Tidal Effects on the Habitability of Exoplanets: The Case of GJ 581 d

Tides may be crucial to the habitability of exoplanets. If such planets form around low-mass stars, then those in the circumstellar habitable zone will be close enough to their host stars to experience strong tidal forces. Tides may result in orbital decay and circularization, evolution toward zero obliquity, a fixed rotation rate (not necessarily synchronous), and substantial internal heating. Due to tidal effects, the range of habitable orbital locations may be quite different from that defined by the traditional concept of a habitable zone (HZ) based on stellar insolation, atmospheric effects, and liquid water on a planet's surface. Tidal heating may make locations within the traditional HZ too hot, while planets outside the traditional zone could be rendered quite habitable due to tides. Here we consider these effects on the exoplanet GJ 581 d.