It is estimated that there may be 200 dwarf planets in the Kuiper belt of the outer Solar System and possibly more than 10,000 in the region beyond. The International Astronomical Union (IAU) has accepted four as official dwarf planets: Pluto, Eris, Haumea, and Makemake, as well as Ceres in the inner Solar System.
Philip Metzger has recently published a case that all dwarf planets are planets. The qualifying feature of a dwarf planet is that it “has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape” is enough to categorize as a planet.
We have 8 official planets and one or two likely planet 9 and 10 candidates which have some evidence but have not been found.
According to Mike Brown of Thu Jun 21 2018 there are:
10 objects which are nearly certainly dwarf planets,
16 objects which are highly likely to be dwarf planets (not recounting the certain),
39 objects which are likely to be dwarf planets (not recounting certain and highly likely),
88 objects which are probably dwarf planets (without recounting above categories)
573 objects which are possibly dwarf planets (without recounting above categories).
Current observations are generally insufficient for a direct determination as to whether a body meets this definition. A dwarf planet may not be the satellite of another body, even though several moons (such as Titan) are larger than the recognized dwarf planets.
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 410–440 km in diameter), Michael Brown estimated that an icy body relaxes into hydrostatic equilibrium at a diameter somewhere between 200 and 400 km.
Ceres is thought to be the only dwarf planet in the asteroid belt. 4 Vesta, the second-most-massive asteroid, appears to have a fully differentiated interior and was therefore in equilibrium at some point in its history, but it is not today. The third-most massive object, 2 Pallas, has a somewhat irregular surface and is thought to have only a partially differentiated interior. Brown has estimated that, because rocky objects are more rigid than icy objects, rocky objects below 900 kilometers (560 mi) in diameter may not be in hydrostatic equilibrium and thus not dwarf planet.
In 2010, Gonzalo Tancredi presented a report to the IAU evaluating a list of 46 candidates for dwarf-planet status based on light-curve-amplitude analysis and the assumption that the object was more than 450 kilometres (280 mi) in diameter. Some diameters are measured, some are best-fit estimates, and others use an assumed albedo of 0.10. Of these, he identified 15 as dwarf planets by his criteria (including the four accepted by the IAU), with another nine being considered possible. To be cautious, he advised the IAU to “officially” accept as dwarf planets the top three not yet accepted: Sedna, Orcus, and Quaoar. Although the IAU had anticipated Tancredi’s recommendations, they have not responded.
Mike Brown considers a large number of trans-Neptunian bodies, ranked by estimated size, to be “probably” dwarf planets. He did 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: We are confident enough in the size estimate to know that each one of these must be a dwarf planet even if predominantly rocky.
Highly likely Anything larger than 600 km is all but certainly round. Even objects significantly smaller are likely round. The predicted and/or measured size of an object in this category would have to be grossly in error or the composition would have to be primarily rocky in order for it not to be a dwarf planet.
Likely: Anything icy larger than 500 km is highly likely to be round. But the size uncertainties are large enough that some of these objects could, in reality, be small enough to be less certain.
Probably: All icy satellite larger than 400 km are round, so we expect these objects to be round if the size estimate is correct.
Possibly: We don’t know where the transition from non-round to round occurs, but in icy satellites it is between 200 and 400 km. Objects this size in the Kuiper belt could thus possible be round, but we don’t know. Probably not: Below 200 km no icy satellite are round. We expect the same in the Kuiper belt. A few of these object could be bigger than expected, however, and could turn out to be large enough to round themselves.
The table also lists the estimated albedo used to determine the size or the calculated albedo from the measured size. Also listed is the absolute magnitude, which in this case refers to how bright the object would be if you were looking at it while you were standing on the surface of the sun and the object were at the distance of the earth. As in the rest of astronomy, smaller magnitudes are brighter and every 5 magnitudes represents a factor of 100.
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