Mini-Neptunes Near M Dwarf Stars May Transform into Rocky Habitable Worlds

Two phenomena that are known to actually inhibit the habitability of planets may actually help chances for life. Scientists have found that tidal forces and vigorous stellar activity may promote life on certain planets orbiting low-mass stars.

Most of the stars in our galaxy are low-mass stars, also known as M dwarfs. These stars are smaller and dimmer than our own sun, which means that planets must orbit closer to them in order to be within the so-called "habitable zone." Because astronomers believe that in the coming years they'll find many Earth-like and "super-Earth" planets in the habitable zones of these stars, it's important to see whether or not they could actually support life.

"There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets," said Rodrigo Luger, one of the researchers, in a news release. "Two important ones are strong tidal effects and vigorous stellar activity."

A tidal force is the star's gravitational tug on an orbiting planet. It's stronger on the near side of the planet than the far side; because it can stretch a world into an egglike shape and cause friction in a planet's interior, it can drive surface volcanism. In some cases, it can even heat the planet into a runaway greenhouse state that boils away its oceans.

Stellar activity can also greatly impact a planet. M dwarfs, when young, emit lots of high-energy X-rays and ultraviolet radiation. This can heat a planet's upper atmosphere and spawn strong winds that can erode the atmosphere away entirely.

Yet these two factors don't completely rule out the existence of life. The scientists used computer models and found that tidal forces and atmospheric escape can shape planets that start out as mini-Neptunes into gas-free and potentially habitable worlds.

"They are initially freezing cold, inhospitable worlds," said Luger. "But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration."

This process, in particular, can bring mini-Neptunes into their star's habitable zone, where they are exposed to higher levels of ultraviolet radiation. This may lead to a loss of the atmospheric gases to space, leaving behind a hydrogen-free rocky world within the habitable zone. Because such a planet is likely to have ice, the movement would cause the ice to melt and lead to oceans.

The findings reveal that it's possible for life to exist within the habitable zone of M dwarf stars. This, in turn, may allow scientists to better hone their search for life on other worlds.

The findings are published in the journal Astrobiology.

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