Hydrostatic Equilibrium

Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force. For instance, the pressure gradient force prevents gravity from collapsing the Earth's atmosphere into a thin, dense shell, while gravity prevents the pressure gradient force from diffusing the atmosphere into space.

Hydrostatic equilibrium is the current distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology. This qualification typically means that the object is symmetrically rounded into a spheroid or ellipsoid shape, where any irregular surface features are due to a relatively thin solid crust. There are 31 such objects (apart from the Sun), sometimes called planemos, that are currently confirmed to exist in the Solar System, seven more which are virtually certain, and a hundred or so more which are likely.