  Stellar activity and rotation frustrate the detection of exoplanets through
the radial velocity technique. This effect is particularly of concern for M
dwarfs, which can remain magnetically active for billions of years. We compile
rotation periods for late-type stars and for the M dwarf planet-host sample in
order to investigate the rotation periods of older field stars across the main
sequence. We show that for stars with masses between 0.25 and 0.5 solar masses
(M4V to M1V), the stellar rotation period typical of field stars coincides with
the orbital periods of planets in the habitable zone. This will pose a
fundamental challenge to the discovery and characterization of potentially
habitable planets around early M dwarfs. Due to the longer rotation periods
reached by mid M dwarfs and the shorter orbital period at which the planetary
habitable zone is found, stars with masses between 0.1 and 0.25 solar masses
(M6V to M4V) offer better opportunities for the detection of habitable planets
via radial velocities.
