Low-mass, flaring stars are often seen by astrobiologists as the last place to find habitable exoplanets, but new research suggests that these angry little suns could in fact turn lifeless ‘mini-Neptunes' into potentially habitable ‘exo-Earths.' GALLERY: The Most Horrific Alien Planets In Our Galaxy
M-type red dwarf stars possess two qualities that are usually considered too harsh to nurture habitable exoplanets: extreme tides and violent space weather.
As the star is smaller, its habitable zone is more compact, so any potentially habitable world would have to orbit very close to its star. Orbiting so close would induce extreme tides on this world, likely rendering it barren and sterile.
"This is the reason we have ocean tides on Earth, as tidal forces from both the moon and the sun can tug on the oceans, creating a bulge that we experience as a high tide," said Rodrigo Luger, of the University of Washington, lead author of a paper published in the journal Astrobiology. "Luckily, on Earth it's really only the water in the oceans that gets distorted, and only by a few feet. But close-in planets, like those in the habitable zones of M dwarfs, experience much stronger tidal forces."
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These tidal forces would physically warp the planet's crust, driving extreme tectonic activity and volcanism, possibly triggering a runaway greenhouse effect, vaporizing any surface water. Tidal locking - where one hemisphere continually faces the star - would also be a problem.
M-type red dwarfs are also known to have extreme space weather, and any planet orbiting within the star's habitable zone would bear the brunt of powerful stellar flares and strong stellar winds. This irradiated environment would ultimately erode any surviving atmosphere away, blowing it into space.
But what's bad for an Earth-like exoplanet may not be so bad for a mini-Neptune, which sports a thick atmosphere during formation.
Mini-Neptunes would form far from their host stars, "with ice molecules joining with hydrogen and helium gases in great quantity to form icy/rocky cores surrounded by massive gaseous atmospheres," writes a UW press release.
"They are initially freezing cold, inhospitable worlds," Luger said. "But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration."
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Inward migration would cause the mini-Neptune to be exposed to the stellar blowtorch and, over millions of years, the mini-Neptune's atmosphere would be blown away. Once lodged in the star's habitable zone, the mini-Neptune's hydrogen-free core may be left behind; objects called "habitable evaporated cores" or HECs.
"Such a planet is likely to have abundant surface water, since its core is rich in water ice," said Luger. "Once in the habitable zone, this ice can melt and form oceans."
For these HECs to become anything remotely ‘habitable', a very delicate balance of atmospheric chemistry and radiation from the star would be needed. But in some models, oxygen-rich atmospheres are possible, incubating liquid water on the rocky surface.
Whether or not such a planet could host any kind of life remains open to debate. One could imagine that if an HEC evolves with a powerful magnetosphere, the worst stellar storms may be deflected. As for the problems with the extreme red dwarf tides, the world may well be tidally locked, where one side of the planet may be inhosptable, but powerful weather systems whip around the planet, creating the most extreme biosphere imaginable where life thrives in isolated regions.
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The possibilities for life are pure speculation until vastly advanced telescopes can be used to detect the atmospheres of small words orbiting close to red dwarf stars, if such worlds exist. But if they do, the life-giving potential of exoplanets orbiting abundant red dwarf stars just became a lot more exciting.
Source: University of Washington