The upper and lower size and mass limits of dwarf planets have not been specified by the IAU. (International Astronomical Union) 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 5×10^20 kg and diameter greater than 800 km”, but this was not retained in the final draft.
Empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object. For a body made of rigid silicates, such as the stony asteroids, the transition to hydrostatic equilibrium should occur at a diameter of approximately 600 km and a mass of some 3.4×10^20 kg. For a body made of less rigid water ice, the limit should be about 320 km and 10^19 kg.
Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered. More than 100,000 Kuiper Belt Objects over 100 km (62 mi) in diameter are believed to exist.
Unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1) are to be named under the assumption that they are dwarf planets.
Mike Brown currently has a list of 82 probable “probably” plutoids. There is also a list of 264 possible objects. Asteroids are not considered.
The terms for varying degrees of likelihood are:
Near certainty: Sufficient confidence to say these must be in hydrostatic equilibrium even if predominantly rocky.
Highly likely: Estimated/measured to be over 600 km. The size would have to be “grossly in error” or they would have to be primarily rocky to not be dwarf planets.
Likely: Estimated/measured to be over 500 km. Uncertainties in measurement mean that some of these will be significantly smaller and thus doubtful.
Probable: Estimated/measured to be over 400 km. Expected to be dwarf planets if they are icy and that figure is correct.
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