Dimmer switch. Too faint to be seen directly, exoplanets are often detected when they eclipse their parent stars.


Mercury, Schmercury. This Planet is Really Toasty

Astronomers today reported the discovery of a planet beyond our solar system that's just twice the diameter of Earth. But this new neighbor is far from habitable: CoRoT-Exo-7b is either a hellish world of erupting volcanoes and sizzling temperatures, or it's covered by a scalding ocean. Either way, the discovery could pave the way toward finding more Earth-sized alien worlds, some of which may actually be habitable.

To date, astronomers have discovered some 330 planets orbiting stars other than our sun. But almost none are rocky like Earth and its inner-solar-system neighbors. Most are gas giants, even bigger versions of Jupiter. The apparent scarcity of Earth-like worlds probably has nothing to do with their actual abundance. Rather, such small objects are difficult to distinguish against the blinding glare of their host stars.

That's where CoRoT comes in. The European spacecraft, launched in 2006, can detect minute decreases in the light output of stars caused when planets pass in front of them. At a CoRoT symposium in Paris today, mission scientists reported the discovery of the smallest planet yet found orbiting a sunlike star. CoRoT-Exo-7b is about 425 light-years away in the constellation Monoceros. The discovery is unexpected because the researchers weren't certain the spacecraft could resolve planets that are so small, says team leader Daniel Rouan of the Observatoire de Paris. What is surprising, and exciting, says planetary scientist Jack Lissauer of NASA's Ames Research Center at Moffett Field, California, is the proximity to its star, so close that one orbit takes only about 20 hours. (Mercury's orbit takes 88 days.) "This is the record-holder," says Lissauer, who was not part of the discovery team.

That proximity to the sun makes for less than ideal conditions on CoRoT-Exo-7b's surface, which the team estimates burns at more than 1000°C. But things may not have always been that way: Recent computer simulations suggest that CoRoT-Exo-7b may have been an icy world that originally formed far away from its parent star, says Lissauer. Then, over billions of years, it migrated inward, finally settling into its extremely close and nearly perfect circular orbit. If so, he explains, the planet may contain substantial quantities of water, and perhaps hydrogen and helium. If there's a rocky surface, he says, it could lie "beneath thousands of kilometers of fluid."

Confirmation will come with further observation. "We can find the density of the planet, which will tell us something about its composition," says team member Malcolm Fridlund of the European Space Agency. "We will study this object for years."

Meanwhile, Lissauer says, the discovery bodes well for NASA's Kepler spacecraft, due to be launched in March. With its larger field of view and more sensitive instruments, it will have an even greater chance of spotting Earth-like alien worlds.