A plot of the flux incident on an exoplanet (in units of the amount on Earth) versus the host star’s temperature. The plot shows two ranges for the habitable zone, conservative green area) and optimistic (yellow area); it also shows where confirmed (blue dots) and unconfirmed (red circles) exoplanets lie in the plot. There are currently twenty known exoplanet candidates smaller than two Earth-radii that fall in their optimistically-defined habitable zones.

A newly published study provides a list of habitable zone exoplanet candidates from the Kepler Data Release 24 Q1-Q17 data vetting process.

In our solar system, the Earth is cozily situated in the middle of the habitable zone which, depending on the model, extends roughly from Venus to Mars. The Kepler mission has as one of its primary goals the determination of the frequency of terrestrial planets in their habitable zones. CfA astronomer Guillermo Torres and his colleagues have now produced a complete catalog of Kepler exoplanet candidates in their habitable zones from the Kepler data releases to date. After reviewing the various criteria for determining the boundaries of the HZ, they report there are 104 candidates within an optimistic (larger) HZ definition, and twenty within a more conservative (smaller) definition of the HZ and which also have radii less than two Earth-radii, making this group in particular potential “Earth-like” candidates.

The scientists also refine the definitions for the HZ for the purpose of more useful statistical diagnostics. They define four overlapping subgroups: candidates in the conservatively defined zone with a radius less than 2 Earth-radii; those in the larger, optimistic zone with this radius; those in the conservative zone with any radius; and those in the optimistic zone with any radius. The current catalog statistics for these four groups are 20, 29, 63, and 104 respectively. They note that these subgroups are useful when conducting specific followup studies; for example, searches for exomoons in the HZ is best done on the fourth group and its giant exoplanets.

Reference: “A Catalog of Kepler Habitable Zone Exoplanet Candidates,” Kane, Stephen R.; Hill, Michelle L.; Kasting, James F.; Kopparapu, Ravi Kumar; Quintana, Elisa V.; Barclay, Thomas; Batalha, Natalie M.; Borucki, William J.; Ciardi, David R.; Haghighipour, Nader; Hinkel, Natalie R.; Kaltenegger, Lisa; Selsis, Franck; Torres, Guillermo, ApJ, 830, 1, 2016.