  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.
