Kepler-22b: A 2.4 Earth-radius Planet in the Habitable Zone of a Sun-like Star

TL;DR

Kepler-22b is the first confirmed planet in the habitable zone of a Sun-like star, with a radius of 2.4 Earth radii, discovered through Kepler photometry and validated with multiple observational methods.

Contribution

This paper reports the discovery and validation of Kepler-22b, the first confirmed habitable-zone planet with a measured radius, using combined photometric, spectroscopic, and radial velocity data.

Findings

Kepler-22b has a radius of approximately 2.4 Earth radii.

The planet orbits its star in the habitable zone with a temperature of 262K.

Radial velocity data constrain the planet's mass to below 124 Earth masses.

Abstract

A search of the time-series photometry from NASA's Kepler spacecraft reveals a transiting planet candidate orbiting the 11th magnitude G5 dwarf KIC 10593626 with a period of 290 days. The characteristics of the host star are well constrained by high-resolution spectroscopy combined with an asteroseismic analysis of the Kepler photometry, leading to an estimated mass and radius of 0.970 +/- 0.060 MSun and 0.979 +/- 0.020 RSun. The depth of 492 +/- 10ppm for the three observed transits yields a radius of 2.38 +/- 0.13 REarth for the planet. The system passes a battery of tests for false positives, including reconnaissance spectroscopy, high-resolution imaging, and centroid motion. A full BLENDER analysis provides further validation of the planet interpretation by showing that contamination of the target by an eclipsing system would rarely mimic the observed shape of the transits. The final validation of the planet is provided by 16 radial velocities obtained with HIRES on Keck 1 over a one year span. Although the velocities do not lead to a reliable orbit and mass determination, they are able to constrain the mass to a 3{\sigma} upper limit of 124 MEarth, safely in the regime of planetary masses, thus earning the designation Kepler-22b. The radiative equilibrium temperature is 262K for a planet in Kepler-22b's orbit. Although there is no evidence that Kepler-22b is a rocky planet, it is the first confirmed planet with a measured radius to orbit in the Habitable Zone of any star other than the Sun.