Changing Gas-Rich Planets into Habitable Worlds

Over the years, discoveries from NASA’s Kepler space telescope have shown that low-mass planets are very common. However, a significant fraction of these low-mass planets are enshrouded by voluminous hydrogen-helium envelopes. For such a planet that resides in the habitable zone of its host star, the removal of its hydrogen-helium envelope is necessary in order for the planet to become habitable.

If such a planet is in orbit within the habitable zone of a Sun-like star, the X-ray/UV flux from the planet’s host star over billion-year timescales is too low to remove the planet’s hydrogen-helium envelope. However, things are different if such a planet is residing in the habitable zone of a red dwarf star. Since the luminosity of a star declines sharply with decreasing mass, a red dwarf star is many times less luminous than a Sun-like star. As a result, the habitable zone around a red dwarf star is situated much closer in, causing a planet in the habitable zone of a red dwarf star to be much nearer to its host star. Furthermore, red dwarf stars tend to be active and subject planets within their habitable zones to much higher X-ray/UV flux over billion year timescales.

Owen & Mohanty (2016) present a study using a rocky planet orbiting in the habitable zone of a red dwarf star. The planet is assumed to be made of 2/3 rock and 1/3 iron. The study shows that if the planet resides on the inner edge of the habitable zone and has less than 90 percent of the Earth’s mass, it can lose enough hydrogen and helium to be subsequently habitable if its initial hydrogen-helium mass fraction is ~1 percent or less. 

The same process can remove an initial hydrogen-helium envelope comprising up to ~1 percent of the planet’s mass for a rocky planet with less than ~80 percent of the Earth’s mass residing at the outer edge of the habitable zone. Basically, a rocky planet in the habitable zone of a red dwarf star that is more massive than the Earth and has a hydrogen-helium envelope that makes up more than one percent of the planet’s mass cannot lose enough of its hydrogen-helium envelope to be habitable.

Low-mass planets are abundant around red dwarf stars, and red dwarf stars are the most common stars in the galaxy. If the evaporation of hydrogen-helium envelopes occurs readily for low-mass planets in the habitable zones around red dwarf stars, then red dwarf stars hosting planetary systems with habitable worlds could be a common phenomenon.

Reference:
Owen & Mohanty (2016), “Habitability of Terrestrial-Mass Planets in the HZ of M Dwarfs. I. H/He-Dominated Atmospheres”, arXiv:1601.05143 [astro-ph.EP]