Researchers from the University of Washington found that tidal forces and intense stellar activity may help uninhabitable mini-Neptunes orbiting low-mass stars acquire just the right conditions to host life. The findings challenge previous theory that the two phenomena were minimizing the odds of a planet to become inhabitable.
“There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets. Two important ones are strong tidal effects and vigorous stellar activity,”
said Rodrigo Luger, one of the authors of the findings.
The scientists published their findings this week in the journal Astrobiology.
Mini-Neptunes, or Super-Earths, are cold planets ten times larger than our Earth, but much smaller than gas giants such as Uranus and Neptune. They usually have thick helium-hydrogen atmospheres with solid cores and a high potential for hosting life.
Mini-Neptunes usually orbit a low-mass star, also known as an M-dwarf. M-dwarfs are much smaller than our sun, and they are surrounded by a swath of empty space called the “habitable zone”. Planets within the habitable zone are more likely to host life than the planets in outer orbits due to the optimum balance between the amount of stellar heat reaching their atmosphere and the liquid water on their surface.
The tidal force of an M-dwarf star can sometimes be so strong that can stretch the nearby orbiting planets into an egg-like shape, while also pulling far-orbiting planets closer to the host star.
“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. 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,”
Mr. Luger explained.
Previous studies showed that the tidal pressure on a close-orbiting planet can trigger volcanic activity, and even heat the planet so hard that all its oceans evaporate into space alongside any chance of habitability. Additionally, intense stellar activity of young M dwarfs can release high amounts of X-rays and UV radiation that can heat up a planet’s atmosphere so much that the resulting winds can virtually erode the atmosphere away.
However, the University of Washington researchers found that these ruthless forces may also contribute to hosting life. Scientists explained that the tidal force of an M-dwarf star can drag a distant icy mini-Neptune into the habitable zone. In the habitable zone, the ice on its core can melt and form oceans from the high amount of stellar radiation. Meanwhile the X-ray radiation can rapidly remove the hydrogen within the planet’s thick atmosphere and leave behind a “habitable evaporated cores.”
“It is all about timing,”
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