|The first five bodies recognized or named as dwarf planets:
A dwarf planet is a planetary-mass object that does not dominate its region of space (as a true planet does) and is not a satellite. That is, it is in direct orbit of its star and is massive enough to be plastic - for its gravity to maintain it in a hydrostatically equilibrious shape (usually a spheroid) - but has not cleared the neighborhood around its orbit of other material. The number of dwarf planets in the Solar System is unknown, as determining whether a potential body is a dwarf planet requires close observation. The interest of dwarf planets to planetary geologists is that, being differentiated and perhaps geologically active bodies, they are likely to display planetary geology.
The term dwarf planet was adopted by the International Astronomical Union (IAU) in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size and finally precipitated by the discovery of Eris, an object even more massive than Pluto. The exclusion of bodies such as Eris and Pluto from the roster of planets has been both praised and criticized.
Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter that for decades were considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid for the smaller bodies and then stopped naming or classifying them as planets.
With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates. It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon. Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets.
In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away. Many of these shared several of Pluto's key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects, the plutinos. It became clear that either the larger of these bodies would also have to be classified as planets, or Pluto would have to be reclassified, much as Ceres had been reclassified after the discovery of additional asteroids. This led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets. Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain classified as a planet.
Eris (then known as ) was discovered in January 2005; it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet. As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006. The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi: they proposed an intermediate category for objects large enough to be round but which had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits.
The IAU's final Resolution 5A preserved this three-category system for the celestial bodies orbiting the Sun. It reads:
|"||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 planet1 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.
The IAU never did establish a process to assign borderline objects, leaving such judgements to astronomers. However, it did subsequently establish guidelines for which IAU committee would get to oversee the naming of possible dwarf planets: unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a minimum diameter of 838 km corresponding to a geometric albedo of 1) were to be named by the dwarf-planet naming committee. At the time (and still as of 2019), the only bodies to meet the naming criterion were Haumea and Makemake.
These five bodies - the three under consideration in 2006 (Pluto, Ceres and Eris) plus the two named in 2008 (Haumea and Makemake) - are commonly presented as the dwarf planets of the Solar System. However, only two of them - Ceres and Pluto - have been observed in enough detail to verify that their current shapes fit what would be expected from hydrostatic equilibrium, and doubts have been raised about whether Haumea has the proper shape for its current rotation. On the other hand, other objects have been called dwarf planets by astronomers. For instance, JPL/NASA characterized as a dwarf planet after observations in 2016, and Simon Porter spoke of "the big eight [TNO] dwarf planets" in 2018.
Although concerns were raised about the classification of planets orbiting other stars, the issue was not resolved; it was proposed instead to decide this only when such objects start to be observed.
The term dwarf planet has itself been somewhat controversial, as it could imply that these bodies are planets, much as dwarf stars are stars. This is the conception of the Solar System that Stern promoted when he coined the phrase. The older word planetoid ("having the form of a planet") has no such connotation, and is also used by astronomers for bodies that fit the IAU definition. Another word occasionally used is quasi-planet.
Michael E. Brown states that planetoid is "a perfectly good word" that has been used for these bodies for years, and that the use of the term dwarf planet for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution. Indeed, the draft of Resolution 5A had called these median bodies planetoids, but the plenary session voted unanimously to change the name to dwarf planet. The second resolution, 5B, defined dwarf planets as a subtype of planet, as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. Resolution 5B was defeated in the same session that 5A was passed. Because of the semantic inconsistency of a dwarf planet not being a planet due to the failure of Resolution 5B, alternative terms such as nanoplanet and subplanet were discussed, but there was no consensus among the CSBN to change it.
In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine, Spanish planeta enano, German Zwergplanet, Russian karlikovaya planeta (? ?), Arabic kaukab qazm (? ), Chinese ?ixíngx?ng (?), Korean waesohangseong (?; ?), but in Japanese they are called junwakusei () meaning "quasiplanets" or "peneplanets".
IAU Resolution 6a of 2006 recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects but neither name was carried forward, perhaps due to objections from geologists that this would create confusion with their pluton.
On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids. The authority of that initial announcement has not been universally recognized:
|"||...in part because of an email miscommunication, the WG-PSN [Working Group for Planetary System Nomenclature] was not involved in choosing the word plutoid. ... In fact, a vote taken by the WG-PSN subsequent to the Executive Committee meeting has rejected the use of that specific term..."||"|
The category of 'plutoid' captured the earlier distinction between the 'terrestrial dwarf' Ceres and the 'ice dwarfs' of the outer Solar system. However, since the Dawn mission of 2015 it has been recognized that Ceres, which originated in the outer Solar system, is analogous to the icy moons of the outer planets and to TNOs such as Pluto rather than to the terrestrial planets, blurring the distinction.
|Body||M/M? (1)||? (2)||µ (3)||? (4)|
Showing the planets and the largest known sub-planetary objects (purple) covering the orbital zones containing likely dwarf planets. All known possible dwarf planets have smaller discriminants than those shown for that zone.
Alan Stern and Harold F. Levison introduced a parameter ? (lambda), expressing the likelihood of an encounter resulting in a given deflection of orbit. The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. This value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where ? > 1 will eventually clear it. A gap of five orders of magnitude in ? was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects.
Using this parameter, Steven Soter and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), whereas dwarf planets lack the mass to do so. Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol µ (mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where µ > 100 is deemed to be cleared.
Jean-Luc Margot refined Stern and Levison's concept to produce a similar parameter ? (Pi). It is based on theory, avoiding the empirical data used by ?. ? > 1 indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets.
Sufficient internal pressure, caused by the body's gravitation, will turn a body plastic, and sufficient plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to have an irregular shape and may be rubble piles. Larger objects, where gravitation is significant but not dominant, are "potato" shaped; the more massive the body is, the higher its internal pressure, the more solid it is and the more rounded its shape, until the pressure is sufficient to overcome its internal compressive strength and it achieves hydrostatic equilibrium. At this point a body is as round as it is possible to be, given its rotation and tidal effects, and is an ellipsoid in shape. This is the defining limit of a dwarf planet.
When an object is in hydrostatic equilibrium, a global layer of liquid covering its surface would form a liquid surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. If the body does not rotate, it will be a sphere, but the faster it rotates, the more oblate or even scalene it becomes. If such a rotating body were to be heated until it melted, its overall shape would not change when liquid. The extreme example of a non-spherical body in hydrostatic equilibrium is Haumea, which is twice as long along its major axis as it is at the poles. If the body has a massive nearby companion, then tidal forces come into effect as well, distorting it into a scalene ellipsoid. An example of this is Jupiter's moon Io, which is the most volcanically active body in the Solar System due to effects of tidal heating. Tidal forces also cause a body's rotation to gradually become tidally locked, such that it always presents the same face to its companion. An extreme example of this is the Pluto-Charon system, where both bodies are tidally locked to each other. Earth's Moon is also tidally locked, as are many satellites of the gas giants.
The upper and lower size and mass limits of dwarf planets have not been specified by the IAU. There is no defined upper limit, and an object larger or more massive than Mercury that has not "cleared the neighbourhood around its orbit" would be classified as a dwarf planet. The lower limit is determined by the requirements of achieving a hydrostatic equilibrium shape, but the size or mass at which an object attains this shape depends on its composition and thermal history. The original draft of the 2006 IAU resolution redefined hydrostatic equilibrium shape as applying "to objects with mass above 5×1020 kg and diameter greater than 800 km", but this was not retained in the final draft.
The number of dwarf planets in the Solar system is not known. The three objects under consideration during the debates leading up to the 2006 IAU acceptance of the category of dwarf planet, Ceres, Pluto and Eris, are universally accepted as dwarf planets, including by those astronomers who continue to classify dwarf planets as planets. In 2015, Ceres and Pluto were determined to have shapes consistent with hydrostatic equilibrium (and thus with being dwarf planets) by the Dawn and New Horizons missions. Eris is universally assumed to be a dwarf planet because it is more massive than Pluto.
In order of discovery, these three bodies are:
Due to the 2008 decision to assign the naming of Haumea and Makemake to the dwarf-planet naming committee and their announcement as dwarf planets in IAU press releases, these two bodies are also generally accepted as dwarf planets, despite the subsequent doubts raised about Haumea:
Four additional bodies meet the criteria of Brown, Tancredi et al. and Grundy et al. for candidate objects:
At the time Makemake and Haumea were named, it was thought that trans-Neptunian objects (TNOs) with icy cores would require a diameter of only perhaps 400 km (250 mi)--about 3% of that of Earth--to relax into gravitational equilibrium. Researchers thought that the number of such bodies could prove to be around 200 in the Kuiper belt, with thousands more beyond. This was one of the reasons (keeping the roster of 'planets' to a reasonable number) that Pluto was reclassified in the first place. However, research since then has cast doubt on the idea that bodies that small could have achieved or maintained equilibrium under common conditions.
Individual astronomers have recognized a number of such objects as dwarf planets or as highly likely to prove to be dwarf planets. Brown maintains a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets. However, Grundy et al. have proposed that dark, low-density bodies smaller than ca. 900-1000 km in diameter may retain internal porosity from their formation, never having fully collapsed into solid planetary bodies, in which case they could not be dwarf-planets.
As of May 20, 2019, Brown's list identifies ten trans-Neptunian objects with diameters greater than 900 km (the four named by the IAU plus , Quaoar, Sedna, Orcus, and Salacia) as "near certain" to be dwarf planets, and another 16, with diameters greater than 600 km, as "highly likely". Notably, may have a larger diameter than Pluto's largest moon Charon (1212 km).
In 2008, Tancredi et al. advised the IAU to officially accept Orcus, Sedna and Quaoar as dwarf planets, though the IAU did not address the issue then and has not since. In addition, Tancredi considered the five TNOs Varuna, Ixion, , , and to mostly likely be dwarf planets as well.
In 2012, Stern stated that there are more than a dozen known dwarf planets, though he did not specify which they were.
However, in 2019 Grundy et al. cast doubt on the possibility that Salacia, Varda and similarly sized objects were solid bodies, and thus the possibility that they could be in hydrostatic equilibrium, while accepting that brighter (albedo > ?0.2) or denser (> ?1.4 g/cc) Orcus and Quaoar probably were solid bodies.
The following Trans-Neptunian objects are agreed by Brown, Tancredi et al. and Grundy et al. to be likely to be dwarf planets. Charon, a moon of Pluto that was proposed as a dwarf planet by the IAU in 2006, is included for comparison. Those objects that have absolute magnitudes greater than +1, and so meet the criteria for the dwarf-planet naming committee of the IAU, are highlighted, as is Ceres, which has been accepted as a dwarf planet by the IAU since they first debated the concept.
Vesta, the next-most-massive body in the asteroid belt after Ceres, was once in hydrostatic equilibrium and is roughly spherical, deviating mainly because of massive impacts that formed Rheasilvia and Veneneia craters after it solidified. Its dimensions are not consistent with it currently being in hydrostatic equilibrium.Triton is more massive than Eris or Pluto, has an equilibrium shape, and is thought to be a captured dwarf planet (likely a member of a binary system), but no longer directly orbits the sun.Phoebe is a captured centaur that, like Vesta, is no longer in hydrostatic equilibrium, but is thought to have been so early in its history due to radiogenic heating.
On March 6, 2015, the Dawn spacecraft began to orbit Ceres, becoming the first spacecraft to orbit a dwarf planet. On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons. Ceres displays such planetary-geologic features as surface salt deposits and cryovolcanos, while Pluto has water-ice mountains drifting in nitrogen-ice glaciers, as well as of course an atmosphere. For both bodies, there is at least the possibility of a subsurface ocean or brine layer.
Dawn has also orbited the former dwarf planet Vesta. Phoebe has been explored by Cassini (most recently) and Voyager 2, which also explored Neptune's moon Triton. These three bodies are thought to be former dwarf planets and therefore their exploration helps in the study of the evolution of dwarf planets.
In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution. Campaigns included car bumper stickers and T-shirts.Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.
NASA has announced that it will use the new guidelines established by the IAU.Alan Stern, the director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets. Thus, in 2011, he still referred to Pluto as a planet, and accepted other dwarf planets such as Ceres and Eris, as well as the larger moons, as additional planets. Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets".
Nineteen moons have an equilibrium shape from having having relaxed under their own gravity at some point in their history, though some have since frozen solid and are no longer in equilibrium. Seven are more massive than either Eris or Pluto. These moons are not physically distinct from the dwarf planets, but do not fit the IAU definition because they do not directly orbit the Sun. (Indeed, Neptune's moon Triton is a captured dwarf planet, and Ceres formed in the same region of the Solar system as the moons of Jupiter and Saturn.) Alan Stern calls planetary-mass moons "satellite planets", one of three categories of planet, together with dwarf planets and classical planets. The term planemo ("planetary-mass object") also covers all three populations.
There has been some debate as to whether the Pluto-Charon system should be considered a double (dwarf) planet. In a draft resolution for the IAU definition of planet, Pluto and Charon were considered dwarf planets in a binary system, given that at the time both were thought to be in hydrostatic equilibrium and that they revolved around a common center of mass located between them (rather than within the larger body).[note 1] The IAU currently states that Charon is not considered to be a dwarf planet and is just a satellite of Pluto, although the idea that Charon might qualify to be a dwarf planet in its own right may be considered at a later date. However, it's no longer clear that Charon is in hydrostatic equilibrium. In addition, the location of the barycenter depends not only on the relative masses of the bodies, but also on the distance between them; the barycenter of the Sun-Jupiter orbit, for example, lies outside the Sun, but they are not considered a double star.