The final definition left the solar system with eight planets.

The 2006 definition of "planet" by the International Astronomical Union (IAU) states that, in the solar system, a planet is a celestial body that: Image File history File links Download high resolution version (1440x904, 1800 KB) Summary Other versions [4] Note:Only description has been changed from Image:Solar_sys. ... Image File history File links Download high resolution version (1440x904, 1800 KB) Summary Other versions [4] Note:Only description has been changed from Image:Solar_sys. ... IAU redirects here. ... This article is about the Solar System. ... This article is about the astronomical term. ... See lists of astronomical objects for a list of the various lists of astronomical objects in Wikipedia. ...

• is in orbit around the Sun,
• has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and
• has "cleared the neighbourhood" around its orbit.

A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet", whilst a non-satellite body fulfilling only the first criterion is termed a "small solar system body" (SSSB). The definition was a controversial one, and has drawn both support and criticism from different astronomers. Two bodies with a slight difference in mass orbiting around a common barycenter. ... Sol redirects here. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ... In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. ... For other uses, see Satellite (disambiguation). ... Artists impression of Pluto (background) and Charon (foreground). ... A Small Solar System Body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets: [1] This encompasses: all minor planets apart from the dwarf planets, : the classical asteroids, (except for 1 Ceres, the...

According to the definition, there are currently eight planets and three dwarf planets known in the solar system. The definition distinguishes planets from smaller bodies, and is not used outside the solar system, where smaller bodies cannot be found yet. Exoplanets are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger. This article is about the astronomical term. ... Artists impression of Pluto (background) and Charon (foreground). ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ...

==Reasons for the debate==????Heller how u doin

Plot of the positions of all known Kuiper belt objects (green), set against the outer planets (blue)

Before the discoveries of the early 21st century, astronomers hadn't really needed a formal definition for planets. With the discovery of Pluto in 1930, astronomers considered the solar system to have nine planets, along with thousands of smaller bodies such as asteroids and comets. Pluto was thought to be larger than Mercury. In 1978, the discovery of Pluto's moon Charon radically changed this picture. By measuring Charon's orbital period, astronomers could accurately calculate Pluto's mass for the first time, which they found to be much smaller than expected.^[1] Pluto's mass was roughly one twentieth of Mercury's, making it by far the smallest planet, smaller even than the Moon, although it was still over ten times as massive as the largest asteroid, Ceres. Then, in the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt Objects, or KBOs.^[2] Many of these shared some of Pluto's key orbital characteristics, and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, and some astronomers stopped referring to Pluto as a planet. Pluto's eccentric and inclined orbit, while very unusual for a planet, also fit in well with the other KBOs. New York City's newly renovated Hayden Planetarium famously did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth and Space in 2000.^[3] Then, starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna and Eris) all comparable to Pluto in terms of size and orbit, it became clear that they all would either have to be called planets or Pluto would have to be reclassified. Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other solar systems. In 2006, the matter came to a head with the measurement of the size of 2003 UB₃₁₃. Eris (as it is now known) turned out to be slightly larger than Pluto, and so was thought to be equally deserving of the status of 'planet'. Image File history File links Size of this preview: 611 × 599 pixelsFull resolution (2708 × 2656 pixel, file size: 314 KB, MIME type: image/png) Legend Red = The Sun Aquamarine = Giant Planet Green = Kuiper Belt Object Orange = Scattered Disk object or Centaur Pink = Trojan of Jupiter Yellow = Trojan of Neptune Axes... Image File history File links Size of this preview: 611 × 599 pixelsFull resolution (2708 × 2656 pixel, file size: 314 KB, MIME type: image/png) Legend Red = The Sun Aquamarine = Giant Planet Green = Kuiper Belt Object Orange = Scattered Disk object or Centaur Pink = Trojan of Jupiter Yellow = Trojan of Neptune Axes... The Kuiper belt (KYE per) is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun, at inclinations consistent with the ecliptic. ... For other uses, see Pluto (disambiguation). ... This article is about the Solar System. ... This article is about the astronomical term. ... This article is about the planet. ... Charon (shair-É™n or kair-É™n (key), IPA , Greek Χάρων), discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. ... This article is about Earths moon. ... For other uses, see Asteroid (disambiguation). ... Spectral type: G[8] Absolute magnitude: 3. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... In astronomy, a plutino is a Pluto-like object, insofar as it has the same relative orbit as Pluto. ... Hayden Planetarium is a public planetarium located on Central Park West, New York City, next to the famous American Museum of Natural History. ... The Rose Center for Earth and Space is a notable part of the American Museum of Natural History in New York City. ... 50000 Quaoar (pronounced kwaa·waar or kwow·ər, English IPA: , Tongva ) [2] is a Trans-Neptunian object orbiting the Sun in the Kuiper belt. ... you are abunch of bull | bgcolour=#FFFFC0 | name=90377 Sedna | image= | caption= Sedna is located in the center of the green circle | discovery=yes | discoverer=M. Brown, C. Trujillo, D. Rabinowitz | discovered=November 14, 2003 | mp_name=90377 Sedna | alt_names= | mp_category=Trans-Neptunian object | epoch=September 26, 1990 (JD 2448160. ... Absolute magnitude: −1. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ...

Historical precedent

The refining understanding of Pluto was echoed by a debate in the 19th century that began with the discovery of Ceres on January 1, 1801. Astronomers immediately declared the tiny object to be the missing planet between Mars and Jupiter, but within four years, the discovery of two more tiny planets in similar orbits had cast doubt on this new thinking. By 1851, the number of planets had grown to 23, and it was clear that hundreds more would eventually be discovered. Astronomers began cataloging them separately, and began calling them "asteroids" instead of "planets."^[4] Spectral type: G[8] Absolute magnitude: 3. ... Asteroids is a popular vector-based video arcade game released in 1979 by Atari. ...

The history of the definition

Because a new planet is discovered infrequently, the IAU did not have any machinery for their definition and naming. After the discovery of Sedna, it set up a 19-member committee with the British astronomer Iwan Williams in the chair, in 2005 to consider the definition of a planet. It proposed three definitions that could be adopted: cultural- a planet is a planet if enough people say it is; structural- a planet is large enough to form a sphere; dynamical- the object is large enough to cause all other objects to eventually leave its orbit.^[5] Another committee, chaired by a historian of astronomy, Owen Gingerich, a historian and astronomer emeritus at Harvard University who led the committee which generated the original definition, and consisting of five planetary scientists and the science writer Dava Sobel was set up to make a firm proposal.^[6] Owen Gingerich Dr. Owen Jay Gingerich (1930-) was Research Professor of Astronomy and of the History of Science at Harvard University and a senior astronomer emeritus at the Smithsonian Astrophysical Observatory. ... Harvard redirects here. ... Dava Sobel (born 1947[1]) is a writer of popular expositions of scientific topics. ...

Draft proposal

The original proposal would have immediately added three planets, shown here in a size comparison to Earth. Leftmost is Eris, then Charon, Ceres, and Earth

Illustration of the draft proposal.

The IAU published the original definition proposal on August 16, 2006. Its form followed loosely the second of three options proposed by the original committee. It stated: Image File history File linksMetadata Three_proposed_planets. ... Image File history File linksMetadata Three_proposed_planets. ... This article is about Earth as a planet. ... Image File history File links ThePlanetDefinition. ... Image File history File links ThePlanetDefinition. ... is the 228th day of the year (229th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...

This definition would have led to three celestial bodies being recognized as planets: Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ...

• Ceres, which had been considered a planet at the time of its discovery, but was subsequently treated as an asteroid
• Charon, a moon of Pluto; the Pluto-Charon system would have been considered a double planet
• Eris, a body in the scattered disk of the outer solar system

A further twelve bodies, pending refinements of knowledge regarding their physical properties, were possible candidates to join the list under this definition. Some objects in this second list were more likely eventually to be adopted as 'planets' than others. Despite what had been claimed in the media,^[7] the proposal did not necessarily leave the solar system with only twelve planets. Mike Brown, the discoverer of Sedna and Eris, has said that at least 53 known bodies in the solar system probably fit the definition, and that a complete survey would probably reveal more than 200.^[8] Spectral type: G[8] Absolute magnitude: 3. ... Charon (shair-ən or kair-ən (key), IPA , Greek Χάρων), discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. ... Pluto and Charon are sometimes informally considered to be a double (dwarf) planet. ... Absolute magnitude: −1. ... The scattered disc (or scattered disk) is a distant region of our solar system, thinly populated by icy planetoids known as scattered disk objects (SDOs), a subset of the broader family of trans-Neptunian objects (TNOs). ... Michael (Mike) E. Brown (born c. ... you are abunch of bull | bgcolour=#FFFFC0 | name=90377 Sedna | image= | caption= Sedna is located in the center of the green circle | discovery=yes | discoverer=M. Brown, C. Trujillo, D. Rabinowitz | discovered=November 14, 2003 | mp_name=90377 Sedna | alt_names= | mp_category=Trans-Neptunian object | epoch=September 26, 1990 (JD 2448160. ...

The definition would have considered a pair of objects to be a double planet system if each component independently satisfied the planetary criteria and the common center of gravity of the system (known as the barycenter) was located outside of both bodies.^[9] Pluto and Charon would have been the only known double planet in the solar system. Other planetary satellites (for example, Earth's moon) might be in hydrostatic equilibrium, but would still not have been defined as a double planet since the barycenter of the system lies within the more massive celestial body (that is, Earth). Pluto and Charon are sometimes informally considered to be a double (dwarf) planet. ... This article or section may contain original research or unverified claims. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it was concentrated. ... For other uses, see Pluto (disambiguation). ... This article is about Earth as a planet. ... This article is about Earths moon. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ...

The twelve "candidate planets" that were possibilities for inclusion under the originally proposed definition.

The term "minor planet" would have been abandoned, replaced by the categories "Small Solar System Body" (SSSB) and a new classification of "pluton". The former would have described those objects underneath the "spherical" threshold. The latter would have been applied to those planets with highly inclined orbits, large eccentricities and an orbital period of more than 200 earth years (that is, those orbiting beyond Neptune). Pluto would have been the prototype for this class. The term "dwarf planet" would have been available to describe all planets smaller than the eight "classical planets" in orbit around the Sun, though would not have been an official IAU classification.^[10] The IAU did not make recommendations in the draft resolution on what separated a planet from a brown dwarf.^[11] A vote on the proposal was scheduled for August 24, 2006.^[12] Image File history File links Iau_dozen. ... Image File history File links Iau_dozen. ... Artists impression of Pluto (background) and Charon (foreground). ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ... is the 236th day of the year (237th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...

Such a redefinition of the term "planet" could also have led to changes in classification for the trans-Neptunian objects 2003 EL₆₁, 2005 FY₉, Sedna, Orcus, Quaoar, Varuna, 2002 TX₃₀₀, Ixion, 2002 AW₁₉₇, and the asteroids Vesta, Pallas, and Hygiea. (also written (136108) 2003 EL61), nicknamed Easter Bunny, is a large Kuiper belt object, roughly one-third the mass of Pluto, discovered by J. L. Ortiz et al. ... (also written (136472) 2005 FY9) is a very large Kuiper belt object, and one of the two largest among the population in the classical KBO orbits. ... you are abunch of bull | bgcolour=#FFFFC0 | name=90377 Sedna | image= | caption= Sedna is located in the center of the green circle | discovery=yes | discoverer=M. Brown, C. Trujillo, D. Rabinowitz | discovered=November 14, 2003 | mp_name=90377 Sedna | alt_names= | mp_category=Trans-Neptunian object | epoch=September 26, 1990 (JD 2448160. ... 90482 Orcus (originally known by the provisional designation 2004 DW) is a Kuiper Belt object (KBO) that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. ... 50000 Quaoar (pronounced kwaa·waar or kwow·ər, English IPA: , Tongva ) [2] is a Trans-Neptunian object orbiting the Sun in the Kuiper belt. ... 20000 Varuna (VAR oo na) is a large classical Kuiper Belt object (KBO). ... (55636) 2002 TX300 (Also written as (55636) 2002 TX300) is a large Trans-Neptunian object discovered in October 15, 2002 by the NEAT program. ... 28978 Ixion (IPA pronunciation: , Wiktionary:Ixion) is a Kuiper belt object discovered on May 22, 2001. ... is a trans-Neptunian object (TNO). ... 4 Vesta (ves-ta) is the second most massive asteroid in the asteroid belt, with a mean diameter of about 530 km and an estimated mass 12% the mass of the entire asteroid belt. ... 2 Pallas (pal-us, Greek Παλλάς) was the first asteroid discovered after 1 Ceres. ... 10 Hygiea (hye-jee-a or hi-jee-a) is the fourth largest Main belt asteroid with a diameter of 407 km. ...

On 18 August the Division of Planetary Sciences of the American Astronomical Society, the world's largest international professional society of planetary scientists, endorsed the draft proposal.^[13] is the 230th day of the year (231st in leap years) in the Gregorian calendar. ... The American Astronomical Society (AAS) is a US society of professional astronomers and other interested individuals, headquartered in Washington, DC. The main aim of the AAS is provide a political voice for its members and organise their lobbying. ...

According to the IAU, the roundness condition generally results in the need for a mass of at least 5×10²⁰ kg, or diameter of at least 800 km.^[10] However, Mike Brown claims that these numbers are only right for rocky bodies like asteroids, and that icy bodies like Kuiper Belt objects reach hydrostatic equilibrium at much smaller sizes, probably somewhere between 200 and 400 km in diameter.^[14] It all depends on the rigidity of the material that makes up the body, which is in turn strongly influenced by its internal temperature.

Advantages

The proposed definition found support among many astronomers as it used the presence of a physical qualitative factor (the object being round) as its defining feature. Most other potential definitions depended on a limiting quantity (e.g. a minimum size or maximum orbital inclination) tailored for the solar system. According to members of the IAU committee this definition did not use man-made limits but instead deferred to "nature" in deciding whether or not an object was a planet.^[15] Inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit and is the angular distance of the orbital plane from the plane of the reference (usually planets equator or the ecliptic), stated in degrees. ...

It also had the advantage of measuring an observable quality. Suggested criteria involving the nature of formation would have been more likely to see accepted planets later declassified as scientific understanding improved.

Additionally, the definition kept Pluto as a planet. Pluto's planetary status was and is fondly thought of by many, and the general public could have been alienated from professional astronomers; there was considerable uproar when the media last suggested, in 1999, that Pluto might be demoted, which was a misunderstanding of a proposal to catalog all trans-Neptunian objects uniformly. ^[16]

Criticism of proposal

The proposed redefinition was criticised as ambiguous: Astronomer Phil Plait^[17] and NCSE writer Nick Matzke^[18] had both written about why they thought the redefinition was not, in general, a good one. It defined a planet as orbiting a star, which would have meant that any planet ejected from its star system or formed outside of one (a rogue planet or interstellar planet) could not have been called a planet, even if it fit all other definitions. A similar situation already applied to the term 'moon', such bodies ceasing to be moons on being ejected from planetary orbit; this usage had widespread acceptance. Philip Plait, physicist , astronomer and writer Philip Plait (a. ... The NCSEs logo The National Center for Science Education (NCSE) is a non-profit organization affiliated with the American Association for the Advancement of Science. ... Nicholas J. Matzke is Public Information Project Director at the National Center for Science Education (NCSE), the leading American pro-science anti-creationist organisation. ... A rogue planet is a planet that either has an extremely elongated orbit around its star so that it is not on the same orbital plane as the other planets in the system, or it is an interstellar planet, a planet that drifts freely through space and doesnt orbit... An interstellar planet is a hypothetical type of rogue planet that has been ejected from its solar system by a proto-gas giant to become an outcast, drifting in interstellar space. ...

Similarly the redefinition did not differentiate between planets and brown dwarf stars. Any attempt to clarify this differentiation was to be left until a later date. This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ...

There had also been criticism of the definition of double planet: at present the Moon is defined as a satellite of the Earth, but over time the Earth-Moon barycenter will drift outwards (see tidal acceleration) and could eventually become situated outside of both bodies. This development would then upgrade the Moon to planetary status at that time, according to the redefinition. The time taken for this to occur, however, would be billions of years, long after many astronomers expect the Sun to expand into a red giant and destroy both Earth and Moon.^[19] Pluto and Charon are sometimes informally considered to be a double (dwarf) planet. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it was concentrated. ... It has been suggested that Tidal friction be merged into this article or section. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ...

In an 18 August Science Friday interview, Mike Brown expressed doubt that a scientific definition was even necessary: is the 230th day of the year (231st in leap years) in the Gregorian calendar. ... Science Friday is a call-in talk show that is part of National Public Radios Talk of the Nation radio program hosted by Ira Flatow every Friday. ...

"The analogy that I always like to use is the word "continent". You know, the word "continent" has no scientific definition ... they're just cultural definitions, and I think the geologists are wise to leave that one alone and not try to redefine things so that the word "continent" has a big, strict definition."^[20]

On 18 August, Owen Gingerich said that correspondence he had received had been evenly divided for and against the proposal.^[21] Animated, colour-coded map showing the various continents. ... is the 230th day of the year (231st in leap years) in the Gregorian calendar. ... Owen Gingerich Dr. Owen Jay Gingerich (1930-) was Research Professor of Astronomy and of the History of Science at Harvard University and a senior astronomer emeritus at the Smithsonian Astrophysical Observatory. ...

Alternative proposal

According to Alan Boss of the Carnegie Institution of Washington, a subgroup of the IAU met on August 18, 2006 and held a straw poll on the draft proposal: only 18 were in favour of it, with over 50 against. The 50 in opposition preferred an alternative proposal drawn up by Uruguayan astronomer Julio Ángel Fernández.^[21] Alan P. Boss is an American astrophysicist. ... The Carnegie Institution of Washington (CIW) is a foundation established by Andrew Carnegie in 1902 to support scientific research. ... is the 230th day of the year (231st in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... A straw poll is an informal type of voting where the results of the poll have little or no direct results, other than to gauge opinion. ... Julio M. Fernandez (born 1954 in Santiago, Chile) is a Professor in the Department of Biological Sciences at Columbia University, New York. ...

Under the alternative proposal the current solar system would have remained unchanged, but Pluto would have been demoted to a dwarf planet. Near-Earth objects (NEO) are asteroids, comets and large meteoroids whose orbit intersects Earths orbit and which may therefore pose a collision danger. ... The centaurs are a class of icy planetoids that orbit the Sun between Jupiter and Neptune, named after the mythical race of centaurs. ... A trans-Neptunian object (TNO) is any object in the solar system with all or most of its orbit beyond that of Neptune. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... Artists impression of Pluto (background) and Charon (foreground). ...

Revised draft proposal

On 22 August the draft proposal was rewritten with two changes from the previous draft. is the 234th day of the year (235th in leap years) in the Gregorian calendar. ...

The first was a generalisation of the name of the new class of planets (previously the draft resolution had explicitly opted for the term 'pluton'), with a decision on the name to be used postponed. Many geologists had been critical of the choice of name for Pluto-like planets,^[23] being concerned about the term pluton which has been used for years within the geological community to represent a form of magmatic intrusion; such formations are fairly common hunks of rock.^[24][25] Confusion was thought undesirable due to the status of planetology as a field closely allied to geology.^[26] Further concerns surrounded use of the word pluton as in many European languages such as French and Spanish, Pluto is itself called Pluton, potentially adding to confusion. Pluton redirects here. ... Devils Tower, an igneous intrusion exposed when the surrounding softer rock eroded away. ...

The second change was a redrawing of the planetary definition in the case of a double planet system. There had been a concern that, in extreme cases where a double body had its secondary component in a highly eccentric orbit, there could have been a drift of the barycenter in and out of the primary body, leading to a shift in the classification of the secondary body between satellite and planet depending on where in its orbit the system was. Thus the definition was reformulated so as to consider a double planet system in existence if its barycenter lay outside both bodies for a majority of the system's orbital period. For other uses, see Satellite (disambiguation). ...

Later on the 22nd two open meetings were held which ended in an abrupt about-face on the basic planetary definition. The position of astronomer Julio Ángel Fernández gained the upper hand among the members attending and was described as unlikely to lose its hold by the 24th. This position would result in only eight major planets, with Pluto ranking as a "dwarf planet" or "planetoid".^[27] The discussion at the first meeting was heated and lively, with IAU members in vocal disagreement with one another over such issues as the relative merits of static and dynamic physics; the main sticking point was over whether or not to include a body's orbital characteristics among the definition criteria. In an indicative vote members heavily defeated the proposals on Pluto-like objects and double planet systems, and were evenly divided on the question of hydrostatic equilibrium. The debate was said to be "still open", with private meetings being held ahead of a vote scheduled for the following day.^[28] Statics is the branch of physics concerned with physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at rest under the action of external forces of equilibrium. ... In physics, dynamics is the branch of classical mechanics that is concerned with the effects of forces on the motion of objects. ...

At the second meeting of the day, following 'secret' negotiations, a compromise began to emerge after the Executive Committee moved explicitly to exclude consideration of extra-solar planets and to bring into the definition a criterion concerning the dominance of a body in its neighbourhood.^[29] Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ...

Final draft proposal

The final, third draft definition proposed on 24 August was: is the 236th day of the year (237th in leap years) in the Gregorian calendar. ...

Illustration of the final proposal.

What exactly it meant to say that a body had "cleared the neighbourhood around its orbit" or that it had a "nearly round" shape was explicitly not defined. These descriptions were used partly to allow for some "common sense" judgement, but primarily to provide language that could readily be used in communicating the definitions to lay audiences. Image File history File links ThePlanetDefinition2. ... Image File history File links ThePlanetDefinition2. ... In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. ... For other uses, see Common sense (disambiguation). ...

Plenary session debate

Voting on the definition took place at the Assembly plenary session during the afternoon. Following a reversion to the previous rules on 15 August, as a planetary definition is a primarily scientific matter every individual member of the Union attending the Assembly was eligible to vote. The number having registered their attendance at the Assembly at the time of the vote stood at 2411,^[30] but out of the over a thousand who attended the session only 424 members chose to vote or indicate their abstention on Resolution 6A (below). is the 227th day of the year (228th in leap years) in the Gregorian calendar. ...

The IAU Executive Committee presented four Resolutions to the Assembly, each concerning a different aspect of the debate over the definition.^[31] Minor amendments were made on the floor for the purposes of clarification.

• Resolution 5A constituted the definition itself as stated above. There was much discussion among members about the appropriateness of using the expression "cleared the neighbourhood" instead of the earlier reference to "dominant body", and about the implications of the definition for satellites. The Resolution was ultimately passed by a near-unanimous vote.

• Resolution 5B sought to amend the above definition by the insertion of the word classical before the word planet in paragraph (1) and footnote [1]. This represented a choice between having a set of three distinct categories of body (planet, "dwarf planet" and SSSB) and the opening of an umbrella of 'planets' over the first two such categories. The Resolution proposed the latter option; it was defeated convincingly, with only 91 members voting in its favour.

• Resolution 6A proposed a statement concerning Pluto: "Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects." After a little quibbling over the grammar involved and questions of exactly what constituted a "trans-Neptunian object", the Resolution was approved by a vote of 237-157, with 30 people indicating their absention. A new category of dwarf planet was thus established.

• Resolution 6B sought to insert an additional sentence at the end of the statement in 6A: "This category is to be called 'plutonian objects'." There was no debate on the question, and in the vote the proposed name was defeated by 186-183; a proposal to conduct a re-vote was rejected. An IAU process will be put in motion to determine the name for the new category.

On a literal reading of the Resolution, "dwarf planets" are by implication of paragraph (1) excluded from the status of 'planet'. Use of the word planet in their title may however cause some ambiguity. For other uses, please see Satellite (disambiguation) A satellite is an object that orbits another object (known as its primary). ... On August 24, 2006, the International Astronomical Union (IAU) decided to reclassify Pluto as a dwarf planet, requiring that a planet must clear the neighbourhood around its orbit. ...

Final definition

Illustration of the outcome of the vote.

The final definition, as passed on 24 August 2006 is: ^[32] Image File history File links ThePlanetDefinition3. ... Image File history File links ThePlanetDefinition3. ... is the 236th day of the year (237th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...

Criticism

Substance

There continues to be criticism regarding the wording of the final draft of the definition. Notably, the lead scientist on NASA's robotic mission to Pluto, Alan Stern, contends that, like Pluto, Earth, Mars, Jupiter and Neptune have not fully cleared their orbital zones either. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be there," he added.^[33] However, his own earlier work on neighbourhood clearing supported the distinction between the largest eight planets and the rest of the solar system.^[34] There is a substantial difference in the extent to which the neighbourhood has been cleared between Pluto and the eight planets. Also, Pluto's position is due to the gravitational effects of Neptune as they are in orbital resonance. New Horizons on the launchpad New Horizons is a robotic spacecraft mission conducted by NASA. It is expected to be the first spacecraft to fly by and study the dwarf planet Pluto and its moons, Charon, Nix and Hydra. ... This page may meet Wikipedias criteria for speedy deletion. ... This article is about Earth as a planet. ... Adjectives: Martian Atmosphere Surface pressure: 0. ... For other uses, see Jupiter (disambiguation). ... For other uses, see Neptune (disambiguation). ... Computer model of the Apollo Asteroid 6489 Golevka Near-Earth asteroids (NEAs) are asteroids whose orbits are close to Earths orbit. ... In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. ... In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other. ...

The debates have clarified that "clearing its orbit" refers to the process that happened during the formation of the planets. It does not talk about the presence of bodies that later strayed into the orbit after the accretions took place, nor to objects in resonant orbits.^{[citation needed]}

The definition may be difficult to apply outside our solar system. Techniques for identifying extrasolar objects generally cannot determine if an object has "cleared its orbit," except indirectly via Stern and Levison's ? parameter, and provide limited information about when the objects were formed. The wording of the new definition is heliocentric in its use of the word Sun instead of star or stars, and is thus not applicable to the numerous objects that have been identified in orbit around other stars. In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. ... Sol redirects here. ... This article is about the astronomical object. ... For the band of the same name, see: Binary Star (band) Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left). ... Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ...

Process

The final vote has come under much criticism because of the relatively small percentage of the 9000-strong membership who participated. Besides the fact that most members do not attend the General Assemblies, this lack was also due to the timing of the vote: the final vote was taken on the last day of the 10-day event, after many participants had left or were preparing to leave. Of over 2,700 astronomers attending the conference, about 800 were present on the day for the significant resolutions^{[citation needed]} on a vote on a subsidiary resolution, the first that required a count, only 424 votes were cast.^[33] There is also the issue of the many astronomers who were unable or who chose not to make the trip to Prague and, thus, cast a vote. Astronomer Marla Geha has clarified that not all members of the Union were needed to vote on the classification issue: only those whose work is directly related to planetary studies.^[35] For other uses, see Prague (disambiguation). ...

Other

Some astrologers have chosen not to follow the new definition.^[36] Hand-coloured version of the anonymous Flammarion woodcut (1888). ...

Impact

It is expected that the decision will have cultural and societal implications. It will affect the "industry of astronomical artifacts and toys."^[37] Educational books need to be revised. The decision was important enough to prompt the editors of the 2007 edition of the World Book Encyclopedia to hold off printing until a final result had been reached.^[37] The new designation also has repercussion in the astrological world and finds mixed receptions.^[36] World Book Encyclopedia is, according to its publisher in the United States, the number-one selling print encyclopedia in the world. ...

See also

• Definition of planet
• Dwarf planet
• Cleared the neighbourhood
• Fusor (astronomy)
• Kuiper belt
• List of solar system objects by planetary discriminant
• List of solar system objects
• Minor planet
• Planemo
• Planet
• Pluto
• Scattered disc
• Small solar system body
• Trans-Neptunian object

Photograph of the planet Neptune and its moon Triton, taken by Voyager 2 as it entered the outer solar system. ... Artists impression of Pluto (background) and Charon (foreground). ... In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. ... A fusor is a term proposed to the IAU by Gibor Basri, Head of Astronomy at the University of Berkeley, to help clarify the nomenclature of celestial bodies. ... The Kuiper belt, derived from data from the Minor Planet Center. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Below is a list of solar system objects with diameter >500km: The Sun, a spectral class G2 star Mercury Venus Earth Moon Mars Jupiter Io Europa Ganymede Callisto complete list of Jupiters natural satellites Saturn Tethys Dione Rhea Titan Iapetus complete list of Saturns natural satellites Uranus Ariel... Minor planets, or asteroids or planetoids, are minor celestial bodies of the Solar system orbiting the Sun (mostly Small solar system bodies) that are smaller than major planets, but larger than meteoroids (commonly defined as being 10 meters across or less[1]), and that are not comets. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... This article is about the astronomical term. ... For other uses, see Pluto (disambiguation). ... Eris, the largest known scattered disc object (center), and its moon Dysnomia (left of center). ... A Small Solar System Body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets: [1] This encompasses: all minor planets apart from the dwarf planets, : the classical asteroids, (except for 1 Ceres, the... A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ...

References

Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 228th day of the year (229th in leap years) in the Gregorian calendar. ... Year 2006