Editing Definition of planet (section)

== History ==
{{see also|Geocentric model|Heliocentrism|Celestial spheres|Classical planet}}

=== Planets in antiquity ===
[[File:Apparent retrograde motion of Mars in 2003.gif|thumb|The motion of 'lights' moving across the background of stars is the basis of the classical definition of planets: wandering stars.]]
While knowledge of the planets predates history and is common to most civilizations, the word ''planet'' dates back to [[ancient Greece]]. Most Greeks believed the Earth to be stationary and at the center of the universe in accordance with the [[geocentric model]] and that the objects in the sky, and indeed the sky itself, revolved around it (an exception was [[Aristarchus of Samos]], who put forward an early version of [[heliocentrism]]). Greek astronomers employed the term {{lang|grc|ἀστέρες πλανῆται}} ({{Transliteration|grc|asteres planetai}}), 'wandering stars',<ref>{{cite web|url=http://www.m-w.com/dictionary/planet|title=Definition of planet|publisher=Merriam-Webster OnLine|access-date=July 23, 2007}}</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|access-date=July 23, 2007}}</ref> to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the {{lang|grc|ἀστέρες ἀπλανεῖς}} ({{Transliteration|grc|asteres aplaneis}}), the '[[fixed stars]]', which stayed motionless relative to one another. The five bodies currently called "planets" that were known to the Greeks were those visible to the naked eye (from [[apparent magnitude|brightest to dimmest]]): [[Venus]], [[Jupiter]], [[Mercury (planet)|Mercury]], [[Mars]], and [[Saturn]]. 
Graeco-Roman [[Timeline of cosmology|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 star-like planets from the Sun and Moon. As the 19th-century German naturalist [[Alexander von Humboldt]] noted in his work ''[[Cosmos (Humboldt)|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 book |title=Cosmos: A Sketch of a Physical Description of the Universe |author=Alexander von Humboldt |year=1849 |work=digitised 2006 |url=https://archive.org/details/cosmosasketchap00humbgoog |page=[https://archive.org/details/cosmosasketchap00humbgoog/page/n337 297] |access-date=2007-07-23 |publisher=H.G. Bohn |isbn=978-0-8018-5503-0}}</ref></blockquote>
In his ''[[Timaeus (dialogue)|Timaeus]]'', written in roughly 360&nbsp;[[BCE]], [[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|access-date= February 22, 2007}}</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, volume II|work=University of Adelaide Library|url=https://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html|year=2004|access-date=February 24, 2007|archive-date=August 23, 2008|archive-url=https://web.archive.org/web/20080823061709/http://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html|url-status=dead}}</ref> In his ''Phaenomena'', which set to verse an astronomical treatise written by the philosopher [[Eudoxus of Cnidus|Eudoxus]] in roughly 350&nbsp;BCE,<ref>{{cite web|title=Phaenomena Book I&nbsp;— ARATUS of SOLI|url=http://www.geocities.com/astrologysources/classicalgreece/phaenomena/index.htm|access-date=June 16, 2007|archive-url = https://web.archive.org/web/20050901044449/http://www.geocities.com/astrologysources/classicalgreece/phaenomena/index.htm |archive-date = September 1, 2005|url-status=dead}}</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|author=Aratus |title=Phaemonema|translator=A. W. & G. R. Mair |work=theoi.com|url=http://www.theoi.com/Text/AratusPhaenomena.html|access-date=June 16, 2007}}</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 |author=Ptolemy|translator=R. Gatesby Taliaterro|publisher= University of Chicago Press|year= 1952|page=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|access-date=February 25, 2007}}</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=Marcus Manilius |translator=G. P. Goold |title=Astronomica|publisher=Harvard University Press|year=1977|page=141}}</ref>

The single view of the seven planets is found in [[Cicero]]'s ''[[Dream of Scipio]]'', written sometime around 53&nbsp;BCE, 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=The Dream of Scipio|author=Cicero|work=Roman Philosophy |translator=Richard Hooker|url=http://www.wsu.edu/~dee/ROME/SCIPIO.HTM|year=1996|access-date=June 16, 2007|archive-url=https://web.archive.org/web/20070703203835/http://www.wsu.edu/~dee/ROME/SCIPIO.HTM |archive-date=July 3, 2007}}</ref> In his ''[[Natural History (Pliny)|Natural History]]'', written in 77&nbsp;CE, [[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|page= 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 ===
[[File:Copernican heliocentrism diagram-2.jpg|thumb|Sketch of the heliocentric model of the Solar System by Copernicus]]
Medieval and Renaissance writers generally accepted the idea of seven planets. The standard medieval introduction to astronomy, [[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|doi=10.1080/0015587x.1943.9717687|volume=54|issue=3}}</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|access-date=February 28, 2007}}</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=Nicolaus Copernicus|work=The Harvard Classics. 1909–14|url=http://www.bartleby.com/39/12.html|access-date=February 23, 2007}}</ref>

=== Earth ===
When Copernicus's [[heliocentric model]] was accepted over the [[geocentric]], Earth was placed among the planets and the Sun and Moon were reclassified, 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>

Copernicus obliquely refers to Earth as a planet in ''De Revolutionibus'' when he says, "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..."<ref name=koper/> [[Galileo Galilei|Galileo]] also 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|access-date=June 14, 2008}}</ref>

=== Modern planets ===
[[File:Uranuslauf November Dezember2022.gif|thumb|Motion of Uranus across the sky as seen by two images taken 25 days apart]]
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. As the asteroids had yet to be discovered, comets were the only moving objects one expected to find in a telescope.<ref name=ken/> 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 (cometary)|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|page=8|isbn=978-0-907322-06-1}}</ref> The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognized 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 a search which did not find the perturbing object (it was later found to be a mathematical artifact caused by an overestimation of Neptune's mass) but did find [[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 name=ken>
{{cite book | author=Croswell, Ken | year=1999 | title=Planet Quest: The Epic Discovery of Alien Solar Systems| publisher= Oxford University Press|pages=48, 66 |isbn=978-0-19-288083-3}}</ref> In 1989, [[Voyager 2]] determined the irregularities to be due to an overestimation of Neptune's mass.<ref>{{cite web|title=Hopes Fade in hunt for Planet X|author=Ken Croswell|year=1993|url=http://kencroswell.com/HopesFadeInHuntForPlanetX.html |access-date=November 4, 2007}}</ref>

=== Satellites ===
{{see also|Satellite planet}}
[[File:Jupiter and the Galilean moons animation.gif|thumb|Motion of the Galilean moons seen from a telescope]]
When Copernicus placed Earth among the planets, he also placed the Moon in orbit around Earth, making the Moon the first [[natural satellite]] to be identified. When [[Galileo Galilei|Galileo]] discovered his four [[Galilean moons|satellites]] of Jupiter in 1610, they lent weight to Copernicus's argument, because if other planets could have satellites, then Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness."<ref>{{cite book|title=Siderius Nuncius|author=Galileo Galilei|publisher=University of Chicago Press|year=1989|page=26|others=Albert van Helden}}</ref> 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 "satellite" (attendant), a word coined by [[Johannes Kepler]].<ref>{{cite web|title=Johannes Kepler: His Life, His Laws and Times|date=September 24, 2016|publisher=NASA|url=https://www.nasa.gov/kepler/education/johannes|accessdate=September 22, 2022|archive-date=June 24, 2021|archive-url=https://web.archive.org/web/20210624003856/https://www.nasa.gov/kepler/education/johannes/|url-status=dead}}</ref><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 ([[Dione (moon)|Dione]] and [[Tethys (moon)|Tethys]]) in 1686, it referred to them strictly as "satellites", though sometimes Saturn as the "primary planet".<ref name="Cassini1686–1692">{{cite journal| doi = 10.1098/rstl.1686.0013| last = Cassini | first = G. D.| author-link = Giovanni Domenico Cassini| year = 1686–1692| 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| journal = Philosophical Transactions of the Royal Society of London| volume = 16| issue = 179–191| pages = 79–85| jstor = 101844| bibcode = 1686RSPT...16...79C| doi-access = free}}<!-- This journal became the Philosophical Transactions of the Royal Society of London in 1775. Are there any earlier publications? Two years seems a long time... --></ref> When William Herschel announced his discovery of two objects in orbit around Uranus in 1787 ([[Titania (moon)|Titania]] and [[Oberon (moon)|Oberon]]), 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" referred to satellites as "secondary planets".<ref name="smith">{{cite book
 |first = Asa | last = Smith
 |year=1868
 |title=Smith's Illustrated Astronomy
 |publisher=Nichols & Hall
 |url=https://archive.org/details/smithsillustrat00smitgoog |page = [https://archive.org/details/smithsillustrat00smitgoog/page/n27 23] |quote = secondary planet Herschel. }}</ref>

=== Minor planets ===
[[File:AnimatedOrbitOf1Ceres.gif|thumb|Orbit of Ceres, which fits with the description of a seemingly "missing" planet between Mars and Jupiter, as predicted by [[Bode's law]].]]
One of the unexpected results of [[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, [[Ceres (dwarf planet)|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://sd-www.jhuapl.edu/weaver_projects/GPD/Contributed_Talks/hilton_gpd_poster.pdf | access-date=May 25, 2006}}</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. The fact that two planets could occupy the same orbit was an affront to centuries of thinking.<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> 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.

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
  |access-date=June 24, 2008 }}</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, 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.minorplanetcenter.org/iau/MPCORB.html|access-date=October 15, 2007}}</ref>

=== Pluto ===
{{main|Planets beyond Neptune}}
The long road from planethood to reconsideration undergone by [[Ceres (dwarf planet)|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 and 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 Pluto's largest moon, [[Charon (moon)|Charon]], which allowed them to determine its mass. Pluto was found to be much tinier than anyone had expected: only one-sixth the mass of Earth's Moon.  However, as far as anyone could yet tell, it was unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar to Pluto in composition, size, and orbital characteristics. They concluded that they had discovered the hypothesized [[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 journal | author=Weissman, Paul R. | title=The Kuiper Belt|  bibcode=1995ARA&A..33..327W|doi = 10.1146/annurev.aa.33.090195.001551 | year=1995 | journal=Annual Review of Astronomy and Astrophysics | volume=33 | pages=327–357 }}</ref>

Pluto's orbit lays within this band and thus its planetary status was thrown into question.  Many scientists concluded that tiny Pluto should be reclassified as a minor planet, just as Ceres had been a century earlier. [[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 | access-date=May 25, 2006 | url-status=dead | archive-url=https://web.archive.org/web/20060427091759/http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=105 | archive-date=April 27, 2006 }}</ref> Those objects under that mass limit would become minor planets. In 1999, [[Brian G. 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=Central Bureau for Astronomical Telegrams
  |url=http://www.icq.eps.harvard.edu/ICQPluto.html
  |access-date=July 3, 2011}}</ref><ref>{{cite news|title=Xena becomes Eris&nbsp;– Pluto reduced to a number|author=Kenneth Chang|work= New York Times|url=http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2006/09/15/MNGS8L67LJ1.DTL|access-date=June 18, 2008|date=September 15, 2006}}</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.bio.aps.anl.gov/~dgore/fun/PSL/Pluto19990203.txt |access-date=May 25, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060923202926/http://www.bio.aps.anl.gov/~dgore/fun/PSL/Pluto19990203.txt |archive-date=September 23, 2006 }} [http://www.bio.aps.anl.gov/ Copy kept] {{Webarchive|url=https://web.archive.org/web/20081005210115/http://www.bio.aps.anl.gov/ |date=October 5, 2008 }} at the [[Argonne National Laboratory]].</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 |access-date=May 25, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20061016080949/http://www.asterism.org/newsletter/l9904-3.htm |archive-date=October 16, 2006 }}</ref>

The discovery of several other [[trans-Neptunian object]]s, 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 more massive 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/ | access-date=May 25, 2006}}</ref> named [[136199 Eris|Eris]].<ref>{{cite web | year=2005 |author1=M. E. Brown |author2=C. A. Trujillo |author3=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 | access-date=August 15, 2006}}</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.nasa.gov/vision/universe/solarsystem/newplanet-072905.html|year=2005|access-date=February 22, 2007|archive-date=March 19, 2015|archive-url=https://web.archive.org/web/20150319203306/http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905.html|url-status=dead}}</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&nbsp;— First Mission to Pluto and the Kuiper Belt; "From Darkness to Light: The Exploration of the Planet Pluto"|author= Bonnie Buratti|work=Jet Propulsion Laboratory|url=http://www.jpl.nasa.gov/events/lectures/nov05.cfm|year=2005|access-date=February 22, 2007}}</ref>