Strange Lava World May Be New Class of Planet

The first rocky planet confirmed to be orbiting another star is truly one strange world, with rock rains, potentially raging volcanoes, and huge temperature differences between its night and day sides. This hellish rock might also be the remnant core of a former gas giant whose atmosphere long ago evaporated away.

CoRoT-7b (named after the French telescope that discovered it) is a so-called "Super-Earth" orbiting a star about 480 light-years from Earth.

Weighing in at just five times the mass of Earth and not quite two times the Earth's radius, this extrasolar planet was the first of the more than 400 that have been found to date that was confirmed to be a rocky world, instead of a gas giant.

But this exoplanet is anything but Earth-like.

CoRoT-7b orbits just 1.6 million miles out from its parent star, or 23 times closer than Mercury is to the sun in our solar system. This close proximity sends temperatures on the star-facing side of the planet up to a hellish 4,000 degrees Fahrenheit.

The planet is tidally-locked, so the same side is always facing its star (just as the moon only presents one face to the Earth). The far side of the planet is therefore always in shadow, and temperatures there dip down as low as minus 350 F.

The possibility that CoRoT-7b's current appearance may just be the shriveled remains of a former gas giant glory also casts another oddity atop the pile.

"CoRoT-7b may be the first in a new class of planet - evaporated remnant cores," said Brian Jackson of NASA's Goddard Space Flight Center in Greenbelt, Md.

Rock rains and volcanoes

The temperatures on the star-facing side of the planet are so hot that they can vaporize rock. Scientists who modeled the atmosphere of CoRoT-7b determined that the planet likely has no volatile gases (carbon dioxide, water vapor, nitrogen), and is instead likely made up of what could be called vaporized rock.

The atmosphere of CoRoT-7b could have weather systems that unlike the watery weather on Earth cause pebbles to condense out of the air and rain rocks onto the molten surface of the planet.

And if the planet doesn't already sound inhospitable to potential alien life, it also could be a volcanic nightmare.

New evidence presented last week at the annual meeting of the American Astronomical Society (AAS) in Washington, D.C., suggests that if CoRoT-7b's orbit is not perfectly circular, gravitational tugs from one of its two sister planets could push and pull the surface, creating friction that heats the interior of the planet. This heating could cause extensive volcanism across the planet's surface, with even more explosive activity than Jupiter's moon Io, which has over 400 volcanoes.

If this turns out to be the case, as scientists' models suggest, CoRoT-7b could be a new class of exoplanet, the Super-Ios, said one of the researchers who made the finding, Rory Barnes of the University of Washington in Seattle. Other rocky planets orbiting close to their stars in tidally-locked orbits could also display such rampant volcanism, Barnes said.

Another study presented at the AAS meeting suggests that the current form of the planet might not even been its original one: It could be the core rocky remains of a Saturn-sized gas giant.

Remnant core

The hot dayside temperatures of the planet not only mean that it could have a vaporized rock atmosphere, but that that atmosphere could be boiling off of the planet altogether. The rate of solar heating on the planet could have already cooked off several Earth masses of material from the planet.

Jackson, of NASA's Goddard Space Flight Center, modeled the planet's mass loss and changes in its orbit and effectively turned back the clock. Jackson and his team found that CoRoT-7b could have once weighed in at 100 Earth masses — about the mass of Saturn — when it first formed. At this point it may have been 50 percent farther from its star than it is now.

The research also showed that regardless of whether the planet started life as a gas giant or a bigger rocky world, it has probably lost several Earth masses of material since it first formed.

"You could say that, one way or the other, this planet is disappearing before our eyes," Jackson said.

Jackson and his team think that other exoplanets close to their sun could also be evaporating. So-called hot Jupiters — gas giants whose orbits hug their star — could also be undergoing mass loss right now, eventually leaving behind rocky cores like CoRoT-7b.

And Super-Ios and remnant cores are not mutually exclusive, Jackson and Barnes said — CoRoT-7b, and other exoplanets — could belong to both classes.

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