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Extreme Neptune-Like Planets Have the Potential to Become Habitable

Jan 29, 2015 12:18 PM EST

In the hunt for habitable planets around the galaxy, scientists may have just gotten some help. Remarkable research has, for the first time, suggested that extreme Neptune-like planets have the potential to become rocky, habitable worlds.

But how is this even possible? According to researchers at the University of Washington, two phenomena would have to occur: tidal forces and vigorous stellar activity. While these events certainly don't guarantee that gas giants like Neptune will suddenly become fit for life, they do give these worlds more of a chance than previously thought.

Experts have been searching for life-supporting planets outside our solar system for some time now. So far, nearly 2,000 exoplanets have been identified, and NASA officials recently affirmed their belief that an astounding 10 to 20 percent of all the stars in the sky host habitable planets.

However, even with all of these options, finding "Goldilocks" planets that are "just right" for life is easier said than done. Taking into account factors such as density, gravity, and distance from their star, scientists have currently ruled out planets that orbit red dwarf stars, and ones that simply can't hold onto their water.

But now new hope has come in the form of mini-Neptunes, which are similar to other gas giants but are only about 10 times the mass of Earth. In particular, doctoral student Rodrigo Luger and research assistant professor Dr. Rory Barnes, who led the research, focused on those around M dwarfs - stars that are smaller and dimmer than the Sun, with close-in habitable zones. (A habitable zone is the space around a star that might allow liquid water - a hallmark sign of life - on an orbiting rocky planet's surface).

Astronomers expect to find many Earth-like and "super-Earth" planets (up to five times the mass of Earth) in the habitable zones of these stars in coming years, so it's important to know if they might indeed support life. (Scroll to read on...)

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

These two factors could be the key to transforming gaseous mini-Neptunes from freezing, inhospitable worlds to ones that are closer-in, gas-free, and potentially habitable.

We typically relate tidal forces with ocean tides on Earth, but for close-in planets like those in the habitable zones of M dwarfs, these forces are so strong that they can essentially pull and stretch a planet into an egg-like shape and cause it to move closer to its star's habitable zone, where they are exposed to higher levels of X-ray and ultraviolet radiation.

This, in turn, causes planets like mini-Neptunes to shed their gaseous envelopments and possibly leave behind a hydrogen-free, rocky world - planets researchers refer to as "habitable evaporated cores."

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

However, Luger and Barnes add that many other conditions would have to be met for mini-Neptunes to become habitable. This includes the development of an atmosphere that could create and recycle nutrients globally.

Timing, they say, is also everything. If hydrogen and helium loss is too slow while a planet is forming, a gaseous envelope would prevail and a rocky, terrestrial world may not form. If the world loses hydrogen too quickly, a runaway greenhouse state could result, with all water lost to space.

"The bottom line is that this process - the transformation of a mini-Neptune into an Earthlike world - could be a pathway to the formation of habitable worlds around M dwarf stars," Luger concluded.

The findings were published in the journal Astrobiology.

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