A. Wandel

Astrophysical Journal 856, 165 Link to Article [DOI: 10.3847/1538-4357/aaae6e]
The Racah Institute of Physics, The Hebrew University of Jerusalem, 91904, Israel

The recent detection of Earth-sized planets in the habitable zone of Proxima Centauri, Trappist-1, and many other nearby M-type stars has led to speculations whether liquid water and life actually exist on these planets. To a large extent, the answer depends on their yet unknown atmospheres, which may, however, be within observational reach in the near future by JWST, ELT, and other planned telescopes. We consider the habitability of planets of M-type stars in the context of their atmospheric properties, heat transport, and irradiation. Instead of the traditional definition of the habitable zone, we define the biohabitable zone, where liquid water and complex organic molecules can survive on at least part of the planetary surface. The atmospheric impact on the temperature is quantified in terms of the heating factor (a combination of greenhouse heating, stellar irradiation, albedo, etc.) and heat redistribution (horizontal energy transport). We investigate the biohabitable domain (where planets can support surface liquid water and organics) in terms of these two factors. Our results suggest that planets orbiting M-type stars may have life-supporting temperatures, at least on part of their surface, for a wide range of atmospheric properties. We apply this analyses to Proxima Cen b and the Trappist-1 system. Finally, we discuss the implications for the search of biosignatures and demonstrate how they may be used to estimate the abundance of photosynthesis and biotic planets.

Nikolaos Georgakarakos¹, Siegfried Eggl², and Ian Dobbs-Dixon¹

Astrophysical Journal 856, 155 Link to Article [DOI: 10.3847/1538-4357/aaaf72]
¹New York University Abu Dhabi, P.O. Box 129188, Saadiyat Island, Abu Dhabi, UAE
²California Institute of Technology, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

The presence of giant planets influences potentially habitable worlds in numerous ways. Massive celestial neighbors can facilitate the formation of planetary cores and modify the influx of asteroids and comets toward Earth analogs later on. Furthermore, giant planets can indirectly change the climate of terrestrial worlds by gravitationally altering their orbits. Investigating 147 well-characterized exoplanetary systems known to date that host a main-sequence star and a giant planet, we show that the presence of “giant neighbors” can reduce a terrestrial planet’s chances to remain habitable, even if both planets have stable orbits. In a small fraction of systems, however, giant planets slightly increase the extent of habitable zones provided that the terrestrial world has a high climate inertia. In providing constraints on where giant planets cease to affect the habitable zone size in a detrimental fashion, we identify prime targets in the search for habitable worlds.