  The habitable zones of main sequence stars have traditionally been defined as
the range of orbits that intercept the appropriate amount of stellar flux to
permit surface water on a planet. Terrestrial exoplanets discovered to orbit M
stars in these zones, which are close-in due to decreased stellar luminosity,
may also undergo significant tidal heating. Tidal heating may span a wide range
for terrestrial exoplanets and may significantly affect conditions near the
surface. For example, if heating rates on an exoplanet are near or greater than
that on Io (where tides drive volcanism that resurface the planet at least
every 1 Myr) and produce similar surface conditions, then the development of
life seems unlikely. On the other hand, if the tidal heating rate is less than
the minimum to initiate plate tectonics, then CO_2 may not be recycled through
subduction, leading to a runaway greenhouse that sterilizes the planet. These
two cases represent potential boundaries to habitability and are presented
along with the range of the traditional habitable zone for main sequence,
low-mass stars. We propose a revised habitable zone that incorporates both
stellar insolation and tidal heating. We apply these criteria to GJ 581 d and
find that it is in the traditional habitable zone, but its tidal heating alone
may be insufficient for plate tectonics.
