Some Earth-Like Exoplanets May Have Started as Mini-Neptunes

Strong irradiation from the host star can cause planets known as mini-Neptunes in the habitable zone to shed their gaseous envelopes and become potentially habitable worlds / Rodrigo Luger/NASA images
Janet Fang 05 Feb 2015, 02:00

Give them a few million years and Neptune-like exoplanets might just become livable and rocky like Earth. According to a new Astrobiology study, some potentially habitable exoplanets began their life far away from their stars as gaseous “mini-Neptunes,” large planets in outer orbits with solid cores and thick hydrogen atmospheres. 

Astronomers looking for potentially habitable Earth-like planets and Super Earths have been targeting systems around low-mass stars called M dwarfs with close-in habitable zones. These are areas around stars that allow for liquid water. “There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets,” says University of Washington’s Rodrigo Luger. “Two important ones are strong tidal effects and vigorous stellar activity.”

Tidal force is the gravitational pull the host star exerts on an orbiting planet. On Earth, the sun and the moon distorts water in the ocean by a few meters. But for close-in planets in the habitable zones of M dwarfs, this tug can stretch its shape and possibly cause it to migrate closer. This stretching creates friction in the interior of the planet, driving surface volcanism and possibly boiling away oceans. Meanwhile, aggressive stellar activity—in the form of high-energy X-rays and UV radiation—can heat a planet’s atmosphere, creating winds so strong they can erode the atmosphere away entirely in just a few hundred million years. 

“But things aren’t necessarily as grim as they may sound,” Luger explains in a news release. Using computer modeling, his team found that the same phenomena of tidal forces and atmospheric escape could shape potentially habitable worlds out of freezing mini-Neptunes. These inhospitable planets typically form far from their host star when ice molecules combine with hydrogen and helium gases, forming icy, rocky cores with gaseous atmospheres. “But planets need not always remain in place,” he adds. “Alongside other processes, tidal forces can induce inward planet migration.”

Once within their star’s habitable zone, the higher X-ray and UV radiation might cause the planets to lose their atmospheric gases. What’s left is a hydrogen-free, rocky planet where oceans could form—something the team calls “habitable evaporated cores.” Of course, a lot of other things would have to work out before a mini-Neptune transforms into an Earth-like world. But by starting out so far from the star (where ice is plentiful) and then moving in closer, these evaporated cores have a better chance of supporting life—compared to rocky planets that form in the habitable zone, Science News explains. These start off dry, and they’re too small to attract water vapor and too warm for ice. 


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