The Viability of Life in Helium-Dominated Exoplanet Atmospheres

Abstract

Helium is the second most abundant element in the Universe, yet helium-dominated atmospheres are rarely considered as environments for life. Two assumptions have contributed to this neglect: helium is expected to escape from rocky planets together with hydrogen, and Earth itself lacks a substantial helium envelope. Recent observations and atmospheric evolution models, indicate that rocky exoplanets can retain helium-dominated atmospheres. Here we make the case for life in such environments by synthesizing a largely overlooked body of laboratory research on life in helium-dominated atmospheres. Every organism studied, including bacteria, fungi, protozoa, microalgae, plants, and animals tolerates helium-dominated atmospheres when supplied with essential metabolic requirements. Across a range of pressures, helium has shown no demonstrated toxicity and no fundamental barrier to metabolism, cell division, photosynthesis, nitrogen fixation, or coordinated multicellular activity, including in humans. Helium-dominated atmospheres therefore constitute a viable and underrecognized planetary environment for life. Helium is chemically inert and spectroscopically unobtrusive, so the bulk atmosphere would neither contribute to the chemical destruction of biosignature gases nor strongly obscure their spectral features. Helium-dominated atmospheres also have low mean molecular weights and large scale heights, strengthening spectral features in transmission and improving remote detectability.