At least nineteen moons have equilibrium shape from having relaxed under self-gravity at some point, though some have since frozen solid and are no longer in equilibrium. Seven are more massive than either Eris or Pluto. These moons are not physically distinct from the dwarf planets, but do not fit the IAU definition because they do not directly orbit the Sun. (Indeed, Neptune's moon Triton is a captured dwarf planet, and Ceres formed in the same region of the Solar System as the moons of Jupiter and Saturn.) Alan Stern calls planetary-mass moons " satellite planets ", one of three categories of planet, together with dwarf planets and classical planets. The term planemo ("planetary-mass object") also covers all three populations.