The Temporal Onset of Habitability For Earth-Like Planets

Abstract

The ability of a planet to maintain surface water, key to life as we know it, depends on solar and planetary energy. As a star ages, it delivers more energy to a planet. As a planet ages it produces less internal heat, which leads to cooling. For the Earth, interior cooling connects to plate tectonics - the surface manifestation of convection within the Earth's interior. This process cycles volatiles (CO2 and water) between surface and interior reservoirs, which affects planetary climate. Cycling rates depend on the efficiency of plate tectonic cooling. That efficiency remains debated and multiple hypotheses have been put forth. Geological proxy data allow us to validate these hypotheses accounting for model and data uncertainty. Multiple models pass the validation test. Those models define a distribution for terrestrial exoplanets akin to Earth, accounting for variations in tectonic efficiency. Feeding this distribution into climate models indicates that the time at which habitable conditions are established can vary by billions of years. Planets of the same absolute age and orbital distance can reside and not reside within the classic habitable zone due to differences in plate tectonic cooling efficiencies. The full model population allows a probability distribution to be constructed for the the time at which habitable conditions are established. The distribution indicates that Earth-like exoplanets, of the same age, can be at different evolutionary stages. It also indicates that planets around stars whose early evolution is unfavorable for life can become habitable later in their energetic histories.