despite the Moon's large relative size, since the barycenter lies within the Earth. As the Moon is slowly [[Orbit of the Moon#Tidal evolution of the lunar orbit|receding from the Earth]], the Earth-Moon system may eventually become a double planet system on the basis of this barycentric definition.
[[Image:Moon trajectory1.png|thumb|200px|A diagram illustrating the [[Moon]]'s co-orbit with the [[Earth]].]]
However, some have suggested that our Moon nonetheless deserves to be called a planet. In 1975, [[Isaac Asimov]] noted that the timing of the Moon's orbit is in tandem with the Earth's own orbit around the Sun — looking down on the [[ecliptic]], the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right.<ref>Asimov, Isaac (1975). ''Just Mooning Around'', In: Of time and space, and other things. Avon.</ref>
Also many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. Jupiter's moon [[Ganymede (moon)|Ganymede]] and Saturn's moon [[Titan (moon)|Titan]] are both larger in terms of diameter (though not mass) than [[Mercury (planet)|Mercury]], and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as [[Io (moon)|Io]] and [[Triton (moon)|Triton]] demonstrate obvious and ongoing geological activity, and Ganymede has a [[magnetic field]]. Just as [[star]]s in orbit around other stars are still referred to as stars, so some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.<ref name=Buie2005>{{cite web
|date=March 2005
|title=Definition of a Planet
|publisher=Southwest Research Insitute
|author=[[Marc W. Buie]]
|url=http://www.boulder.swri.edu/~buie/pluto/planetdefn.html
|accessdate=2008-07-07}}</ref><ref name=stern2008>{{cite web
|date=June 15, 2008
|title=IAU Snobbery
|publisher=NASA Watch (not a NASA Website)
|url=http://www.nasawatch.com/archives/2008/06/iau_snobbery.html
|accessdate=2008-07-05}}</ref><ref>{{cite book|author=Serge Brunier|title=Solar System Voyage|publisher=Cambridge University Press|year=2000|pages=160–165}}</ref> Indeed Mike Brown makes just such a claim in his dissection of the issue, noting that there is little case for describing an object 400 km across with little internal geological activity as a planet if a 5000 km object with methane lakes, [[cryovolcanism]] and storms (i.e. [[Titan (moon)|Titan]]) is called a moon.<ref name=Brown />
=== Extrasolar planets and brown dwarfs ===
{{main|Extrasolar planet|Brown dwarf}}
The [[International Astronomical Union|IAU's]] definition of planet applies only to objects within our own [[Solar System]].<ref>{{cite web| year=2006 | title=Why Planets Will Never Be Defined| url=http://www.space.com/aol/061121_exoplanet_definition.html
| accessdate=2008-02-13}}</ref> The more than 200 extrasolar planets (planet-sized objects in [[orbit]] around other [[star]]s) were excluded as too complex an issue to be resolved during the congress. However, any future definition will need to include them, as their discovery has widened the debate on the nature of planethood in unexpected ways. Many of these planets are of considerable size, approaching the mass of small stars, while many newly-discovered brown dwarfs are conversely small enough to be considered planets.<ref>{{cite web| year=2006 | title=IAU General Assembly: Definition of Planet debate| url=http://astro2006.meta.mediastream.cz/Astro2006-060822-01.asx
| accessdate=2006-09-24}} </ref>
[[Image:Brown Dwarf Gliese 229B.jpg|thumb|left|The brown dwarf [[Gliese 229B]] in orbit around its star.]]
Traditionally, the defining characteristic for starhood has been an object's ability to [[Nuclear fusion|fuse]] [[hydrogen]] in its core. However, stars such as brown dwarfs have always challenged that distinction. Too small to commence sustained hydrogen fusion, they have been granted star status on their ability to fuse [[deuterium]]. However, due to the relative rarity of that [[isotope]], this process lasts only a tiny fraction of the star's lifetime, and hence most brown dwarfs would have ceased fusion long before their discovery.<ref>{{citation | 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> [[Binary star]]s and other multiple-star formations are common, and many brown dwarfs orbit other stars. Therefore, since they do not produce energy through fusion, they could be described as planets. Indeed, astronomer [[Adam Burrows (astronomer)|Adam Burrows]] of the [[University of Arizona]] claims that "from the theoretical perspective, however different their modes of formation, extrasolar giant planets and brown dwarfs are essentially the same."<ref>{{cite web | author=Burrows, Adam, Hubbard, W.B., Lunine, J., Leibert, James | year=2001 | title=The Theory of Brown Dwarfs and Extrasolar Giant Planets| work=Department of Astronomy and Steward Observatory, and Lunar and Planetary Laboratory, The University of Arizona| url=http://zenith.as.arizona.edu/~burrows/papers/rmp/RMP-final.pdf
| accessdate=2006-06-09}} </ref> Burrows also claims that such stellar remnants as [[white dwarfs]] should not be considered stars,<ref>Croswell p. 119</ref> a stance which would mean that an orbiting [[white dwarf]], such as [[Sirius B]] could be considered a planet. However, the current convention among astronomers is that any object massive enough to have possessed the capability to fuse during its lifetime should be considered a star.<ref> {{cite book | author=Croswell, Ken | year=1999 | title=Planet Quest: The Epic Discovery of Alien Solar Systems| publisher= Oxford University Press p. 119 (ISBN 0-19-288083-7)}}</ref>
The confusion does not end with brown dwarfs. Maria Rosa Zapatario-Osorio et al. have discovered many objects in young [[star cluster]]s of masses below that required to sustain fusion of any sort (currently calculated to be roughly 13 Jupiter masses).<ref> {{cite web |year= 2000| author= Zapatero M. R. Osorio, V. J. S. Béjar, E. L. Martín, R. Rebolo, D. Barrado y Navascués, C. A. L. Bailer-Jones, R. Mundt | title=Discovery of Young, Isolated Planetary Mass Objects in the Sigma Orionis Star Cluster| work= Division of Geological and Planetary Sciences, California Institute of Technology| url=http://www.sciencemag.org/cgi/content/short/290/5489/103 | accessdate=2006-05-25}} </ref> These have been described as "[[interstellar planet|free floating planets]]" because current theories of Solar System formation suggest that planets may be ejected from Solar Systems altogether if their orbits become unstable.<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=249–265| year=1987| doi=10.1016/0019-1035(87)90104-7}}</ref>
[[Image:Sol Cha-110913-773444 Jupiter.jpg|thumb|The solitary [[sub-brown dwarf]] [[Cha 110913-773444]] (middle), the least massive [[brown dwarf]] yet found, set to scale against the Sun (left) and the planet [[Jupiter]] (right).]]
However, it is also possible that these "free floating planets" could have formed in the same manner as stars.<ref>{{cite web |year= 2000| title=Rogue planet find makes astronomers ponder theory| work=Reuters| url=http://archives.cnn.com/2000/TECH/space/10/06/space.planets.reut/| accessdate=2006-05-25}} </ref> The material difference between a low-mass star and a large [[gas giant]] is not clearcut; apart from size and relative temperature, there is little to separate a gas giant like Jupiter from its host star. Both have similar overall compositions: hydrogen and [[helium]], with trace levels of heavier [[chemical element|elements]] in their [[atmosphere]]s. The generally accepted difference is one of formation; stars are said to have formed from the "top down"; out of the gases in a nebula as they underwent gravitational collapse, and thus would be composed almost entirely of hydrogen and helium, while planets are said to have formed from the "bottom up"; from the accretion of dust and gas in orbit around the young star, and thus should have cores of [[silicate]]s or ices.<ref> {{cite web |year= 2004| author=G. Wuchterl| title=Giant planet formation| work=Institut für Astronomie der Universität Wien| url=http://www.springerlink.com/content/j6357423q7324013/| accessdate=2006-10-04}} </ref> As yet it is uncertain whether gas giants possess such cores. If it is indeed possible that a gas giant could form as a star does, then it raises the question of whether such an object, even one as familiar as Jupiter or Saturn, should be considered an orbiting low-mass star rather than a planet.
In 2003, the IAU officially released a statement<ref>{{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> to define what constitutes an extrasolar planet and what constitutes an orbiting star. To date, it remains the only official decision reached by the IAU on this issue.
{{cquote|
#Objects with [[true mass]]es below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/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 dwarfs", no matter how they formed nor where they are located.
#Free-floating objects in young star clusters 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).}}
[[Image:hubbledwarf.jpg|left|thumb|CHXR 73 b, a star which lies at the border between planet and brown dwarf]]
Like defining a planet by having [[clearing the neighbourhood|cleared its neighbourhood]], this definition creates ambiguity by making location, rather than formation or composition, the determining characteristic for planethood. A free-floating object with a mass below 13 Jupiter masses is a "sub-brown dwarf," whereas such an object in orbit round a fusing star is a planet, even if, in all other respects, the two objects may be identical.
This ambiguity was highlighted in December 2005, when the [[Spitzer Space Telescope]] observed [[Cha 110913-773444]] (above), the least massive brown dwarf yet found, only eight times Jupiter's mass with what appears to be the beginnings of its own [[star system]]. Were this object found in orbit round another star, it would have been termed a planet.<ref>{{cite web |year= 2005| author= Clavin, Whitney| title=A Planet With Planets? Spitzer Finds Cosmic Oddball| work=Spitzer Science Center| url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html
| accessdate=2006-05-25}}</ref>
It was highlighted again in September 2006, when the [[Hubble Space Telescope]] imaged [[CHXR 73 b]] (left), an object orbiting a young companion star at a distance of roughly 200 AU. At 12 Jovian masses, CHXR 73 b is just under the threshold for deuterium fusion, and thus technically a planet; however, its vast distance from its parent star suggests it could not have formed inside the small star's [[protoplanetary disc]], and therefore must have formed, as stars do, from gravitational collapse.<ref>{{cite web|title=Planet or failed star? Hubble photographs one of the smallest stellar companions ever seen|work=ESA Hubble page|url=http://www.spacetelescope.org/news/html/heic0610.html|year=2006|accessdate=2007-02-23}}</ref>
===Criteria for determining "plutoids"===
The current criterion established by the IAU for classifying an object as a "plutoid" demands that its absolute magnitude be higher than H= +1. What that effectively means is that any new plutoids will be determined not by size, but by their brightness. As Mike Brown noted in his blog, brightness is not an absolute indicator that an object has achieved hydrostatic equilibrium:
:''If you take Pluto and cover it with dirt it would no longer be a Plutoid. Or take something much smaller and cover it was snow instead of rocks and it might be a Plutoid. Or, my favorite example, if you take Eris, which is currently the intrinsically brightest object, bring it closer to the sun (where it will be in 290 years), melt some of the ice on the surface, and exposure some of the darker substrate, it might just get dark enough to no longer be a Plutoid. Now you see it; now you don’t.''<ref>{{cite web|title=Plutoid fever|work=Mike Brown's Planets|author=Mike Brown|year=2008|url=http://www.mikebrownsplanets.com/2008/06/plutoid-fever.html|accessdate=2008-06-13}}</ref>
=== Semantics ===
Finally, from a purely linguistic point of view, there is the dichotomy that the IAU created between 'planet' and 'dwarf planet'. The term 'dwarf planet' arguably contains two words, a noun (planet) and an adjective (dwarf). Thus, the term could suggest that a dwarf planet is a type of planet, even though the IAU explicitly defines a dwarf planet as ''not'' so being. By this formulation therefore, 'dwarf planet' and 'minor planet' are best considered [[compound noun]]s. Benjamin Zimmer, of [http://www.languagelog.org languagelog.org], summarised the confusion: "The fact that the IAU would like us to think of dwarf planets as distinct from 'real' planets lumps the lexical item 'dwarf planet' in with such oddities as '[[Welsh rabbit]]' (not really a rabbit) and '[[Rocky Mountain oysters]]' (not really oysters)."<ref name=langlog>{{cite web|title=New planetary definition a "linguistic catastrophe"!| work=Language log| url=http://itre.cis.upenn.edu/~myl/languagelog/archives/003504.html| accessdate=2006-10-04}}</ref> As [[Dava Sobel]], the historian and popular science writer who participated in the IAU's initial decision in October 2006, noted in an interview with [[National Public Radio]], "A dwarf planet is not a planet, and in astronomy, there are dwarf stars, which are stars, and dwarf galaxies, which are galaxies, so it's a term no one can love, dwarf planet."<ref>{{cite web|title=A Travel Guide to the Solar System|year=2006|work=National Public Radio|url=http://www.sciencefriday.com/pages/2006/Oct/hour2_102706.html|accessdate=2006-11-18}}</ref> Mike Brown noted in an interview with the Smithsonian that, "Most of the people in the dynamical camp really did not want the word "dwarf planet," but that was forced through by the pro-Pluto camp. So you’re left with this ridiculous baggage of dwarf planets not being planets."<ref name=smithsonian>{{cite web|title=Pluto's Planethood: What Now?|work=Smithsonian Air and Space|url=http://www.airspacemag.com/issues/2006/august-september/FEATURE-PlutoDebate.php?page=1|year=2006|accessdate=2007-08-21}}</ref>
Conversely, astronomer Robert Cumming of the Stockholm Observatory notes that, "The name 'minor planet' been more or less synonymous with 'asteroid' for a very long time. So it seems to me pretty insane to complain about any ambiguity or risk for confusion with the introduction of 'dwarf planet'."<ref name=langlog />
== Notes ==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}Olbers proposed that these new discoveries were the [[Fifth planet#disruption theory|fragments of a planet]], later dubbed [[Phaeton (planet)|Phaeton]], that had formerly revolved around the sun but had been destroyed by impact with a comet.<ref name=police /> As more "pieces" continued to be found, Olbers's new hypothesis continued to gain popularity.<ref>{{cite web|title=A Brief History of Asteroid Spotting|work=Open2.net|url=http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/history.html|accessdate=2007-05-15}}</ref> Though a few [[fringe science|fringe]] groups maintain to this day that Olbers' theory was correct,<ref>{{cite web|title=Olbers' planet: the history continues indeed|auhtor=AV Bagrov|work=Istoriko-Astronomicheskie Issledovaniya|url=http://adsabs.harvard.edu/abs/2003IAIss..28...72B|year=2003|accessdate=2007-05-16}}</ref> it has for the most part been discarded by the scientific community, [[Superseded scientific theories|superseded]] by the [[accretion disc|accretion model]], which holds that the asteroid belt is a remnant of the Sun's [[protoplanetary disc]] which failed to coalesce into a planet due to the gravitational interference of Jupiter.<ref>{{cite journal | author=Petit, J.-M.; Morbidelli, A.; Chambers, J.| title=The Primordial Excitation and Clearing of the Asteroid Belt | journal=Icarus| year=2001| volume=153| pages=338–347| url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf| format=PDF| accessdate=2007-03-22 | doi=10.1006/icar.2001.6702}}</ref></li>
<li>{{Note_label|B|b|none}} Defined as the region occupied by two bodies whose orbits cross a common distance from the Sun, if their orbital periods differ less than an order of magnitude. In other words, if two bodies occupy the same distance from the Sun at one point in their orbits, and those orbits are of similar size, rather than, as a [[comet|comet's]] would be, extending for several times the other's distance, then they are in the same orbital zone.<ref>{{cite web| url=http://arxiv.org/abs/astro-ph/0608359| title= What is a Planet?| first=Steven| last=Soter| date=2006-08-16| accessdate=2006-08-24}} submitted to The Astronomical Journal, [[16 August]] [[2006]]</ref></li>
<li>{{Note_label|C|c|none}}In 2002, in collaboration with dynamicist Harold Levison, Stern wrote, "we define an ''überplanet'' as a planetary body in orbit around a [[star]] that is dynamically important enough to have cleared its neighboring planetesimals ... And we define an ''unterplanet'' as one that has not been able to do so," and then a few paragraphs later, "our [[Solar System]] clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are [[Pluto]] and [[Ceres (planet)|Ceres]]."<ref name="Stern 2002">{{cite journal | last=Stern | first=S. Alan | coauthors=and Levison, Harold F. | year=2002 | title=Regarding the criteria for planethood and proposed planetary classification schemes | url=http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf | format=[[PDF]] | journal=Highlights of Astronomy| volume=12 | pages=205–213, as presented at the XXIVth General Assembly of the IAU–2000 [Manchester, UK, 7–18 August 2000]}}</ref> While this may appear to contradict Stern's objections, Stern noted in an interview with Smithsonian Air and Space that, unlike the IAU's definition, his definition still allows unterplanets to be planets: "I do think from a dynamical standpoint, there are planets that really matter in the architecture of the solar system, and those that don’t. They’re both planets. Just as you can have wet and dry planets, or life-bearing and non-life-bearing planets, you can have dynamically important planets and dynamically unimportant planets."<ref name=smithsonian /></li>
<li>{{Note_label|D|d|none}}The density of an object is a rough guide to its composition: the lower the density, the higher the fraction of ices, and the lower the fraction of rock. The n denser objects, Vesta and Juno, are composed almost entirely of rock with very little ice, and have a density close to the [[Moon|Moon's]], while the less dense, such as Proteus and Enceladus, are composed mainly of ice.<ref> {{cite web | year=1997 | author= Righter, Kevin; Drake, Michael J. | title=A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites| work= METIC| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997M%26PS...32..929R&db_key=AST&data_type=HTML&format=&high=4374b9c9ce01530| accessdate=2006-05-25}} </ref><ref> {{cite web | year=2003 | author= Johanna Torppa, Mikko Kaasalainen, Tadeusz Michałowski, Tomasz Kwiatkowski, Agnieszka Kryszczyńska, Peter Denchev, and Richard Kowalski | title=Shapes and rotational properties of thirty asteroids from photometric data| work= Astronomical Observatory, Adam Mickiewicz University,| url=http://www.rni.helsinki.fi/~mjk/thirty.pdf | accessdate=2006-05-25}}</ref></li></ol></div>
== See also ==
{{Wiktionary|Planet}}
*[[Timeline of discovery of Solar System planets and their natural satellites]]
*[[Mesoplanet]]
*[[Natural kind]]
*[[Planemo]]
*[[planetar (astronomy)|Planetar]]
*[[Planetesimal]]
*[[Planets in astrology]]
*[[Rogue planet]]
== References ==
{{reflist|2}}
== Bibliography and external links ==
<div class="references-small">
* [http://uk.youtube.com/watch?v=FqX2YdnwtRc Why Isn't Pluto a Planet Any More?] The planet debate short and sweet.
* [http://www.sciam.com/article.cfm?chanID=sa006&articleID=93385350-E7F2-99DF-3FD6272BB4959038&pageNumber=2&catID=2 What is a planet?] -Steven Soter
* [http://www.space.com/scienceastronomy/061121_exoplanet_definition.html Why Planets will never be defined: Robert Roy Britt on the outcome of the IAU's decision]
* {{cite web | last = Nunberg | first = G. | title = Dwarfing Pluto | publisher = NPR | url = http://www.npr.org/templates/story/story.php?storyId=5723794 | date = 2006-08-28 | lastaccess = 2007-04-13}} An examination of the redefinition of Pluto from a linguistic perspective.
*[http://news.bbc.co.uk/1/hi/sci/tech/4798205.stm Q&A New planets proposal] Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
*[http://www.ifa.hawaii.edu/faculty/jewitt/kb/pluto.html David Jewitt's Kuiper Belt page- Pluto]
*[http://www.cfa.harvard.edu/icq/ICQPluto.html Dan Green's webpage: What is a planet?]
*[http://www.space.com/scienceastronomy/solarsystem/planet_confusion_001101-2.html What is a Planet? Debate Forces New Definition]
* [http://www.infoplease.com/ipa/A0776605.html?mail-07-29 The Flap Over Pluto]
* [http://slate.msn.com/id/2123839 "You Call That a Planet?: How astronomers decide whether a celestial body measures up."]
* David Darling. ''The Universal Book of Astronomy, from the Andromeda Galaxy to the Zone of Avoidance.'' 2003. John Wiley & Sons Canada (ISBN 0-471-26569-1), p. 394
* ''Collins Dictionary of Astronomy'', 2nd ed. 2000. HarperCollins Publishers (ISBN 0-00-710297-6), p. 312-4.
*[http://xxx.lanl.gov/abs/astro-ph/0607184 Catalogue of Planetary Objects. Version 2006.0] O.V. Zakhozhay, V.A. Zakhozhay, Yu.N. Krugly, 2006
*[http://kuffner-sternwarte.at/im_brennp/archiv2006/Planeten_Definition_Resolution_5_6_7.html The New Proposal, Resolution 5, 6 and 7] 2006-08-22
* [http://www.astronomy2006.com/media-stream-archive.php IAU 2006 General Assembly: video-records of the discussion and of the final vote on the Planet definition.]
</div>
[[Category:Planetary science]]
[[Category:Planets of the Solar System]]
[[Category:Planets]]
[[Category:History of astronomy]]
[[et:Planeedi definitsioon]]
[[es:Definición de planeta]]
[[ko:행성의 정의]]
[[it:Definizione di pianeta]]
[[mk:Дефиниција на планета]]
[[pt:Definição de planeta]]
[[fi:Planeetan määritelmä]]
[[sv:Definition av planet]]
[[zh:行星定義]]