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International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU) defined in August 2006 that, in the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, a ''
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
'' is a
celestial body An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
that: # is in
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
around the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, # has sufficient mass to assume
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planeta ...
(a nearly round shape), and # has "
cleared the neighbourhood "Clearing the neighbourhood" (or dynamical dominance) around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise ...
" around its orbit. A non-
satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
body fulfilling only the first two of these criteria (such as
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
, which had hitherto been considered a planet) is classified as a ''
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
''. According to the IAU, "planets and dwarf planets are two distinct classes of objects" – in other words, "dwarf planets" are not planets. A non-satellite body fulfilling only the first criterion is termed a ''
small Solar System body A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other obje ...
'' (SSSB). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The decision was a controversial one, and has drawn both support and criticism from
astronomers An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either obse ...
. The IAU has stated that there are eight known planets in the Solar System. However, it is now known that Mercury does not meet criterion 2, but it is nonetheless universally considered to be a planet.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. It has also been argued that the definition is problematic because it depends on the location of the body: if a
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
-sized body were discovered in the inner Oort cloud, it would not have enough mass to clear out a neighbourhood that size and meet criterion 3. The definition distinguishes planets from smaller bodies and is not applicable outside the Solar System. To date, there is no accepted definition of
extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s, or exoplanets. In 2007, an IAU working group issued a position statement that proposes to distinguish exoplanets from
brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s on the basis of mass, but there has been no IAU-wide resolution or vote associated with this position statement. A separate proposal to extend the IAU definition to exoplanets has not been formally reviewed by the IAU.


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 Ceres on January 1, 1801. Astronomers immediately declared the tiny object to be the "missing planet" between Mars and Jupiter. Within four years, however, the discovery of two more objects with comparable sizes and orbits had cast doubt on this new thinking. By 1851, the number of "planets" had grown to 23 (the eight recognised today, plus fifteen between Mars and Jupiter), and it was clear that hundreds more would eventually be discovered. Astronomers began cataloguing them separately and began calling them "asteroids" instead of "planets". With the discovery of
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
by
Clyde Tombaugh Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was cons ...
in 1930, astronomers considered the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
to have nine
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s, along with thousands of smaller bodies such as
asteroids An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
and
comets A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
. Pluto was thought to be larger than Mercury. Tombaugh discovered Pluto while working at the
Lowell Observatory Lowell Observatory is an astronomical observatory in Flagstaff, Arizona, United States. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landma ...
founded by
Percival Lowell Percival Lowell (; March 13, 1855 – November 12, 1916) was an American businessman, author, mathematician, and astronomer who fueled speculation that there were canals on Mars, and furthered theories of a ninth planet within the Solar System. ...
, one of many astronomers who had theorized on the existence of the large
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
Planet X Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's ...
, and Tombaugh had been searching for Planet X when he found Pluto. Almost immediately after its discovery, however, astronomers questioned whether Pluto could be Planet X.
Willy Ley Willy or Willie is a masculine, male given name, often a diminutive form of William (given name), William or Wilhelm (name), Wilhelm, and occasionally a nickname. It may refer to: People Given name or nickname * Willie Aames (born 1960), American ...
wrote a column in 1956 titled "The Demotion of Pluto", stating that it "simply failed to live up to the advance publicity it received as 'Planet X' before its discovery. It has been a disappointment all along, for it did not turn out to be what one could reasonably have expected". In 1978, Pluto's moon
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the ...
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. 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 The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, 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, now known as
Kuiper Belt object The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 time ...
s, or KBOs. Many of these shared some of Pluto's key orbital characteristics and are now called
plutinos In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the l ...
. 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 in the Solar System, fits in well with the other KBOs. New York City's newly renovated
Hayden Planetarium The Rose Center for Earth and Space is a part of the American Museum of Natural History in New York City. The Center's complete name is The Frederick Phineas and Sandra Priest Rose Center for Earth and Space. The main entrance is located on the no ...
did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth and Space in 2000. Starting in 2000, with the discovery of at least three bodies (
Quaoar Quaoar (50000 Quaoar), provisional designation , is a dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. A non-resonant object ( cubewano), it measures approximately in diameter, about half the diameter of Pluto. Th ...
, Sedna, and Eris) all comparable to Pluto in terms of size and
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
, it became clear that either they all had to be called planets or Pluto would have to be reclassified. Astronomers also thought it likely 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
planetary system A planetary system is a set of gravitationally bound non- stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also consi ...
s. In 2006, the first measurement of the volume of Eris erroneously (until the ''
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
'' mission to Pluto) showed it to be slightly larger than Pluto, and so was thought to be equally deserving of the status of "planet". Because new planets are discovered infrequently, the IAU did not have any mechanism for their definition and naming. After the discovery of Sedna, it set up a 19-member committee in 2005, with the British astronomer Iwan Williams in the chair, 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. Another committee, chaired by a historian of astronomy,
Owen Gingerich Owen Jay Gingerich (; born 1930) is professor emeritus of astronomy and of the history of science at Harvard University and a senior astronomer emeritus at the Smithsonian Astrophysical Observatory. In addition to his research and teaching, he has ...
, a historian and astronomer emeritus at
Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of highe ...
who led the committee which generated the original definition, and consisting of five planetary scientists and the science writer
Dava Sobel Dava Sobel (born June 15, 1947) is an American writer of popular expositions of scientific topics. Her books include ''Longitude'', about English clockmaker John Harrison, and '' Galileo's Daughter'', about Galileo's daughter Maria Celeste, and ...
, was set up to make a firm proposal.


Proposals


First 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 that: 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 In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the ...
, a moon of Pluto; the Pluto-Charon system would have been considered a
double planet In astronomy, a double planet (also binary planet) is a binary satellite system where both objects are planets, or planetary-mass objects, that share an orbital axis external to both planetary bodies. Although up to a third of the star syst ...
* 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, 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. The definition would have considered a pair of objects to be a
double planet In astronomy, a double planet (also binary planet) is a binary satellite system where both objects are planets, or planetary-mass objects, that share an orbital axis external to both planetary bodies. Although up to a third of the star syst ...
system if each component independently satisfied the planetary criteria and the common
center of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...
of the system (known as the ''
barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
'') was located outside of both bodies.
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
and Charon would have been the only known double planet in the Solar System. Other planetary satellites (such as the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
or Ganymede) might be in
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planeta ...
, but would still not have been defined as a component of a double planet, since the barycenter of the system lies within the more massive celestial body. The term "minor planet" would have been abandoned, replaced by the categories "
small Solar System body A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other obje ...
" (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 A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
" 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. The IAU did not make recommendations in the draft resolution on what separated a planet from a
brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
. A vote on the proposal was scheduled for August 24, 2006. Such a definition of the term "planet" could also have led to changes in classification for the
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
s , , Sedna,
Orcus Orcus ( la, Orcus) was a god of the underworld, punisher of broken oaths in Etruscan and Roman mythology. As with Hades, the name of the god was also used for the underworld itself. In the later tradition, he was conflated with Dis Pater. A ...
,
Quaoar Quaoar (50000 Quaoar), provisional designation , is a dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. A non-resonant object ( cubewano), it measures approximately in diameter, about half the diameter of Pluto. Th ...
,
Varuna Varuna (; sa, वरुण, , Malay: ''Baruna'') is a Vedic deity associated initially with the sky, later also with the seas as well as Ṛta (justice) and Satya (truth). He is found in the oldest layer of Vedic literature of Hinduism, su ...
, ,
Ixion In Greek mythology, Ixion ( ; el, Ἰξίων, ''gen''.: Ἰξίονος means 'strong native') was king of the Lapiths, the most ancient tribe of Thessaly. Family Ixion was the son of Ares, or Leonteus, or Antion and Perimele, or the not ...
, and , and the asteroids Vesta,
Pallas Pallas may refer to: Astronomy * 2 Pallas asteroid ** Pallas family, a group of asteroids that includes 2 Pallas * Pallas (crater), a crater on Earth's moon Mythology * Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena * Pa ...
, and Hygiea. On 18 August the Committee of the Division of Planetary Sciences (DPS) of the
American Astronomical Society The American Astronomical Society (AAS, sometimes spoken as "double-A-S") is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC. The primary objective of the AAS is to promote the adv ...
endorsed the draft proposal. 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 kg, or diameter of at least 800 km. However, Mike Brown claimed that these numbers are only right for rocky bodies like asteroids, and that icy bodies like
Kuiper Belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
objects reach hydrostatic equilibrium at much smaller sizes, probably somewhere between 200 and 400 km in diameter. It all depends on the rigidity of the material that makes up the body, which is in turn strongly influenced by its internal temperature. Assuming that Methone's shape reflects the balance between the
tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
exerted by Saturn and the moon's gravity, its tiny 3 km diameter suggests Methone is composed of icy fluff. The IAU's stated radius and mass limit are not too far off from what as of 2019 is believed to be the approximate limit for objects beyond Neptune that are fully compact, solid bodies, with (''r'' = , ''m'' = ) and possibly (''r'' = , ''m'' unknown) being borderline cases both for the 2006 Q&A expectations and in more recent evaluations, and with being just above the expected limit.


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 Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Ea ...
) 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. 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, especially in the United States since Pluto was found by American astronomer
Clyde Tombaugh Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was cons ...
, 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 object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
s uniformly.


Criticism

The proposed definition was criticised as ambiguous: Astronomer
Phil Plait Philip Cary Plait (born September 30, 1964), also known as The Bad Astronomer, is an American astronomer, skeptic, and popular science blogger. Plait has worked as part of the Hubble Space Telescope team, images and spectra of astronomical objec ...
and NCSE writer
Nick Matzke Nicholas J. Matzke is the former Public Information Project Director at the National Center for Science Education (NCSE) and served an instrumental role in NCSE's preparation for the 2005 ''Kitzmiller v. Dover Area School District'' trial. One of h ...
both wrote about why they thought the definition 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 A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
) 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 Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
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
double planet In astronomy, a double planet (also binary planet) is a binary satellite system where both objects are planets, or planetary-mass objects, that share an orbital axis external to both planetary bodies. Although up to a third of the star syst ...
: at present the Moon is defined as a satellite of the Earth, but over time the Earth-Moon
barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
will drift outwards (see
tidal acceleration Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away f ...
) 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 definition. 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 A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around o ...
and destroy both Earth and Moon. In an 18 August 2006 ''
Science Friday ''Science Friday'' (known as ''SciFri'' for short) is a weekly call-in talk show that broadcasts each Friday on public radio stations, distributed by WNYC Studios, and carried on over 400 public radio stations. ''SciFri'' is hosted by award-wi ...
'' 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 A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...
". 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." On 18 August,
Owen Gingerich Owen Jay Gingerich (; born 1930) is professor emeritus of astronomy and of the history of science at Harvard University and a senior astronomer emeritus at the Smithsonian Astrophysical Observatory. In addition to his research and teaching, he has ...
said that correspondence he had received had been evenly divided for and against the proposal.


Alternative proposal

According to
Alan Boss Alan P. Boss (born in Lakewood, Ohio) is a United States astrophysicist and planetary scientist. Life and career Educated at the University of South Florida and the University of California, Santa Barbara, Boss is a prominent scientist in stellar ...
of the
Carnegie Institution of Washington The Carnegie Institution of Washington (the organization's legal name), known also for public purposes as the Carnegie Institution for Science (CIS), is an organization in the United States established to fund and perform scientific research. Th ...
, 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
Uruguay Uruguay (; ), officially the Oriental Republic of Uruguay ( es, República Oriental del Uruguay), is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast; while bordering ...
an astronomers Gonzalo Tancredi and
Julio Ángel Fernández Julio Ángel Fernández Alves (born 5 April 1946) is a Uruguayan astronomer and teacher, member of the department of astronomy at the Universidad de la República in Montevideo. He is also a member of PEDECIBA, (the program for development of ba ...
. Under this proposal, Pluto would have been demoted to a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
.


Revised draft proposal

On 22 August 2006 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, being concerned about the term ''
pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
'', which has been used for years within the geological community to represent a form of magmatic
intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
; such formations are fairly common balls of rock. Confusion was thought undesirable due to the status of planetology as a field closely allied to geology. Further concerns surrounded use of the word ''pluton'' as in major languages such as
French French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and accents ** French people, a nation and ethnic group identified with Franc ...
and
Spanish Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many Latin American countries **Spanish cuisine Other places * Spanish, Ontario, Can ...
, 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 A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
and planet depending on where the system was in its orbit. 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 August 22, two open meetings were held which ended in an abrupt about-face on the basic planetary definition. The position of astronomer
Julio Ángel Fernández Julio Ángel Fernández Alves (born 5 April 1946) is a Uruguayan astronomer and teacher, member of the department of astronomy at the Universidad de la República in Montevideo. He is also a member of PEDECIBA, (the program for development of ba ...
gained the upper hand among the members attending and was described as unlikely to lose its hold by August 24. This position would result in only eight major planets, with Pluto ranking as a "dwarf planet". 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 Static may refer to: Places *Static Nunatak, a nunatak in Antarctica United States * Static, Kentucky and Tennessee *Static Peak, a mountain in Wyoming **Static Peak Divide, a mountain pass near the peak Science and technology Physics *Static el ...
and
dynamic Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyn ...
physics; the main sticking point was 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. 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 An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
and to bring into the definition a criterion concerning the dominance of a body in its neighbourhood.


Final draft proposal

The final, third draft definition proposed on 24 August 2006 read:


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
Jocelyn Bell Burnell Dame Susan Jocelyn Bell Burnell (; Bell; born 15 July 1943) is an astrophysicist from Northern Ireland who, as a postgraduate student, discovered the first radio pulsars in 1967. The discovery eventually earned the Nobel Prize in Physics in ...
. During this session, IAU members cast votes on each resolution by raising yellow cards. A team of students counted the votes in each section of the auditorium, and astronomer
Virginia Trimble Virginia Louise Trimble (born 1943) is an American astronomer specializing in the structure and evolution of stars and galaxies, and the history of astronomy. She has published more than 600 works in Astrophysics, and dozens of other works in ...
compiled and tallied the vote counts. The IAU Executive Committee presented four Resolutions to the Assembly, each concerning a different aspect of the debate over the definition.IAU General Assembly Newspaper, 24 August 2006
/ref> 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 A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...
. 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 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 abstentions. A new category of dwarf planet was thus established. It would be named " 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. 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: The IAU further resolves: 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.
orig link
)


Closing issues


Substance

Alan Stern Sol Alan Stern (born November 22, 1957) is an American engineer and planetary scientist. He is the principal investigator of the ''New Horizons'' mission to Pluto and the Chief Scientist at Moon Express. Stern has been involved in 24 suborbita ...
, the lead scientist on NASA's robotic mission to Pluto, contended that
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
,
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
,
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...
, and
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...
have not fully cleared their orbital zones, just like Pluto. Earth orbits with 10,000
near-Earth asteroids A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun ( perihelion) is less than 1.3 astronomical units (AU ...
. 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." 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
Trojan asteroid In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger body, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its Lagrangian points and . Trojans can shar ...
s), these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Earth accretes or ejects near-Earth asteroids on million-year time scales, thereby clearing its orbit. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called plutinos, into a 3:2 resonance (i.e., they orbit the Sun twice for every three Neptune orbits). Since the orbits of these objects are entirely dictated by Neptune's gravity, Neptune is therefore gravitationally dominant. On June 11, 2008, the IAU announced that the subcategory of dwarf planets with trans-Neptunian orbits would be known as " plutoids". In an accompanying press release, the IAU said that: This subcategory includes
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
,
Haumea , discoverer = , discovered = , earliest_precovery_date = March 22, 1955 , mpc_name = (136108) Haumea , pronounced = , adjectives = Haumean , note = yes , alt_names = , named_after = Haumea , mp_category = , orbit_ref = , epoc ...
, Makemake and Eris. 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 orbit-clearing criterion. The wording of the 2006 definition is
heliocentric Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth ...
in its use of the word ''
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
'' instead of ''
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
or
stars A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...
'', and is thus not applicable to the numerous
objects Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ai ...
which have been identified in orbit around other stars. A separate "working" definition for extrasolar planets was, however, recommended by a working group of the IAU in 2003 and includes the criterion: "The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System."


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 Prague ( ; cs, Praha ; german: Prag, ; la, Praga) is the capital and largest city in the Czech Republic, and the historical capital of Bohemia. On the Vltava river, Prague is home to about 1.3 million people. The city has a temperate ...
and, thus, cast no vote. Only 424 astronomers were present for the vote, which is less than 5% of the astronomer community. However, sampling 400 representative members out of a population of 9,000 statistically yields a result with good accuracy (confidence interval better than 5%). 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.


Impact

The decision generated cultural and societal implications, affecting the "industry of astronomical artifacts and toys." 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 The ''World Book Encyclopedia'' is an American encyclopedia. The encyclopedia is designed to cover major areas of knowledge uniformly, but it shows particular strength in scientific, technical, historical and medical subjects. ''World Book'' wa ...
'' to hold off printing until a final result had been reached.


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 Christine Lavin (born January 2, 1952) is a New York City-based singer-songwriter and promoter of contemporary folk music. She has recorded numerous solo albums, and has also recorded with other female folk artists under the name Four Bitchin ...
. A good-natured protest against suggestions that Pluto is not a planet. * "Pluto" (1998), song by
2 Skinnee J's 2 Skinnee J's (also spelled Two Skinnee J's or Too Skinnee J's) is an American hip hop band from Brooklyn, New York, whose music has been categorized as nerdcore and rap rock. The band was founded in 1991 by Special J (vocals), Rabbi J-Slim (voc ...
. An impassioned defense of Pluto's status as a planet. * ''Thing a Week'', August 25, 2006 podcast by
Jonathan Coulton Jonathan William Coulton (born December 1, 1970), often called "JoCo" by fans, is an American folk/comedy singer-songwriter, known for his songs about geek culture and his use of the Internet to draw fans. Among his most popular songs are " Co ...
. Featured a song "I'm Your Moon", from Charon's point of view, about Pluto being reclassified as a dwarf planet. * "Bring Back Pluto" (2007), song by
Aesop Rock Ian Matthias Bavitz (born June 5, 1976), better known by his stage name Aesop Rock, is an American rapper and producer from Long Island, New York. He was at the forefront of the new wave of underground and alternative hip hop acts that emer ...
on the album ''
None Shall Pass ''None Shall Pass'' is the fifth studio album by American hip hop artist Aesop Rock. It was released on Definitive Jux on August 28, 2007. Music The album features production by Blockhead, El-P, Rob Sonic, and Aesop Rock himself. Guest appearan ...
''. Hip-hop song supporting Pluto's status as the 9th planet in the Solar System. * "Pluto" (2009), song by
Robbie Fulks Robert "Robbie" Fulks (born March 25, 1963) is an American alternative country singer-songwriter, instrumentalist and long-time Chicago, Illinois resident. He has released 13 albums over a career spanning more than 30 years. His 2016 record '' Up ...
, part of his release "50-vc. Doberman." About Pluto's reclassification, remembered as a 9th planet from the times of the singer's youth, and re-presents Pluto as an unforgotten monarch of the
Kuiper Belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
. * "Ode to Pluto" is the final track on Terra Lumina's self-titled debut album, mentioning the change of classification. * Pluto's demotion is alluded to in "The Lonesome Friends of Science" on
John Prine John Edward Prine (; October 10, 1946 – April 7, 2020) was an American singer-songwriter of country-folk music. He was active as a composer, recording artist, live performer, and occasional actor from the early 1970s until his death. He ...
's ''
The Tree of Forgiveness ''The Tree of Forgiveness'' is the eighteenth and final studio album by American country folk singer John Prine. The album was released on April 13, 2018. It is the last album released by Prine before his death on April 7, 2020. Background The al ...
'', in which the planets (including non-existent Vulcan) are anthropomorphized, and Pluto is "uninvited to the interplanetary dance." * "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.


Plutoed

The verb ''to pluto'' (
preterite The preterite or preterit (; abbreviated or ) is a grammatical tense or verb form serving to denote events that took place or were completed in the past; in some languages, such as Spanish, French, and English, it is equivalent to the simple ...
and
past participle In linguistics, a participle () (from Latin ' a "sharing, partaking") is a nonfinite verb form that has some of the characteristics and functions of both verbs and adjectives. More narrowly, ''participle'' has been defined as "a word derived from ...
: ''plutoed'') was coined in the aftermath of the 2006 IAU decision. In January 2007, the
American Dialect Society The American Dialect Society (ADS), founded in 1889, is a learned society "dedicated to the study of the English language in North America, and of other languages, or dialects of other languages, influencing it or influenced by it." The Society p ...
chose ''plutoed'' as its 2006
Word of the Year The word(s) of the year, sometimes capitalized as "Word(s) of the Year" and abbreviated "WOTY" (or "WotY"), refers to any of various assessments as to the most important word(s) or expression(s) in the public sphere The public sphere (german: Ö ...
, defining ''to pluto'' as "to demote or devalue someone or something, as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet." 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".


See also

*
Clearing the neighbourhood "Clearing the neighbourhood" (or dynamical dominance) around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise ...
* Fusor (astronomy) * Geophysical planet definition *
List of Solar System objects The following is a list of Solar System objects by orbit, ordered by increasing distance from the Sun. Most named objects in this list have a diameter of 500 km or more. *The Sun, a spectral class G2V main-sequence star *The inner Solar Sy ...
* List of former planets *
Minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
* Planemo *
Small Solar System body A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other obje ...


References


External links


IAU 2006 General Assembly: Result of the IAU Resolution votes
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
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 article on the new definition from SPACE.com
Dwarf planet discoverer Mike Brown explains the concept behind the new definition


{{DEFAULTSORT:IAU Definition Of Planet Redefinition of planet Planets Dwarf planets History of astronomy Definition of planet Pluto's planethood