  Terrestrial planets are more likely to be detected if they orbit M dwarfs due
to the favorable planet/star size and mass ratios. However, M dwarf habitable
zones are significantly closer to the star than the one around our Sun, which
leads to different requirements for planetary habitability and its detection.
We review 1) the current limits to detection, 2) the role of M dwarf spectral
energy distributions on atmospheric chemistry, 3) tidal effects, stressing that
tidal locking is not synonymous with synchronous rotation, 4) the role of
atmospheric mass loss and propose that some habitable worlds may be the
volatile-rich, evaporated cores of giant planets, and 5) the role of planetary
rotation and magnetic field generation, emphasizing that slow rotation does not
preclude strong magnetic fields and their shielding of the surface from stellar
activity. Finally we present preliminary findings of the NASA Astrobiology
Institute's workshop "Revisiting the Habitable Zone." We assess the
recently-announced planet Gl 581 g and find no obvious barriers to
habitability. We conclude that no known phenomenon completely precludes the
habitability of terrestrial planets orbiting cool stars.
