On Alien World, It Rains Rocks

Andrea Thompson
Senior Writer

On Earth, strange things, including frogs and fish, sometimes fall from the sky, but on a distant extrasolar planet, the weather could be even weirder: When a front moves in, small rocks rain down on the surface, a new study suggests.

The exoplanet, COROT-7b, was discovered in February by the COROT space telescope launched by the French and European space agencies. Last month is became the first planet outside our solar system to be confirmed as a rocky body — most other known exoplanets are gas giants.

The planet is nearly twice the size of Earth and about five times the mass of our world. Calculations have indicated it has a density about that of Earth's, which means it is likely made up of silicate rocks, just as Earth's crust is.

The planet is likely much less hospitable to life though, as it is only about 1.6 million miles (2.6 million km) away from its parent star — 23 times closer than Mercury sits to the sun.

Locked and scorching

Because the planet is so close to the star, it is gravitationally locked to it in the same way the Moon is locked to Earth. One side of the planet always faces its star, just as one side of the Moon always faces Earth.

This star-facing side has a temperature of about 4,220 degrees Fahrenheit (2,326 degrees Celsius) — hot enough to vaporize rock.

So unlike the much cooler Earth, COROT-7b has no volatile gases (carbon dioxide, water vapor, nitrogen) in its atmosphere. Instead it's atmosphere consists of what might be called vaporized rock.

"The only atmosphere this object has is produced from vapor arising from hot molten silicates in a lava lake or lava ocean," said Bruce Fegley Jr., of Washington University in St. Louis.

Rocky weather forecast

To find out what COROT-7b's atmosphere might be like, Fegley and his colleagues modeled it. They found that 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.

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

The rock rains form similarly to Earth's watery 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," Fegley explained. "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."

Menagerie of minerals

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 from COROT-7.

These large clouds may be detectable by Earth-based telescopes. The sodium, for example, should glow in the orange part of the spectrum, like a giant but very faint sodium vapor streetlamp.

Observers have recently spotted sodium in the atmospheres of two other exoplanets.

The new modeling finding is detailed in the Oct. 1 issue of The Astrophysical Journal.