  Potentially habitable planets can orbit close enough to their host star that
the differential gravity across their diameters can fix the rotation rate at a
specific frequency, a process called tidal locking. Tidally locked planets on
circular orbits will rotate synchronously, but those on eccentric orbits will
either librate or rotate super-synchronously. I calculate how habitable planets
evolve under two commonly-used models and find, for example, that one model
predicts that the Earth's rotation rate would have synchronized after 4.5 Gyr
if its initial rotation period was 3 days, it had no satellites, and it always
maintained the modern Earth's tidal properties. Lower mass stellar hosts will
induce stronger tidal effects on potentially habitable planets, and tidal
locking is possible for most planets in the habitable zones of GKM dwarf stars.
For fast rotating planets, both models predict eccentricity growth and that
circularization can only occur once the rotational frequency is similar to the
orbital frequency. The orbits of potentially habitable planets of very late M
dwarfs (<0.15 solar masses) are very likely to be circularized within 1 Gyr and
hence those planets will be synchronous rotators. Proxima b is almost assuredly
tidally locked, but its orbit may not have circularized yet, so the planet
could be rotating super-synchronously today. The evolution of the isolated and
potentially habitable Kepler planet candidates is computed and about half could
be tidally locked. Finally, projected TESS planets are simulated over a wide
range of assumptions, and the vast majority of all cases are found to tidally
lock within 1 Gyr. These results suggest that the process of tidal locking is a
major factor in the evolution of most of the potentially habitable exoplanets
to be discovered in the near future. [abridged]
