Potential long-term habitable conditions on planets with primordial H-He atmospheres.

Introduction: Planets that retain a primordial, H-He dominated atmosphere can have hydrogen act as a greenhouse gas: the collision induced absorption (CIA) of infra-red light increases with pressure. This mechanism could replace regular greenhouse gasses in exoplanets to allow warm enough temperatures for a liquid water layer. Prior work[1, 2] demonstrated that either stellar insolation or intrinsic heat could be a sufficient energy source. In the case of the latter, a ‘habitable zone’ for such planets could extend up to infinity[3, 4]. In this work we aim to study the long-term potential for habitability of planets with hydrogen-dominated atmospheres. We simulate planets with varying properties to investigate how these influence the likeliness of habitable conditions. Importantly, we include for the first time long-term temporal evolution of both the planet and the host-star, to estimate how long liquid water could remain.

Methods: We model the thermal structure of a silicate-iron planet with a H-He-dominated atmosphere and vary the core mass, initial envelope mass, and semimajor-axis. An evolution model for the host-star's luminosity is included, as well as a model for the evolution of the planet's intrinsic heat and radius. The intrinsic heat model includes a radiogenic component based on Earth's abundance of radioactive materials as well as cooling and contraction of both the silicate-iron core and the gaseous envelope. We furthermore include a thermal XUV-driven atmosphere loss model. By comparing the pressure and temperature at the bottom of the atmosphere with the water phase diagram, we determine when liquid water can exist.

Results: We find that terrestrial and super-Earth planets with masses of about 1 - 10 Earth masses can maintain liquid water conditions for more than 9 billion years at radial distances larger than 2 AU. The required surface pressures are typically between ~100 bar (as on Venus) and 1 kbar (as in the oceanic trenches on Earth). Hydrodynamic escape can reduce this duration significantly for planets with a smaller envelope than 10-5 Earth mass within 10 AU, while planets with an envelope larger than 10-3 Earth mass remain mostly unaffected by this mechanism. Planets that receive a negligible amount of stellar radiation can maintain the conditions as long as the internal heat source is sufficient, which is at maximum 80 billion years in our simulation.

Conclusions: Under the assumptions of our model we show that there is a wide range of conditions under which liquid water can exist and remain on the order of 10 billion years. This raises the question whether most potentially habitable planets are very different from Earth. Our model is simplified and future work should investigate the formation and retention of a liquid water ocean in more detail. The pathway and the conditions towards planets with the right initial conditions should be studied in the future as well. This will enable us to better predict the occurence rate of such a planet and the chances of observing them at the current age of the universe.

References: [1] Stevenson D. J. (1999) Nature 400:32. [2] Pierrehumbert R. & Gaidos E. (2011). The Astrophysical Journal Letters 734:L13. [3] Seager S. (2013) Science, 340:577-581. [4] Madhusudhan, N. et al. (2021) The Astrophysical Journal, 918:1.