  Exoplanet habitability is traditionally assessed by comparing a planet's
semi-major axis to the location of its host star's "habitable zone," the shell
around a star for which Earth-like planets can possess liquid surface water.
The Kepler space telescope has discovered numerous planet candidates near the
habitable zone, and many more are expected from missions such as K2, TESS and
PLATO. These candidates often require significant follow-up observations for
validation, so prioritizing planets for habitability from transit data has
become an important aspect of the search for life in the universe. We propose a
method to compare transiting planets for their potential to support life based
on transit data, stellar properties and previously reported limits on planetary
emitted flux. For a planet in radiative equilibrium, the emitted flux increases
with eccentricity, but decreases with albedo. As these parameters are often
unconstrained, there is an "eccentricity-albedo degeneracy" for the
habitability of transiting exoplanets. Our method mitigates this degeneracy,
includes a penalty for large-radius planets, uses terrestrial mass-radius
relationships, and, when available, constraints on eccentricity to compute a
number we call the "habitability index for transiting exoplanets" that
represents the relative probability that an exoplanet could support liquid
surface water. We calculate it for Kepler Objects of Interest and find that
planets that receive between 60-90% of the Earth's incident radiation, assuming
circular orbits, are most likely to be habitable. Finally, we make predictions
for the upcoming TESS and JWST missions.
