Now that we have explored the properties of the planets and their moons in Chapters 7 and 8, we finish studying the objects orbiting the Sun in this chapter. In Section 5-8, we saw that astronomers have several overlapping ways of classifying the bodies orbiting the Sun. To briefly review, recall that a planet is a celestial body that (1) is in orbit around the Sun, (2) has sufficient mass for its self-gravity to pull itself into a nearly spherical shape (although rotation causes many planets to be wider at their equators than at their poles), and (3) also has enough mass for its gravitational force to clear the neighborhood around its orbit. A dwarf planet satisfies (1) and (2), but it does not have enough gravity to clear its orbital neighborhood of debris. (Note that the dwarf planet Haumea is egg-shaped, rather than being uniformly wider at its equator, like Earth. This is apparently due to Haumea’s exceptionally rapid rotation.) All objects in the solar system that are not planets, dwarf planets, or moons are called small solar system bodies (SSSBs).

So far, Pluto, Ceres (also an asteroid), Eris (named after a Greek goddess), Makemake (pronounced mah-key mah-key and named after a god of the Rapa Nui people of Easter Island), and Haumea (pronounced how-may-uh and named after a goddess of Hawaii) have earned the title of dwarf planet. The latter three bodies were discovered in 2005. Dozens of other solar system objects are being evaluated for inclusion in this class. Bear in mind that as the properties of more solar system objects are pinned down, the definitions of dwarf planets and SSSBs may be refined.