Much is made of finding exoplanets in habitable zones, locations with orbital semi-major axes permitting water in the liquid state. Habitability may be compromised if such bodies become tidally locked, orbiting within the star's tidal lock radius. The illustration below shows the tidal lock radius for various stellar spectral classes:
The diagram shows Mercury within the tidally locked zone although its really in a 3:2 spin-orbit resonance. How is the tidal lock radius calculated?
Best Answer
From Wikipedia (which cites the paywalled http://dx.doi.org/10.1006/icar.1996.0117), we get http://en.wikipedia.org/wiki/Tidal_locking#Timescale
Now, a is the semi-major axis (or orbital radius) of the object (I'm not totally sure if the logic changes for elliptical orbits). Anyways, we can easily rearrange the equation to express a (or the radius) in terms of $t_{lock}$ (or the time) and all the other variables.
Then (using the Wikipedia variable names) we get $$a = (\frac{3t_{lock}Gm_p^2 k_2 r^5}{wIQ})^{1/6}$$
So then if we know t (which is basically the age of the stellar system in question – which we can get a rough idea of since we do have a rough idea of how each star is), then we can get a value of a for that particular t.