Large Interferometer For Exoplanets (LIFE): VI. Detecting rocky exoplanets in the habitable zones of Sun-like stars

Abstract

While previous studies have shown a strong preference for a future mid-infrared nulling interferometer space mission to detect planets within the HZ around M dwarfs, we here focus on a more conservative approach toward the concept of habitability and present yield estimates for two stellar samples consisting of nearby (d<20 pc) Sun-like stars (4800-6300 K) and nearby FGK-type stars (3940-7220 K) accessible to such a mission. Our yield estimates are based on recently derived occurrence rates of rocky planets from the Kepler mission and our LIFE exoplanet observation simulation tool LIFEsim, which includes all main astrophysical noise sources, but no instrumental noise sources as yet. Depending on a pessimistic or optimistic extrapolation of the Kepler results, we find that during a 2.5-year search phase, LIFE could detect between ~10-16 (average) or ~5-34 (including 1$\sigma$ uncertainties) rocky planets (0.5-1.5 R${}_\rm{Earth}$) within the optimistic HZ of Sun-like stars and between ~4-6 (average) or ~1-13 (including 1$\sigma$ uncertainties) exo-Earth candidates (EECs) assuming four collector spacecraft equipped with 2 m mirrors and a conservative instrument throughput of 5%. With D=3.5 m or 1 m mirrors, the yield $Y$ changes strongly, following approximately $Y \propto D^{3/2}$. With the larger sample of FGK-type stars, the yield increases to ~16-22 (average) rocky planets within the optimistic HZ and ~5-8 (average) EECs. Furthermore, we find that in addition to the mirror diameter, the yield depends strongly on the total throughput, but only weakly on the exozodiacal dust level and the accessible wavelength range of the mission. When the focus lies entirely on Sun-like stars, larger mirrors (~3 m with 5% total throughput) or a better total throughput (~20% with 2 m mirrors) are required to detect a statistically relevant sample of ~30 rocky planets within the optimistic HZ.