A new paper was published this month by University of Washington doctoral student Rodrigo Luger and co-author Rory Barnes, a research assistant professor. The paper describes how ‘mini-Neptunes’ could transform into super-Earths inside the habitable zone of M dwarf stars.
“There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets,” according to Luger. “Two important ones are strong tidal effects and vigorous stellar activity.”
Here’s a quick refresher on tidal forces. A tidal force is a star’s gravitational pull on an orbiting planet. It’s stronger on the side facing the star and weaker on the far side. On Earth, tidal forces affect the world’s oceans a little bit.
On a planet near a M dwarf star? That effect is a lot more pronounced. M dwarf stars are dimmer and smaller than our sun. As a result, the habitable zone is a lot closer to the host star. And, the tidal forces are a lot stronger. A University of Washington press release explains what can happen with these extreme tidal forces.
This stretching causes friction in a planet’s interior that gives off huge amounts of energy. This can drive surface volcanism and in some cases even heat the planet into a runaway greenhouse state, boiling away its oceans, and all chance of habitability.
So, what do gas planets have to do with all of this? Luger and his colleagues found tidal forces and ‘atmospheric escape’ can sometimes transform these mini-Neptunes into gas-free, and possibly habitable worlds.
Gas planets generally form in the outer orbits of their host star. But, tidal forces in conjunction with other processes can bring the gas planet closer to the host star. As the gas planet enters the host star’s habitable zone, it is bombarded with higher levels of X-ray and UV radiation.
This can lead to the gas planet shedding its atmospheric gases. What you are left with is a rocky world right where it would need to be for life to form.
“Such a planet is likely to have abundant surface water, since its core is rich in water ice,” Luger said. “Once in the habitable zone, this ice can melt and form oceans.” With liquid water on a planet inside the habitable zone, comes the prospects of possible life.
Several other factors would come into play for these planets to be habitable. First, an atmosphere is needed. Second, if the new world loses hydrogen too quickly, runaway greenhouse gases could boil all the water off – leaving a desolate, rocky planet behind.
“The bottom line is that this process — the transformation of a mini-Neptune into an Earthlike world — could be a pathway to the formation of habitable worlds around M dwarf stars,” Luger said.