Planets and Dwarf Planets

• Planet - This is a celestial body that: (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape (due to a hydrostatic equilibrium); and (c) has cleared the neighborhood around its orbit. As now defined, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

• Dwarf Planet - This is a celestial body that: (a) is in orbit around the Sun; and (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape (due to a hydrostatic equilibrium); but (c) has not cleared the neighborhood around its orbit; and (d) is not a satellite of another planet. As defined, the Solar System has probably has at least one dwarf planet in the Main Asteroid Belt (Ceres), Edgeworth-Kuiper Belt (Pluto and Eris); and Oort Cloud (possibly Sedna). In addition, the "dwarf planet" Pluto is recognized as a prototype of a new class of trans-Neptunian objects.

• Smaller Solar System Bodies - This category collectively refers to all other celestial bodies orbiting our Sun, Sol (including asteroids, comets, and satellites).

  Currently, the International Astronomical Union (IAU) has recognized five dwarf planets, Ceres, the trans-Neptunian object Eris (2003 UB313), Pluto, Haumea, and Makemake.