When were the First Exocontinents?

Abstract

Earth’s biosphere was able to increase as continents emerged. I assess when continents could first appear on hypothetical rocky planets of nearby stars. Radiogenic heating of the planetary mantle is evaluated through stellar abundances of iron and silicon (core and mantle proxies) and thorium and potassium (mantle heating proxies). The heat per unit mantle mass is compared to a threshold on Earth after which large-scale continents appeared. Longer delays are inferred for some exoplanets, particularly those with high thorium, but earliest continents could have arisen 2 Gyr before those on Earth, among thin disk stars. In the thick disk, continents could appear 4–5 Gyr pre-Earth. Hence, subsolar-metalicity systems could be an important focus in searching for planets where life could be more advanced than on Earth. At least ∼2 worlds with such old continents are expected to be accessible to future space telescopes, such as Habitable Worlds Observatory.