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A Happy New Year to all my readers. May next year’s science be as good as the science of 2009.
The most popular story of the year is also one of the most bizarre: a planet where it rains rocks.
Exoplanet COROT-7b is 500 light years away, orbiting a star in the Monoceros constellation. It’s orbit is what makes it so unique: it circles its star at a distance that is 23-times closer than Mercury’s orbit around the Sun.
“It’s amazing how close this planet is to its star,” says Bruce Fegley Jr., Ph.D., professor of earth and planetary sciences at Washington University in St. Louis. “It’s just a hot little bugger.”
The side that faces the sun is estimated at 2600 Kelvin, about 4500 degrees Fahrenheit. That’s as hot as the tungsten filament in a light bulb. At that temperature, rocks boil. “If you had the Earth at this temperature, all the rocks would be molten,” says Fegley.
Earlier studies showed that when silicate is vaporized, elements of the rock remain in the atmosphere. And, just like water on Earth, when those vapors condense, it rains. But on COROT-7b, the rain is pebbles.
At least that’s what the model suggests. “It’s the only object where you may, and I stress may, have an atmosphere composed of silicate vapor,” says Fegley. “As far as we know it’s a unique atmosphere…. It’s the most exciting thing since sliced white bread.”
And what makes it all the more exciting is that the model uses code Fegley created with the late Alastair Cameron, a great astrophysicist at Harvard University 20 years ago to explain why Mercury appeared to have such a high metal content, somewhere between 60-70 percent.
“[Al] had an idea that maybe during the early history of the planet and the solar nebula, the planet that became Mercury got really hot and boiled off the silicates, and so that’s what we modeled” says Fegley. That model is very similar to COROT-7b, another planet were silicates are undoubtedly boiled off. But Mercury’s story didn’t end there. Cameron had another theory. “[He thought] that maybe there was a big impact that knocked off the silicate. These two models actually come up with different conclusions as to the composition of the planet in terms of the abundance of certain elements.”
Those models are 20 years old. Today, they are close to being tested. The Messenger mission to Mercury should be able to tell the composition of the planet, should be able to tell which of the models is correct.
In the meantime, the 20-year-old code is still proving highly useful, a fact that Fegley truly likes.
“I had no idea [the model would be used for exoplanets], because we’d never heard of such a thing,” he says. “I didn’t think at that time that we’d take [the model for Mercury] and apply it to something so exotic.”
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