List of possible dwarf planets
[2] However, consideration of the surprisingly low densities 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.
[3][4] The International Astronomical Union (IAU) defines dwarf planets as being in hydrostatic equilibrium, and notes six bodies in particular: Ceres in the inner Solar System and five in the trans-Neptunian region: Pluto, Eris, Haumea, Makemake, and Quaoar.
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 Gonggong, Orcus, and Sedna.
In practice the requirement for hydrostatic equilibrium is often loosened to include all gravitationally rounded objects, even by the IAU, as otherwise even Mercury would not be a planet.
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".
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.
[10] 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.
[1] 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.
However, Grundy et al.[3] 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.
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 450 kilometres (280 mi) in diameter.
"[20] The terms for varying degrees of likelihood he split these into: Beside the five older accepted by the IAU plus Quaoar, the 'nearly certain' category includes Gonggong, Sedna, Orcus, 2002 MS4, and Salacia.
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.
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.
In 2023, Emery et al. wrote that near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 suggests that Sedna, Gonggong and Quaoar internally melted and differentiated and are chemically evolved, like the larger dwarf planets Pluto, Eris, Haumea, and Makemake, but unlike "all smaller KBOs".
On the other hand, the surfaces of Sedna, Gonggong and Quaoar have low abundances of CO and CO2, similar to Pluto, Eris and Makemake but in contrast to smaller bodies.
This suggests that the threshold for dwarf planethood in the trans-Neptunian region is a diameter of ~1000 km (thus including only Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar and possibly Sedna).
Emery et al. suggest that Sedna, Quaoar, and Gonggong went through internal melting, differentiation, and chemical evolution like the larger dwarf planets, but that all smaller KBOs did not.
They hypothesised that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon Weywot.
[31] Two moons are included for comparison: Triton likely formed as a TNO before it was captured by Neptune, and Charon is larger than some dwarf planet candidates.
