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2006 definition of planet

The 2006 definition of "planet" by the International Astronomical Union (IAU) states that, in the solar system, a planet is a celestial body that:

  • is in orbit around the Sun,
  • has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  • has "cleared the neighborhood" 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 been both criticised and supported by different astronomers.

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 can't 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.

Reasons for the debate

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, and 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.[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. New York City's newly renovated Hayden Planetarium famously did not include Pluto in its exhibit of the planets.[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'.

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]

Early Proposals

Debate within the IAU led Julio Fernández and Gonzalo Tancredi of Uruguay[5] to suggest proposals to redefine the term "planet" so as to include other objects beyond the traditional nine planets that had been historically considered part of the solar system.[6] In its final form the proposal was denoted as Resolutions 5A, 5B, 6A and 6B for GA-XXVI. Members of the IAU's General Assembly voted on the proposal on August 24, 2006 in Prague, Czech Republic, with the vote removing Pluto's status as a planet and reclassifying it as a dwarf planet.[7] Pluto had long been exceptional among the planets, being small, distant, and eccentric in orbit.

In its original form the redefinition would have kept Pluto as a planet and recognized three new planets: Ceres, Charon, and Eris. It was presumed that, after more observation and discussion, astronomers would accept more objects in the solar system as meeting the new definition. On August 22, however, the original redefinition (which recognized twelve solar system planets, including Pluto), was dealt a fatal blow in two open IAU meetings. Jay Pasachoff of Williams College, who attended both meetings, was quoted as saying, "I think that today can go down as 'the day we lost Pluto' ".[8]

August 16

The IAU published the original definition proposal on August 16, 2006. Its form followed loosely the second of three options considered by a 19-member IAU-appointed panel in 2005. It stated:

"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

This definition would have led to three celestial bodies being recognized as planets:

  • 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,[9] 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.[10]

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.[11] 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).

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 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.[12] The IAU did not make recommendations in the draft resolution on what separated a planet from a brown dwarf.[13] A vote on the proposal was scheduled for August 24, 2006.[14]

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.

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.[15]

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.[12] 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.[16] It all depends on the rigidity of the material that makes up the body, which is in turn strongly influenced by its internal temperature.


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.[17]

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. [18]

Criticism of proposal

The proposed redefinition was criticized as ambiguous: Astronomer Phil Plait[19] and NCSE writer Nick Matzke[20] 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.

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.

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.[21]

In an 18 August Science Friday interview, Mike Brown expressed doubt that a scientific definition was even necessary:

"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."[22]

On 18 August, Owen Gingerich, a historian and astronomer emeritus at Harvard who led the committee which generated the original definition, said that correspondence he had received had been evenly divided for and against the proposal.[23]

August 18

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.[23]

"(1) A planet is a celestial body that (a) is by far the largest object in its local population[1], (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) does not produce energy by any nuclear fusion mechanism [3].

(2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of our Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities.

(3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as "Small Solar System Bodies".[4]

Definitions and clarifications

  1. The local population is the collection of objects that cross or closely approach the orbit of the body in consideration.
  2. This generally applies to objects with sizes above several hundred kilometers, depending on the material strength.
  3. This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
  4. This class currently includes most of the Solar System asteroids, Near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets.[24]"

Under the alternative proposal the current solar system would have remained unchanged, but Pluto would have been demoted to a dwarf planet.

August 22

On 22 August the draft proposal was rewritten with two changes from the previous draft.

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,[25] 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.[26][27] Confusion was thought undesirable due to the status of planetology as a field closely allied to geology.[28]

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.

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.

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".[29] 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.[30]

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.[31]

August 24

Final draft

The final, third draft definition proposed on 24 August was:

"The IAU...resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet [1] 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 [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [3] orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies."

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.

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,[32] 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).

The IAU Executive Committee presented four Resolutions to the Assembly, each concerning a different aspect of the debate over the definition.[33] 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.

Final definition

The final definition, as passed on 24 August 2006 is: [34]

The IAU...resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

(1) A planet [1] 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 [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".


[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects[1].


[1] An IAU process will be established to select a name for this category.




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.[35] However, his own earlier work on neighborhood clearing supported the distinction between the largest eight planets and the rest of the solar system.[36] There is a substantial difference in the extent to which the neighborhood 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.

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.

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 Λ 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.


The final vote has come under 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 on a vote on a subsidiary resolution, the first that required a count, only 424 votes were cast.[35] 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.[37]


Some astrologers have chosen not to follow the new definition.[38]


It is expected that the decision will have cultural and societal implications. It will affect the “industry of astronomical artifacts and toys”.[39] 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.[39] The new designation also has repercussion in the astrological world and finds mixed receptions.[38]

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