The effective definition of a planet is a body too small to sustain nuclear reactions in its interior, but large enough that its thermal decay time is more than five billion years, the age of the solar system. By this definition, there are fifteen planets in the solar system: all the conventional planets but Pluto plus the seven large satellites. The evolution of a planet is rather insensitive to its dynamical circumstances (with one striking exception); once a planet is formed, evolution is largely controlled by its interior.