What makes a planet habitable?

The Milky Way Galaxy teems with planetary systems, most of which are unlike our own (1). It is tempting to assume that life can only originate on a planet that is similar to Earth, but different kinds of planets may be able to sustain Earth-like features that could be important for habitability. To focus the search for extraterrestrial life, scientists must assess which features of Earth are essential to the development and sustenance of life for billions of years and whether the formation of such planets is common. External effects such as stellar variability and orbital stability can affect habitability, but internal planetary processes that sustain a clement surface are essential to life; these processes are, however, difficult to characterize remotely. A combination of observations, experiments, and modeling are needed to understand the role of planetary interiors on habitability and guide the search for extraterrestrial life.