Exploring Kepler Giant Planets in the Habitable Zone

TL;DR

This paper assesses the occurrence rates of giant planets in the habitable zone, explores the potential for habitable moons, and evaluates their detectability for future observational missions.

Contribution

It provides new estimates of giant planet occurrence rates in the habitable zone and analyzes the potential for large moons to exist and be detected.

Findings

Giant planet occurrence rates are 6.5% for G stars, 11.5% for K stars, and 6% for M stars.

If each giant planet has one large moon, moons are less common than terrestrial planets in the habitable zone.

Multiple moons per planet could make habitable moons as common or more common than terrestrial planets.

Abstract

The Kepler mission found hundreds of planet candidates within the habitable zones (HZ) of their host star, including over 70 candidates with radii larger than 3 Earth radii ($R_\oplus$) within the optimistic habitable zone (OHZ) (Kane et al. 2016). These giant planets are potential hosts to large terrestrial satellites (or exomoons) which would also exist in the HZ. We calculate the occurrence rates of giant planets ($R_p =$~3.0--25~$R_\oplus$) in the OHZ and find a frequency of $(6.5 \pm 1.9)\%$ for G stars, $(11.5 \pm 3.1)\%$ for K stars, and $(6 \pm 6)\%$ for M stars. We compare this with previously estimated occurrence rates of terrestrial planets in the HZ of G, K and M stars and find that if each giant planet has one large terrestrial moon then these moons are less likely to exist in the HZ than terrestrial planets. However, if each giant planet holds more than one moon, then the occurrence rates of moons in the HZ would be comparable to that of terrestrial planets, and could potentially exceed them. We estimate the mass of each planet candidate using the mass-radius relationship developed by Chen & Kipping (2016). We calculate the Hill radius of each planet to determine the area of influence of the planet in which any attached moon may reside, then calculate the estimated angular separation of the moon and planet for future imaging missions. Finally, we estimate the radial velocity semi-amplitudes of each planet for use in follow up observations.