There are no specific size or mass limits of dwarf planets, as those are not defining features. There is no clear upper limit: an object very far out in the Solar System that is more massive than Mercury might not have had time to clear its neighbourhood; such a body would fit the definition of dwarf planet rather than planet. Indeed, Mike Brown set out to find such an object. The lower limit is determined by the requirements of achieving and retaining hydrostatic equilibrium, but the size or mass at which an object attains and retains equilibrium depends on its composition and thermal history, not simply its mass. An IAU 2006 press release question-and-answer section estimated that objects with mass above 0.5 × 10 kg and radius greater than 400 km would "normally" be in hydrostatic equilibrium (the shape... would normally be determined by self-gravity), but that all borderline cases would need to be determined by observation. This is close to what as of 2019 is believed to be roughly the limit for objects beyond Neptune that are fully compact, solid bodies, with Salacia ( r = 423 ± 11 km , m = (0.492 ± 0.007) × 10 kg ) being a borderline case both for the 2006 Q&A expectations and in more recent evaluations, and with Orcus being just above the expected limit. No other body with a measured mass is close to the expected mass limit, though several without a measured mass approach the expected size limit.