  A key goal of the Kepler mission is the discovery of Earth-size transiting
planets in "habitable zones" where stellar irradiance maintains a temperate
climate on an Earth-like planet. Robust estimates of planet radius and
irradiance require accurate stellar parameters, but most Kepler systems are
faint, making spectroscopy difficult and prioritization of targets desirable.
The parameters of 2035 host stars were estimated by Bayesian analysis and the
probabilities p(HZ) that 2738 candidate or confirmed planets orbit in the
habitable zone were calculated. Dartmouth Stellar Evolution Program models were
compared to photometry from the Kepler Input Catalog, priors for stellar mass,
age, metallicity and distance, and planet transit duration. The analysis
yielded probability density functions for calculating confidence intervals of
planet radius and stellar irradiance, as well as p(HZ). Sixty-two planets have
p(HZ) > 0.5 and a most probable stellar irradiance within habitable zone
limits. Fourteen of these have radii less than twice the Earth; the objects
most resembling Earth in terms of radius and irradiance are KOIs 2626.01 and
3010.01, which orbit late K/M-type dwarf stars. The fraction of Kepler dwarf
stars with Earth-size planets in the habitable zone (eta-Earth) is 0.46, with a
95% confidence interval of 0.31-0.64. Parallaxes from the Gaia mission will
reduce uncertainties by more than a factor of five and permit definitive
assignments of transiting planets to the habitable zones of Kepler stars.
