Spin-Orbit Variations, Greenhouse Gases, and Superhabitable Conditions on Terrestrial Worlds in Multi-Planet Systems

Heller and Armstrong (Astrobiology, 2014) describe a number of ways in which an Earth-like world (“Earth twins” with a tidal heat flux Ft less than 0.04 W/m2, and within their star’s habitable zone) might be “superhabitable,” with surface environments that are more amenable to life than the common benchmark of modern Earth. Over Earth history, several time periods are known from the geologic record that could meet these criteria, providing multiple opportunities to explore known superhabitable and inhabited planetary conditions from an astrophysical perspective.