Researchers: Rain on newly found planet could be pebbles

ST. LOUIS -- The rain on the recently discovered planet of Corot-7b comes in the form of pebbles, not raindrops, researchers who studied its atmosphere have suggested.

Researchers at Washington University in St. Louis believe the planet's blazing heat would lead rock to vaporize. Instead of water clouds, they think "rock clouds" that might resemble a cloud of sand grains would form. They believe they would rain small pebbles into lakes of molten lava below.

"The atmosphere is totally composed of vapor you'd get from boiling rock," planetary sciences professor Bruce Fegley said Wednesday. "Depending on the weather, you'd get lava raining back down or pebbles coming down like snow or hail."

Fegley, who studied Corot-7b's atmosphere with research assistant Laura Schaefer, added that the way weather might work on the planet remains unknown.

European scientists who announced the discovery of the extrasolar planet, or "exoplanet," earlier this year asked the Washington University researchers to create models of its atmosphere. The St. Louis researchers' work appears in the Oct. 1 issue of The Astrophysical Journal.

While astronomers have found about 400 planets outside our solar system, Corot-7b has been attracting attention because it has been confirmed as a rocky planet about as dense as the Earth, rather than a ball of gas like other exoplanets.

Corot-7b is far too hot to sustain life, but the exoplanet is believed to have an average density about the same as the Earth, so likely has silicate rocks like those in the Earth's crust.

The planet is about 1.6 million miles from the star it orbits, close enough that it is gravitationally locked to it in the same way the moon is locked to the Earth. The side of the planet that always faces its star can get as hot as 4,220 degrees Fahrenheit. The side that doesn't is much colder.

The Washington University researchers used calculations to model Corot-7b's atmosphere. They didn't know the planet's composition, but said they got similar results using four different compositions.

"We don't know about the atmosphere (of Corot-7b) yet, but it could have this silicate atmosphere," said Kristen Menou, an associate professor of astronomy at Columbia University who didn't work on the St. Louis research but studies exoplanets.

Discoveries about exoplanets have been so surprising and diverse, they have been providing more incentive for scientists to study chemistry and changes in planetary environments that are very different from the earth.

"I think it will generate more interest," Menou said.