Rocks fall like rain on exoplanet

We may like to moan about the weather on this planet, but at least it doesn’t rain rocks. It’s not so on Corot-7b, though, where simulations indicate that pebbles condense out of the air and rain into lakes of molten lava below.

Discovered last February by the Corot space telescope, the exoplanet Corot-7b is so close to its star that its day-face is hot enough to melt rock. Models suggest the planet’s atmosphere consists of the components of rock in gaseous form, with lava or boiling oceans on its surface. 

Last month a consortium of European observatories was able to calculate that Corot-7b has an average density about the same as Earth’s. But it’s much closer to its star, and gravitationally locked so that one side always faces its star. This star-facing side has a temperature of about 2,600 degrees Kelvin.

Perhaps because they were cooked off, 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 vapor arising from hot molten silicates in a lava lake or lava ocean,” says Professor Bruce Fegley of Washington University.

The team used thermochemical equilibrium calculations to model Corot-7b’s atmosphere. Because the scientists didn’t know the exact composition of the planet, they ran the program with four different starting compositions. “We got essentially the same result in all four cases,” says Fegley.

“Sodium, potassium, silicon monoxide and then oxygen — either atomic or molecular oxygen — make up most of the atmosphere.” But there are also smaller amounts of the other elements found in silicate rock, such as magnesium, aluminum, calcium and iron.

The atmosphere has its own weird 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,” explains 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.”

Even more strangely, the kind of rock condensing out of the cloud depends on the altitude. The atmosphere works the same way as fractionating columns, with the heaviest components (with the highest boiling point) at the bottom, and the lightest (and most volatile) rising to the top.

The exoplanet’s atmosphere condenses out minerals such as enstatite, corundum, spinel, and wollastonite. Elemental sodium and potassium, which have very low boiling points in comparison with rocks, do not rain out but would instead stay in the atmosphere, where they would form high gas clouds buffeted by the stellar wind.

These large clouds may be detectable by Earth-based telescopes. Observers have recently spotted sodium in the atmospheres of two other exoplanets.

The work appears in The Astrophysical Journal.

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