A French spacecraft designed to discover new worlds beyond our solar system has made one of its most significant finds yet—a planet that looks like a cousin to those in our own celestial backyard. COROT 9 b, named by astronomical convention for the instrument that discovered it, the COROT (for COnvection, ROtation and planetary Transits) satellite, is less massive than Jupiter and orbits a star, called COROT 9, at about the same distance Mercury orbits the sun. The new world is of fairly average size, but it is the most temperate exoplanet yet whose properties are well known in orbit around a sunlike star.

A largely European research team reports the discovery the March 18 issue of Nature. (Scientific American is part of Nature Publishing Group.)

Like NASA's Kepler spacecraft, launched in 2009, the three-year-old COROT tracks the brightness of stars with a photometer, looking for periodic dimming that might be attributable to the passage of a planet across the face of its host star. Actually confirming a planetary cause of that dimming takes painstaking follow-up work at telescopes on the ground. Most often the researchers look for Doppler shifts in the host star's light as the planet's gravity regularly tugs the star nearer to and then farther from Earth.

The degree of dimming starlight during the passage of a planet across its star, a type of partial eclipse known as a transit, indicates the body's diameter. The velocity at which the star wobbles under the planet's influence, on the other hand, reveals the object's mass. With both transit and stellar-wobble observations of a planet, astronomers can paint a fairly complete picture of a world they have only indirectly observed.

"With transits we can learn much more about the planets than with any other method to find planets," says lead study author Hans Deeg, an astronomer at Spain's Institute of Astrophysics of the Canary Islands. "It's the only method currently where we can measure the size of the planets fairly reliably." On its own, a measurement of the star's wobble can only reveal a lower limit to the planet's mass, and in some cases the true mass turns out to be many times greater than that lower bound.

Residing 1,500 light-years away in the constellation Serpens Cauda, COROT 9 b has about the same diameter as Jupiter and is about 85 percent as massive. It keeps a much greater distance from its host star than the other transiting planets discovered to date, almost all of which reside in scalding hot orbits less than 10 million kilometers from their stars. The newfound world circles its star at about 60 million kilometers, leaving it with a relatively mild temperature that Deeg's group estimates to be between minus 20 degrees Celsius and 150 degrees C, depending on its atmospheric makeup. For comparison, many exoplanets are so close to their stars that their temperatures exceed 1,000 degrees C. The plentiful population of massive exoplanets in star-nuzzling orbits has been dubbed the "hot Jupiters"; COROT 9 b might be called a warm Jupiter—or even a cool one, if its true temperature turns out to be at the lower end of the estimated range.

"This is the first one that we can study in some detail that is relatively cool and which doesn't vary very much in its temperature," Deeg says. The only known transiting planet with a comparably long orbit, called HD 80606 b, has an extremely eccentric orbit; the distance between HD 80606 b and its star varies greatly throughout the planet's orbit, driving temperature changes of several hundred degrees in a matter of hours.

To confirm that the transits of COROT 9 b recorded in 2008 by the COROT satellite were indeed planetary, the researchers first took two follow-up readings of the star using a spectrograph in France, a rough sketch that was consistent with the presence of a planet. Next they ruled out a false positive, usually caused by an eclipsing binary-star system in the background, with two relatively small telescopes on the ground, which offer better spatial resolution than COROT does. But most importantly, the team detected the host star's telltale wobble using several nights of observations on the world-class HARPS spectrograph at La Silla Paranal Observatory in Chile.

The stepwise confirmation process highlights the directives behind follow-up work—to confirm exoplanet discoveries efficiently and inexpensively, explains Natalie Batalha, a professor of physics and astronomy at San Jose State University who did not contribute to the new research. "You employ cheaper resources to vet out the false positives before you send these candidates to the really expensive, big telescopes like Keck [in Hawaii] or the HARPS telescope," Batalha says.

Deeg notes that COROT 9 b, which takes 95 days to circle its star, is nearing the upper limit of the orbital period that COROT can detect—the spacecraft shifts its view every 150 days, and its detectors must record a minimum of two transits in an observation window to know the planet's orbital period. Kepler, which will keep a continuous watch on a patch of stars for more than three years, is better suited to finding planets like our own in terms of orbital periods as well as other parameters, although it will likely be a few years before it moves from the hot objects it has already discovered to cooler, potentially habitable worlds, whose transits are subtler and less frequent. A truly Earth-like planet will circle its star at a remove comparable to the Earth–sun distance, so it will pass in front of its star just once a year. (The laws of planetary motion derived by Johannes Kepler, the 17th-century German astronomer and namesake of the Kepler mission, show that the duration of an orbit depends on its size.)

Batalha, a Kepler co-investigator, says her team can learn a lot from COROT. Just one year into its mission, Kepler has already discovered five confirmed exoplanets and, Batalha says, has identified hundreds of additional candidates that await further investigation. "As a Kepler scientist, COROT is kind of paving the way with regard to space-based transit detection," Batalha says. "So I'm always really interested in seeing how they're dealing with a large volume of data and how they follow up those candidates and lead themselves to confirmation."