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Study: Cosmic forces can transform mini-Neptunes into Earth-like exoplanets

By Brooks Hays
An artistic rendering depicts a mini-Neptune's transformation into an Earth-like "evaporated core." Photo by Rodrigo Luger/NASA.
An artistic rendering depicts a mini-Neptune's transformation into an Earth-like "evaporated core." Photo by Rodrigo Luger/NASA.

SEATTLE, Jan. 29 (UPI) -- A great many astronomers are dedicated to the task of finding habitable exoplanets -- rocky planets that are close enough to a star that water can exist in liquid form, but not so close that it all evaporates.

Researchers have done a pretty decent job of finding these sorts of planets -- with the help of Kepler, of course. But new research suggests their stock of potential Earth-like alien worlds could be more extensive than originally thought.

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When it comes to stars smaller and less intense than our sun, features like tidal forces and stellar activity (factors previously thought to inhibit habitability) may actually help transform large gaseous exoplanets like Neptune into Earth-like exoplanets capable of hosting life.

"There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets," explained Rodrigo Luger, a doctoral student at the University of Washington in Seattle. "Two important ones are strong tidal effects and vigorous stellar activity."

Previously, these two factors were thought to be primarily a danger to the chances of habitability.

Tidal forces only affect Earth's water, but for planets orbiting closer to red dwarf stars (the smaller, less intense versions of our sun) these same tidal forces are stronger. The stretching and pulling of these forces can coax latent energy out of a planet's core, encouraging extreme volcanism that boils away an exoplanet's water -- and any chance it has of hosting life along with it.

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Excessive stellar activity can have similar effects. The high-energy X-rays and ultraviolet radiation emanating from a young dwarf star can burn off the atmosphere of a close-orbiting exoplanet. Once the atmosphere is gone, it's not long before the water boils away too.

But these same factors can also encourage habitability. The same stellar forces (tidal and electromagnetic) that prove antagonistic towards the habitability of inner worlds, can transform cold outer worlds into more inviting places -- turning the Neptunes and Uranuses of the cosmos into Earths, so to speak.

"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."

This planet migration can cause Neptune-like worlds to drift into the habitable zone of a red dwarf. As it comes closer to the host star, electromagnetic radiation can help burn away the thick atmosphere that once surrounded its icy core -- what's left is what scientists call "habitable evaporated cores."

"Such a planet is likely to have abundant surface water, since its core is rich in water ice," Luger concluded. "Once in the habitable zone, this ice can melt and form oceans."

Luger and his research partner Rory Barnes say there are likely thousands of habitable evaporated cores out there waiting to be discovered.

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The new study was published this week in the journal Astrobiology.

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