Candidate Planets in the Habitable Zones of Kepler Stars

TL;DR

This study estimates the likelihood of Kepler candidate planets being in habitable zones using Bayesian analysis, identifying 62 promising Earth-like planets and estimating that about 46% of Kepler dwarf stars host such planets.

Contribution

It introduces a Bayesian method to assess habitable zone probabilities for Kepler planets, improving target prioritization and stellar parameter estimation.

Findings

62 planets have >50% probability of being in habitable zones.

Approximately 46% of Kepler dwarf stars may host Earth-like planets in habitable zones.

Gaia parallaxes will significantly reduce uncertainties in future analyses.

Abstract

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.