Image: CoRot-7b
ESO
An artist's conception shows CoRoT-7b, a searingly hot planet around a distant star that is about five times Earth's mass, but orbits only about 1.6 million miles from that sun. Scientists have determined that CoRoT-7b is a rocky world, perhaps covered with lava.
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updated 9/16/2009 1:15:44 PM ET 2009-09-16T17:15:44

One of the smallest exoplanets yet discovered has just been confirmed as a rocky world, scientists announced.

The planet, called CoRoT-7b, is the first planet beyond our solar system with a proven density similar to Earth's, astronomers say. Most known exoplanets are large gas giants like Jupiter.

"We have indications that other exoplanets could be rocky, but it's the first time that the density of such a planet has been measured," said study team member Claire Moutou of the Laboratoire d'Astrophysique de Marseille in France. "We are really sure it's rocky."

Though its terrestrial surface renders CoRoT-7b more similar to Earth than many other exoplanets are, it's still a far cry from a familiar setting. The planet orbits extremely close to its star — about 1.6 million miles (2.5 million kilometers), or 23 times closer than Mercury is to the sun. At this range, the planet's surface temperatures are scorching, with highs above 2,000 degrees Fahrenheit (1,000 degrees Celsius) on the star-facing side.

CoRoT-7b's close proximity to its star means that the planet is likely to be tidally locked, with one side always facing its sun and the other side always in darkness.

"Probably the day side is very hot and is pure lava, boiling, and the other side probably is very cold, and it could be rocky with some mountains," Moutou told SPACE.com. "It's not possible that there is liquid water."

Crowded universe
CoRoT-7b was discovered in February 2009 by the CoRoT space telescope, a European collaboration. The tiny planet was discovered orbiting a star slightly smaller and cooler than our sun, about 500 light-years away. As the planet passed in front of its star, it eclipsed a small portion of the star's light, causing a dip in brightness.

This dip was enough to tell that a planet existed, and to estimate the planet's distance from its host star and its radius, which is about 80 percent larger than Earth's. But to learn its density, which would reveal whether it is a rocky or gas planet, scientists had to make a precise measurement of the parent star's velocity, which is slightly warped by the planet's small mass.

To make this measurement, astronomers used the High Accuracy Radial-velocity Planet Searcher, or HARPS, a spectrograph on the European Southern Observatory's 3.6-meter telescope at La Silla Observatory in Chile. The new data revealed that CoRoT-7b has a mass about five times that of Earth, making it one of the lightest exoplanets yet found.

Slideshow: Month in Space With the planet's mass and radius, the researchers calculated its density (about 4.7 grams per cubic centimeter), which placed it in firm rocky territory.

"This is the first proof of the detection of a rocky planet," planet-formation theorist Alan Boss of the Carnegie Institution of Washington told Space.com.

"It shows that rocky planets really are commonplace," said Boss, who was not involved in the new research. "The estimates are that about 30 percent of sunlike stars have these hot and warm super-Earths, and now that we know the density of one of them, it is easy to make the claim that most of the rest of them are probably rocky too. The evidence is becoming overwhelming that we live in a crowded universe."

Looking for life
Finding a rocky planet with an Earthlike density takes us one step closer to discovering another planet similar to our own. A twin Earth beyond the solar system could offer the best chance of finding life elsewhere in the universe, scientists say.

Although CoRoT-7b's lack of liquid water means it's unlikely to host life, the planet's discovery is still a promising sign. CoRoT and NASA's Kepler space observatory are both up there as you read this, seeking such a discovery.

"We are searching for any kind of exoplanets," Moutou said. "We're trying not to be biased by our own system, but of course we would be very interested to find a planet where life could develop. This one is not habitable, but some future planets of this kind could allow life to develop. This is our longtime goal, to find an analog to Earth."

The research team, led by Didier Queloz of the Geneva Observatory in Switzerland, described the results in a paper to be published in the Oct. 22 issue of the journal Astronomy and Astrophysics.

More on extrasolar planets | CoRoT

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