Long-Lived Liquid Water on Cold, Massive Terrestrial Planets

Artistic impression of the planet K2-18 b, its red dwarf host star, and another planet in the system. Credit: ESA/Hubble, M. Kornmesser.

Liquid water is a prerequisite for life as we know it. Consequently, scientists have long sought stable liquid water in our solar system to search for life. This idea has been extended to extrasolar planets that reside within the so-called habitable zone of their stellar systems, where liquid water could exist for an Earth-like planet. However, while Earth’s current nitrogen- and oxygen-rich atmosphere is critical for allowing liquid water to exist through a greenhouse effect, the atmosphere’s composition has changed over time. Earth’s primordial atmosphere was collected from the nebular gases, composed of hydrogen (H) and helium (He). While Earth lost this early atmosphere, more massive extrasolar rocky planets would retain their primordial envelopes over planetary lifetimes. A critical question emerges: Could liquid water exist on these non-Earth-like planets, thus allowing the potential for life?

To answer this question, Marit Mol Lous, Ravit Helled, and Christoph Mordasini (Universität Bern and University of Zürich) conducted simulations focusing on the evolution of stars and the atmospheres of planets orbiting those stars. While many of these atmospheres are lost due to heating and ultraviolet-induced hydrodynamic escape — both a function of stellar luminosity — favorable conditions for the existence of liquid water, perhaps stable for billions of years, can exist for some planets. Stable liquid water is facilitated by the warming effects of dihydrogen (H2). In sufficiently large atmospheres, H2 molecules collide, inducing a dipole moment that allows them to absorb infrared radiation released from the rocky interior of these massive planets. Consequently, compared to Earth-like atmospheres, the energy from the star is less important, allowing “cold” massive rocky planets with liquid water to orbit cooler stars or to orbit far from their stars, which are both conditions that reside outside of classically defined habitable zones. These results allow for a broad new range of exoplanets having the potential for liquid water and habitability. READ MORE