Free Energy Budgets and Power Constraints on Cumulative Biosphere Size

A planet’s energy supply places an upper bound on the size of the biosphere it can support. We estimate the free-energy budgets for 28 Solar System bodies and 1,463 exoplanets from measured planetary, orbital, and stellar properties. We compare these estimates with the energy requirements of Earth’s biosphere. Over its history, Earth’s biosphere has used about 0.35% of the planet’s available free energy. This suggests that bulk energy supply has not limited Earth’s biosphere size. The situation is different for ocean worlds, where a subsurface biosphere must rely on internal energy. Europa’s internal free-energy budget is about 86 times too small to support an Earth-equivalent biosphere, and Enceladus’s is about 24,000 times too small. Among detected exoplanets, most cumulative energy shortfalls result from young system ages. After accounting for differences in system age, eight exoplanets require greater utilization than Earth’s time-averaged benchmark, including small, temperate worlds around old M and K dwarfs. At Earth-equivalent stellar irradiation, an Earth-like planet has sufficient energy around star types from G to M5. This comparison does not account for differences in the wavelengths of light produced by different stars. The energy budgets calculated here do not predict whether life exists, but place an upper bound on biosphere size and can therefore help define the sensitivity required to detect life.