  The determination of an exoplanet as rocky is critical for the assessment of
planetary habitability. Observationally, the number of small-radius, transiting
planets with accompanying mass measurements is insufficient for a robust
determination of the transitional mass or radius. Theoretically, models predict
that rocky planets can grow large enough to become gas giants when they reach
~10 Earth-masses, but the transitional mass remains unknown. Here I show how
transit data, interpreted in the context of tidal theory, can reveal the
critical radius that separates rocky and gaseous exoplanets. Standard tidal
models predict that rocky exoplanets' orbits are tidally circularized much more
rapidly than gaseous bodies', suggesting the former will tend to be found on
circular orbits at larger semi-major axes than the latter. Well-sampled
transits can provide a minimum eccentricity of the orbit, allowing a
measurement of this differential circularization. I show that this effect
should be present in the data from the Kepler spacecraft, but is not apparent.
Instead, it appears that there is no evidence of tidal circularization at any
planetary radius, probably because the publicly-available data, particularly
the impact parameters, are not accurate enough. I also review the bias in the
transit duration toward values that are smaller than that of planets on
circular orbits, stressing that the azimuthal velocity of the planet determines
the transit duration. The ensemble of Kepler planet candidates may be able to
determine the critical radius between rocky and gaseous exoplanets, tidal
dissipation as a function of planetary radius, and discriminate between tidal
models.
