Limiting Values
The qualifying feature of dwarf planets is that they "have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical shape)." Except for Pluto and Ceres, current observations are insufficient for a direct determination if a body meets this definition. However, Michael Brown estimates that an icy body relaxes into hydrostatic equilibrium at a diameter somewhere between 200 and 400 km. Thus, all the TNOs listed below are estimated to be at least 400 km in diameter, though not all bodies estimated to be that size are included. The lists are further complicated by bodies such as 1999 TC36 that were at first assumed to be large single objects but were later discovered to be smaller binary or triple systems.
Ceres is the only identified dwarf planet in the asteroid belt. The most likely other possibility is 4 Vesta, the second-most-massive asteroid; Vesta appears to have a fully differentiated interior and was therefore in equilibrium at some point in its history, though it apparently 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 estimates that, since rocky objects are more rigid than than icy objects, rocky objects below 900 km in diameter may not be in hydrostatic equilibrium and thus not dwarf planets.
Read more about this topic: List Of Dwarf Planet Candidates
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