Stars Can Strip Gas Giants Naked

April 22, 2009

A recently discovered rocky "super Earth" could be the stripped-down core of a gas giant that hugged its star too tightly, scientists say.

COROT-7b, discovered in February, orbits a star about 457 light-years from Earth.

With a mass five to eight times that of Earth's, the distant world is one of the smallest planets yet found outside our solar system. At first glance, COROT-7b would appear to be a rocky Earthlike world orbiting close to its host star.

But new computer models show that the extrasolar planet, or exoplanet, may once have been a Neptune-size gas giant whose atmosphere was slowly blown into space by radiation from its star.

(Related: "'Hot Jupiters' Could Give Rise to Earthlike Worlds, Study Says.")

Study leader Helmut Lammer, of the Space Research Institute of the Austrian Academy of Sciences, and colleagues wanted to know how solar winds and space weather might influence planets that are very close to their stars.

According to their model, a gas giant at least the size of Jupiter can have its atmosphere torn away entirely if it orbits closer than 2 percent of an astronomical unit (AU) to a sunlike star.

One AU is equal to about 93 million miles (150 million kilometers), or the distance between Earth and the sun.

Smaller gas planets the size of Neptune or Uranus can be reduced to rocky pits if their orbits are less than 5 percent of an AU, the team announced this week at the European Week of Astronomy and Space Science conference at the U.K.'s University of Hertfordshire.

As gas giants undergo this extreme mass loss, Lammer said, they come to resemble planet-size comets, as radiation from their stars stretches their escaping atmospheres into long, tenuous tails.

Stripped Slowly

Star-hugging gas giants are most at risk of being stripped naked when their stars are young and active, the model predicts.

COROT-7b orbits its sunlike, orange-red star at a distance of just more than one percent of an AU.

For now, astronomers are uncertain as to whether COROT-7b started life as a terrestrial world or a gas giant. The new model simply means scientists can't rule out the possibility that the planet was once a gassy world, Lammer says.

If it had been a Neptune-like planet, COROT-7b would have lost its atmosphere over a period of about 500 million years, he noted.

Scientists have long speculated about extreme mass loss in gas giants, said Jean Schneider, an astrophysicist at the Paris Observatory in France, who was not involved in the study.

The new study is the first to model this phenomenon in detail, Schneider said.

One way to test Lammer's theory, Schneider added, would be to measure how fast gas loss occurs on known close-orbiting gas giants and compare the rates to those predicted by the new model.

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