Empirical observations suggest that the lower limit may vary according to the composition of the object. For example, in the asteroid belt, Ceres, with a diameter of 975 km, is the only object presently known to be self-rounded, while 2 Pallas at approximately 600 km appears to be partially but incompletely differentiated. Therefore, it has been suggested that the limit where other rocky-ice bodies like Ceres become rounded might be somewhere around 900 km. The rocky body Vesta, at 530 km appears to have achieved equilibrium, only to have it disrupted by a massive impact after it solidified. More icy bodies like trans-Neptunian objects have less rigid interiors and therefore more easily relax under their self-gravity into a rounded shape. The smallest icy body known to have achieved hydrostatic equilibrium is Mimas, while the largest irregular one is Proteus; both average slightly more than 400 km (250 mi) in diameter. Mike Brown (a leading researcher in this field and discoverer of Eris) suggests that the lower limit for an icy dwarf planet is therefore likely to be somewhere under 400 km.
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