There may be no place like home, but a recently discovered planet beyond the solar system has some awfully familiar traits. Astronomers report that new measurements provide the first solid evidence for a rocky extrasolar planet and the orb has a composition similar to that of Earth’s interior.

The planet, about 500 light-years from Earth, closely orbits its parent star and is much too hot to support life, about 2,000˚ Celsius on its sunny side. However, the diameter and newly determined mass of the body, dubbed COROT-7b when it was found in February, indicate that the planet has a bulk composition highly similar to Earth’s. For example, the planet likely has a silicate mantle and an iron core, Didier Queloz of the Geneva Observatory in Sauverny, Switzerland, and his colleagues report in an upcoming Astronomy & Astrophysics.

“This is truly the first rocky world beyond the solar system, and we know there’s more to come,” comments theorist Sara Seager of MIT. “This is a day we've been waiting for, for a long time.” The new find, along with about a dozen other known heavyweight versions of Earth, may help astronomers understand how terrestrial planets form around other stars and how common they are. Although planet hunters ultimately hope to find Earthlike planets in life-friendly orbits, for now scientists are happy to settle for discovering even uninhabitable analogs of Earth. 

In February, Queloz’s team announced it had found the planet — the smallest extrasolar planet yet known, with a diameter of about 1.8 times the diameter of Earth. The scientists were able to pin down the size of the planet because the orb periodically passes in front of its parent star as seen from Earth, blocking a tiny amount of starlight. These passages, or transits, were recorded by the COROT satellite (SN: 2/28/09, p. 9). 

But at that time, the scientists had only a rough estimate of the mass of the planet, ranging between five and 11 times the mass of Earth. Since then, the team has more accurately measured the tug of the tiny planet on its parent star using the HARPS spectrograph in La Silla, Chile. The team now finds that the planet has a mass about five times that of Earth.

The new mass measurement, in combination with the diameter, reveals that the planet has an average density of about 5.6 grams per cubic centimeter, almost identical to that of Earth.

“This mostly likely means that it has to be a rocky planet,” comments Alan Boss of the Carnegie Institution for Science in Washington, D.C. “This is a big deal.” 

Because the planet’s star is both faint and variable, astronomers cannot use the starlight to determine if the planet has an atmosphere or to infer the composition of the planet’s surface, Seager says. But other systems, with brighter, steadier stars, should allow more detailed studies of this type of planet, known as a superEarth.  

With the recently launched Kepler satellite joining COROT in hunting for small, transiting planets, “it’s only a matter of time before we have a large number of them,” Seager says.