A planet that rocks

October 1, 10:07 AMScience News ExaminerMeg Marquardt

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Rendition of the molten surface of COROT-7b, the planet where it rains rocks.  Credit: ESO/L. Calcada. (source)

Imagine the following scenario: 

You are preparing to leave for work and shout down the hall to your significant other. “Honey, what’s the weather supposed to be like?” 

And the reply: “50% chance of rock.”

 Best pack the steel umbrella.

 If there were life on the newly-discovered exoplanet COROT-7b, they would face a unique problem. Unlike Earth, where precipitation falls in various forms of water, on COROT-7b it actually rains rocks.

Researchers at Washington University in St. Louis [WUSTL] have been running models of the planet that was discovered in February. According to these models, the 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.” [WUSTL]

 COROT-7b is among only a handful of rocky exoplanets (planets outside our solar system). Most are gas supergiants, like Jupiter. However, other than its rocky surface, COROT-7b is far from Earth-like. Besides the strange pebble showers, it is so close to its sun that its orbit is like the Moon around Earth. One face is always pointed towards its sun. That side is thought to have a temperature of 4220 degrees Fahrenheit.

 At that heat, rocks will vaporize. The vaporization of the its surface is why Dr. Bruce Fegley Jr of WUSTL believes the precipitation on COROT-7b is pebbly in nature. “The only atmosphere this object has is produced from vapor arising from hot molten silicates in a lava lake or lava ocean," Fegley says. [WUSTL]

 Published in The Astrophysical Journal, the study utilized a computer system originally developed in 1986. Though the exact composition of the planet is unknown, running the program with various constraints yielded consistent results: rock showers. "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." [WUSTL]

 It is nearly impossible to imagine a deluge of pebbles falling from the sky, or turning on the morning forecast to hear reports of “rocking” instead of “raining.” But the research goes to show just how unusual the universe can be.

 The WUSTL press release has an even more detailed explanation of the process. Be sure to investigate it here.