What are dwarf planets and how many are there?

An illustration of the known dwarf planets and their moons. From left to right: Pluto (with Charon, Hydra, Myx, Kerberos, and Styx), Eris (with Dysnomia), Makemake, Ceres, and Haumea (with Hiaka and Namaka). (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via…