From the wikipedia article about Ceres[1]:

> A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet"[2].

[1] https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)

[2] https://www.webcitation.org/62D6DNhLH?url=http://www.iau.org...