Effects of a Binary Companion Star on Habitability of Tidally Locked Planets around an M-type Host Star

• Published in: The Astrophysical journal Vol. 880; no. 2; pp. 107 - 124
• Main Authors: Okuya, Ayaka, Fujii, Yuka, Ida, Shigeru
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.08.2019; IOP Publishing
• Subjects: astrobiology; Astrophysics; Atmosphere; binaries: general; Binary stars; Climate; Cold; Cold traps; Energy balance; Extrasolar planets; G stars; Habitability; Irradiation; Locking; M stars; Planetary atmospheres; Planetary orbits; Planets; planets and satellites: atmospheres; planets and satellites: terrestrial planets; Stars & galaxies; Stellar systems; Temperate climates; Trapping; Two dimensional models; Water; Water vapor
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/ab29e7

Planets in the "habitable zones" around M-type stars are important targets for characterization in future observations. Due to tidal locking in synchronous spin-orbit rotations, the planets tend to have a hot dayside and a cold nightside. On the cold nightside, water vapor transferred from the dayside can be frozen in ("cold trapping") or the major atmospheric constituent could also condense ("atmospheric collapse") if the atmosphere is so thin that the redistribution of heat is not efficient, in the case of a single M-type star. Motivated by the abundance of binary star systems, we investigate the effects of irradiation from a G-type companion star on the climate of a tidally locked planet around an M-type star using the 2D energy balance model. We find that the irradiation from the G-type star is more effective at warming up the nightside of the planet than the dayside. This contributes to the prevention of the irreversible trapping of water and atmosphere on the cold nightside, broadening the parameter space where tidally locked planets can maintain surface liquid water. Tidally locked ocean planets with 0.3 bar atmospheres or land planets with 3 bar atmospheres can produce a temperate climate with surface liquid water when they are also irradiated by a companion star with a separation of 1-4 au. We also demonstrate that planets with given properties can be in the Earth-like temperate climate regime or in a completely frozen state under the same total irradiation.