It's raining pebbles on recently discovered exoplanet

WASHINGTON: A simulation developed by scientists at Washington University in St Louis has suggested that the atmosphere of COROT-7b, an exoplanet discovered last February by the COROT space telescope, is made up of the ingredients of rocks, which results in a rain of pebbles on the surface of the planet.

Theoretical models suggest the planet has an atmosphere of the components of rock in gaseous form and lava or boiling oceans on its surface.

According to models by scientists at Washington University in St Louis, COROT-7b's atmosphere is made up of the ingredients of rocks and when "a front moves in," pebbles condense out of the air and rain into lakes of molten lava below.

The model was developed by Laura Schaefer, research assistant in the Planetary Chemistry Laboratory, and Bruce Fegley Jr, professor of earth and planetary sciences in Arts and Sciences.

COROT-7b is less than twice the size of Earth and only five times it's mass.

In August 2009 a consortium of European observatories led by the Swiss reported the discovery of COROT-7c, a second planet orbiting COROT-7. Using the data from both planets, they were able to calculate that COROT-7b has an average density about the same as Earth's.

"This means it is almost certainly a rocky planet made up of silicate rocks like those in Earth's crust," said Fegley.

COROT-7b's atmosphere has none of the volatile elements or compounds that make up Earth's atmosphere, such as water, nitrogen and carbon dioxide.

"The only atmosphere this object has is produced from vapour arising from hot molten silicates in a lava lake or lava ocean," Fegley said. To find out what might that atmosphere be like, Schaefer and Fegley have used thermo chemical equilibrium calculations to model COROT-7b's atmosphere.
"Sodium, potassium, silicon monoxide and then oxygen - either atomic or molecular oxygen - make up most of the atmosphere," said Fegley.

But there are also smaller amounts of the other elements found in silicate rock, such as magnesium, aluminium, calcium and iron.

The peculiar atmosphere has its own singular weather. "As you go higher the atmosphere gets cooler and eventually you get saturated with different types of 'rock' the way you get saturated with water in the atmosphere of Earth," explained Fegley.

"But instead of a water cloud forming and then raining water droplets, you get a 'rock cloud' forming and it starts raining out little pebbles of different types of rock," he added.