IAU definition of planet
The
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has "cleared the neighbourhood" around its orbit.
A non-
The IAU has stated that there are
The working definition of an exoplanet is as follows:[5][6]
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+√621)) are "planets" (no matter how they formed).
- The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
Background
The process of new discoveries spurring a contentious refinement of Pluto's categorization echoed a debate in the 19th century that began with the discovery of
Tombaugh discovered Pluto while working at the
In 1978, Pluto's moon Charon was discovered. 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.[9] Pluto's mass was roughly one twenty-fifth of Mercury's, making it by far the smallest planet, smaller even than the Earth's Moon, although it was still over ten times as massive as the largest asteroid, Ceres.
In the 1990s, astronomers began finding other objects at least as far away as Pluto, known as
Starting in 2000, with the discovery of at least three bodies (
Because new planets are discovered infrequently, the IAU did not have any mechanism for their definition and naming. After the discovery of
- 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.[12]
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.[13]
Proposals
First draft proposal
The IAU published the original definition proposal on August 16, 2006.[14] Its form followed loosely the second of three options proposed by the original committee. It stated that:[14]
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 more celestial bodies being recognized as planets, in addition to the previously accepted nine:
- 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
- scattered diskof 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,[15] 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.[16]
The definition would have considered a pair of objects to be a
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.[18] The IAU did not make recommendations in the draft resolution on what separated a planet from a brown dwarf.[19] A vote on the proposal was scheduled for August 24, 2006.[15]
Such a definition of the term "planet" could also have led to changes in classification for the trans-Neptunian objects Haumea, Makemake, Sedna, Orcus, Quaoar, Varuna, 2002 TX300, Ixion, and 2002 AW197, and the asteroids Vesta, Pallas, and Hygiea.
On 18 August the Committee of the Division of Planetary Sciences (DPS) of the American Astronomical Society endorsed the draft proposal.[20] The DPS Committee represents a small subset of the DPS members, and no resolution in support of the IAU definition was considered or approved by the DPS membership.
According to an IAU draft resolution, the roundness condition generally results in the need for a mass of at least 5×1020 kg, or diameter of at least 800 km.
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 human-made limits but instead deferred to "nature" in deciding whether or not an object was a planet.[24]
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.[citation needed]
Additionally, the definition kept Pluto as a planet. Pluto's planetary status was and is fondly thought of by many, especially in the United States since Pluto was found by American astronomer Clyde Tombaugh, 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.[25]
Criticism
The proposed definition was criticised as ambiguous: Astronomer Phil Plait and NCSE writer Nick Matzke both wrote about why they thought the definition was not, in general, a good one.[26][27] 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) could not have been called a planet, even if it fit all other criteria. However, a similar situation already applies to the term 'moon'—such bodies ceasing to be moons on being ejected from planetary orbit—and this usage has widespread acceptance. Another criticism was that the definition 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 proposed definition of
In an 18 August 2006 Science Friday interview, Mike Brown expressed doubt that a scientific definition was even necessary. He stated, "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."[30]
On 18 August, Owen Gingerich said that correspondence he had received had been evenly divided for and against the proposal.[31]
Alternative proposal
According to
(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], and (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 the 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, belongs 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
- The local population is the collection of objects that cross or closely approach the orbit of the body in consideration.
- This generally applies to objects with sizes above several hundred kilometers, depending on the material strength.
- This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
- This class currently includes most of the Solar System asteroids,
Trans-Neptunian Objects (TNOs), and comets.[32]
Under this proposal, Pluto would have been demoted to a dwarf planet.
Revised draft proposal
On 22 August 2006 the draft proposal was rewritten with two changes from the previous draft.
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 the system was in its orbit.[39] 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.[citation needed]
Later on August 22, two open meetings were held which ended in an abrupt about-face on the basic planetary definition. The position of astronomer
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
Final draft proposal
The final, third draft definition proposed on 24 August 2006 read:
The IAU...resolves that planets and other bodies in the 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.
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 plenary session was chaired by astronomer
The IAU Executive Committee presented four Resolutions to the Assembly, each concerning a different aspect of the debate over the definition.[45] 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 approved 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[46] 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",[plutoid" and more narrowly defined by the IAU Executive Committee on 11 June 2008.
- 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 was then to be put in motion to determine the name for the new category.[46]
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 under the Resolution 5A of the 26th General Assembly, is:[48][49]
The IAU...resolves that planets and other bodies, except satellites, in the Solar System be defined into three distinct categories in the following way:
(1) A
cleared the neighbourhoodaround 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".
Footnotes:
[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:
Trans-Neptunian Objects[1].Footnote:
[1] An IAU process will be established to select a name for this category.
The IAU also resolved that "planets and dwarf planets are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets.[46]
Closing issues
Substance
Alan Stern, the lead scientist on NASA's robotic mission to Pluto, contended that Earth, Mars, Jupiter, and Neptune have not fully cleared their orbital zones, just like Pluto. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. Stern has asserted: "If Neptune had cleared its zone, Pluto wouldn't be there."[50]
Some astronomers counter this opinion by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Although Jupiter does coexist with a large number of small bodies in its orbit (the
On June 11, 2008, the IAU announced that the subcategory of dwarf planets with trans-Neptunian orbits would be known as "
Plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit.
This subcategory includes
Some aspects of the definition are as yet difficult to apply outside the Solar System. Techniques for identifying extrasolar objects generally cannot determine whether an object has "cleared its orbit", except indirectly via an
Process
The final vote was criticized because of the relatively small percentage of the 9000-strong IAU 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. Many astronomers were also unable or chose not to make the trip to Prague and, thus, cast no vote. Only 424 astronomers were present for the vote, which is less than 5% of the astronomer community.[50] However, sampling 400 representative members out of a population of 9,000 statistically yields a result with good accuracy (confidence interval better than 5%).[55] 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.[56]
Impact
The decision generated cultural and societal implications, affecting the "industry of astronomical artifacts and toys."[57] Most educational books that included the definition were printed after 2006. 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.[57]
Popular culture
The impact of the revised definition, particularly the change in the status of Pluto, has been reflected in popular culture. A number of musical contributions have commemorated the change:
- "Planet X" (1996), song by Christine Lavin. A good-natured protest against suggestions that Pluto is not a planet.[citation needed]
- "Pluto" (1998), song by 2 Skinnee J's. An impassioned defense of Pluto's status as a planet.[citation needed]
- Thing a Week, August 25, 2006 podcast by Jonathan Coulton. Featured a song "I'm Your Moon", from Charon's point of view, about Pluto being reclassified as a dwarf planet.[citation needed]
- "Bring Back Pluto" (2007), song by Aesop Rock on the album None Shall Pass. Hip-hop song supporting Pluto's status as the 9th planet in the Solar System.[citation needed]
- "Pluto" (2009), song by Kuiper Belt.[citation needed]
- "Ode to Pluto" is the final track on Terra Lumina's self-titled debut album, mentioning the change of classification.[citation needed]
- Pluto's demotion is alluded to in "The Lonesome Friends of Science" on John Prine's The Tree of Forgiveness, in which the planets (including non-existent Vulcan) are anthropomorphized, and Pluto is "uninvited to the interplanetary dance."[citation needed]
- "134340" (2018), a song by BTS from album Love Yourself: Tear, feature the breakup of a sad relationship by the Pluto status as a real planet.[58]
Plutoed
The verb to pluto (
Society president Cleveland Evans stated the reason for the organization's selection of plutoed: "Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the Roman god Pluto, but we still have a sense of connection with the former planet".[61]
See also
- Clearing the neighbourhood
- Fusor (astronomy)
- Geophysical planet definition
- List of Solar System objects
- List of former planets
- Minor planet
- Planemo
- Small Solar System body
- How I Killed Pluto and Why It Had It Coming - Memoir by astronomer Michael E. Brown about the event.
References
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- ^ Brown, Mike [@plutokiller] (February 10, 2023). "The real answer here is to not get too hung up on definitions, which I admit is hard when the IAU tries to make them sound official and clear, but, really, we all understand the intent of the hydrostatic equilibrium point, and the intent is clearly to include Mercury & the moon" (Tweet) – via Twitter.
- ^ Sean Solomon, Larry Nittler & Brian Anderson, eds. (2018) Mercury: The View after MESSENGER. Cambridge Planetary Science series no. 21, Cambridge University Press, pp. 72–73.
- ^ "Official Working Definition of an Exoplanet". IAU position statement. Retrieved 29 November 2020.
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- .
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External links
- IAU 2006 General Assembly: Result of the IAU Resolution votes International Astronomical Union official site.
- IAU 2006 General Assembly: video-records of the discussion and of the final vote on the Planet definition.
- Planet Definition Questions & Answers Sheet, International Astronomical Union Official Site
- Q&A: The IAU's Proposed Planet Definition Q&A article on the new definition from SPACE.com
- Dwarf planet discoverer Mike Brown explains the concept behind the new definition
- Cronología de los hechos relativos a la Definición de Planeta adoptada por la Asamblea de la Unión Astronómica Internacional