Tides, planetary companions, and habitability: habitability in the habitable zone of low-mass stars

Abstract

Earth-scale planets in the classical habitable zone (HZ) are more likely to\nbe habitable if they possess active geophysics. Without a constant internal\nenergy source, planets cool as they age, eventually terminating tectonic\nactivity and rendering the planet sterile to life. However, for planets\norbiting low-mass stars, the presence of an outer companion could generate\nenough tidal heat in the HZ planet to prevent such cooling. The range of mass\nand orbital parameters for the companion that give adequate long-term heating\nof the inner HZ planet, while avoiding very early total desiccation, is\nprobably substantial. We locate the ideal location for the outer of a pair of\nplanets, under the assumption that the inner planet has the same incident flux\nas Earth, orbiting example stars: a generic late M dwarf ($T_{eff}=2670 K$) and\nthe M9V/L0 dwarf DEN1048. Thus discoveries of Earth-scale planets in the HZ\nzone of old small stars should be followed by searches for outer companion\nplanets that might be essential for current habitability.\n