  Stable, hydrogen-burning, M dwarf stars comprise about 75% of all stars in
the Galaxy. They are extremely long-lived and because they are much smaller in
mass than the Sun (between 0.5 and 0.08 MSun), their temperature and stellar
luminosity are low and peaked in the red. We have re-examined what is known at
present about the potential for a terrestrial planet forming within, or
migrating into, the classic liquid-surface-water habitable zone close to an M
dwarf star. Observations of protoplanetary disks suggest that planet-building
materials are common around M dwarfs, but N-body simulations differ in their
estimations of the likelihood of potentially-habitable, wet planets residing
within their habitable zones, which are only ~ 1/5 to 1/50 of the width of that
for a G star. Particularly in light of the claimed detection of the planets
with masses as small as 5.5 and 7.5 MEarth orbiting M stars, there seems no
reason to exclude the possibility of terrestrial planets. Tidally locked
synchronous rotation within the narrow habitable zone doesn't necessarily lead
to atmospheric collapse, and active stellar flaring may not be as much of an
evolutionarily disadvantageous factor as has previously been supposed. We
conclude that M dwarf stars may indeed be viable hosts for planets on which the
origin and evolution of life can occur. A number of planetary processes such as
cessation of geothermal activity, or thermal and non-thermal atmospheric loss
processes may limit the duration of planetary habitability to periods far
shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes
sense to include M dwarf stars in programs that seek to find habitable worlds
and evidence of life.
