The number of dwarf planets in the Solar System is unknown. Estimates have run as high as 200 in the Kuiper belt and over 10,000 in the region beyond. However, consideration of the surprisingly low densities and albedos of many large trans-Neptunian objects, as well as spectroscopic analysis of their surfaces, suggests that the number of dwarf planets may be much lower, perhaps only nine among bodies known so far. The International Astronomical Union (IAU) defines dwarf planets as being in hydrostatic equilibrium, and notes six bodies in particular: in the inner Solar System and five in the trans-Neptunian region: , , , , and .Accepted in a 2022–2023 annual report Only Pluto and Ceres have been confirmed to be in hydrostatic equilibrium, due to the results of the New Horizons and Dawn missions. Eris is generally assumed to be a dwarf planet because it is similar in size to Pluto and even more massive. Haumea and Makemake were accepted as dwarf planets by the IAU for naming purposes and will keep their names if it turns out they are not dwarf planets. Smaller trans-Neptunian objects have been called dwarf planets if they appear to be solid bodies, which is a prerequisite for hydrostatic equilibrium: planetologists generally include at least , , and . Quaoar was labelled as a dwarf planet in a 2022–2023 annual report, though it does not appear to be in hydrostatic equilibrium. In practice the requirement for hydrostatic equilibrium is often loosened to include all gravitationally rounded objects, even by the IAU, as otherwise Mercury would not be a planet.
Limiting values
Beside directly orbiting the Sun, the qualifying feature of a dwarf planet is that it have "sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape". Current observations are generally insufficient for a direct determination as to whether a body meets this definition. Often the only clues for trans-Neptunian objects (TNO) is a crude estimate of their diameters and albedos. Icy satellites as large as 1,500 km in diameter have proven to not be in equilibrium, whereas dark objects in the outer solar system often have low densities that imply they are not even solid bodies, much less gravitationally controlled dwarf planets.
, which has a significant amount of ice in its composition, is the only accepted dwarf planet in the asteroid belt, though there are unexplained anomalies. 4 Vesta, the second-most-massive asteroid and one that is basaltic in composition, appears to have a fully differentiated interior and was therefore in equilibrium at some point in its history, but no longer is today. The third-most massive object, 2 Pallas, has a somewhat irregular surface and is thought to have only a partially differentiated interior; it is also less icy than Ceres. Michael Brown has estimated that, because rocky objects such as Vesta are more rigid than icy objects, rocky objects below in diameter may not be in hydrostatic equilibrium and thus not dwarf planets. The two largest icy outer-belt asteroids 10 Hygiea and 704 Interamnia are close to equilibrium, but in Hygiea's case this may be a result of its disruption and the re-aggregation of its fragments, while Interamnia is now somewhat away from equilibrium due to impacts.
Based on a comparison with the icy moons that have been visited by spacecraft, such as Mimas (round at 400 km in diameter) and Proteus (irregular at 420 km in diameter), Brown estimated that an icy body relaxes into hydrostatic equilibrium at a diameter somewhere between 200 and 400 km. However, after Brown and Tancredi made their calculations, better determination of their shapes showed that Mimas and the other mid-sized ellipsoidal moons of Saturn up to at least Iapetus (which, at 1,471 km in diameter, is approximately the same size as Haumea and Makemake) are no longer in hydrostatic equilibrium; they are also icier than TNOs are likely to be. They have equilibrium shapes that froze in place some time ago, and do not match the shapes that equilibrium bodies would have at their current rotation rates. Thus Rhea, at 1528 km in diameter, is the smallest body for which gravitational measurements are consistent with current hydrostatic equilibrium. Ceres, at 950 km in diameter, is close to equilibrium, but some deviations from equilibrium shape remain unexplained. Much larger objects, such as Earth's moon and the planet Mercury, are not near hydrostatic equilibrium today, though the Moon is composed primarily of silicate rock and Mercury of metal (in contrast to most dwarf planet candidates, which are ice and rock). Saturn's moons may have been subject to a thermal history that would have produced equilibrium-like shapes in bodies too small for gravity alone to do so. Thus, at present it is unknown whether any trans-Neptunian objects smaller than Pluto and Eris are in hydrostatic equilibrium. Nonetheless, it does not matter in practice, because the precise statement of hydrostatic equilibrium in the definition is universally ignored in favour of roundness and solidity.
The majority of mid-sized TNOs up to about in diameter have significantly lower densities (~ ) than larger bodies such as Pluto (1.86 g/cm3). Brown had speculated that this was due to their composition, that they were almost entirely icy. However, Grundy et al. point out that there is no known mechanism or evolutionary pathway for mid-sized bodies to be icy while both larger and smaller objects are partially rocky. They demonstrated that at the prevailing temperatures of the Kuiper Belt, water ice is strong enough to support open interior spaces (interstices) in objects of this size; they concluded that mid-size TNOs have low densities for the same reason that smaller objects do—because they have not compacted under self-gravity into fully solid objects, and thus the typical TNO smaller than in diameter is (pending some other formative mechanism) unlikely to be a dwarf planet.
Assessment by Tancredi In 2010, Gonzalo Tancredi presented a report to the IAU evaluating a list of 46 trans-Neptunian candidates for dwarf planet status based on light-curve-amplitude analysis and a calculation that the object was more than in diameter. Some diameters were measured, some were best-fit estimates, and others used an assumed albedo of 0.10 to calculate the diameter. Of these, he identified 15 as dwarf planets by his criteria (including the 4 accepted by the IAU), with another 9 being considered possible. To be cautious, he advised the IAU to "officially" accept as dwarf planets the top three: Sedna, Orcus, and Quaoar. Although the IAU had anticipated Tancredi's recommendations, by late 2023 only Quaoar had been accepted.
Assessment by Brown Brown's categories Min. Number of objects Nearly certain > 900 km 10 Highly likely 600–900 km 17 (27 total) Likely 500–600 km 41 (68 total) Probably 400–500 km 62 (130 total) Possibly 200–400 km 611 (741 total)Source: Mike Brown, as of October 22, 2020
Mike Brown considers 130 trans-Neptunian bodies to be "probably" dwarf planets, ranked them by estimated size. He does not consider asteroids, stating "in the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round."
The terms for varying degrees of likelihood he split these into: Near certainty: diameter estimated/measured to be over . Sufficient confidence to say these must be in hydrostatic equilibrium, even if predominantly rocky. 10 objects as of 2020. Highly likely: diameter estimated/measured to be over . The size would have to be "grossly in error" or they would have to be primarily rocky to not be dwarf planets. 17 objects as of 2020. Likely: diameter estimated/measured to be over . Uncertainties in measurement mean that some of these will be significantly smaller and thus doubtful. 41 objects as of 2020. Probably: diameter estimated/measured to be over . Expected to be dwarf planets, if they are icy, and that figure is correct. 62 objects as of 2020. Possibly: diameter estimated/measured to be over . Icy moons transition from a round to irregular shape in the 200–400 km range, suggesting that the same figure holds true for KBOs. Thus, some of these objects could be dwarf planets. 611 objects as of 2020. Probably not: diameter estimated/measured to be under 200 km. No icy moon under 200 km is round, and the same may be true of KBOs. The estimated size of these objects would have to be in error for them to be dwarf planets.
Beside the five older accepted by the IAU plus Quaoar, the 'nearly certain' category includes , , , Máni, and . Note that although Brown's site claims to be updated daily, these largest objects have not been updated since late 2013, and no objects have been added to the list since late 2020; indeed, the current best diameter estimates for Salacia and Máni are less than 900 km. (Orcus is just above the threshold.)How many dwarf planets are there in the outer solar system? (updates daily), updated 2013-11-01
Assessment by Grundy et al.
Grundy et al. propose that dark, low-density TNOs in the size range of approximately are transitional between smaller, porous (and thus low-density) bodies and larger, denser, brighter, and geologically differentiated planetary bodies (such as dwarf planets). Bodies in this size range should have begun to collapse the interstitial spaces left over from their formation, but not fully, leaving some residual porosity.
Many TNOs in the size range of about have oddly low densities, in the range of about , that are substantially less than those of dwarf planets such as Pluto, Eris and Ceres, which have densities closer to 2. Brown has suggested that large low-density bodies must be composed almost entirely of water ice since he presumed that bodies of this size would necessarily be solid. However, this leaves unexplained why TNOs both larger than 1,000 km and smaller than 400 km, and indeed comets, are composed of a substantial fraction of rock, leaving only this size range to be primarily icy. Experiments with water ice at the relevant pressures and temperatures suggest that substantial porosity could remain in this size range, and it is possible that adding rock to the mix would further increase resistance to collapsing into a solid body. Bodies with internal porosity remaining from their formation could be at best only partially differentiated, in their deep interiors (if a body had begun to collapse into a solid body, there should be evidence in the form of fault systems from when its surface contracted). The higher albedos of larger bodies are also evidence of full differentiation, as such bodies were presumably resurfaced with ice from their interiors. Grundy et al. propose therefore that mid-size, low-density and low-albedo bodies such as Salacia, Varda, Gǃkúnǁʼhòmdímà, and Uni are not differentiated planetary bodies like Orcus, Quaoar, and Charon. The boundary between the two populations would appear to be in the range of about , although Grundy et al. also suggest that might constitute an upper limit to retaining significant porosity.
If Grundy et al. are correct, then very few known bodies in the outer Solar System are likely to have compacted into fully solid bodies, and thus to possibly have become dwarf planets at some point in their past or to still be dwarf planets at present. Pluto–Charon, Eris, Haumea, Gonggong, Makemake, Quaoar, and Sedna are either known (Pluto) or strong candidates (the others). Orcus is again just above the threshold by size, though it is bright.