Smallest known transiting planet discovered

Finding could help to understand composition and structure of planets beyond the solar system that could have structures like Earth’s

Astronomers report today that they have found an extrasolar planet no more than 11 times Earth’s mass with a diameter about twice that of Earth. The discovery may ultimately provide groundbreaking information about the composition and structure of terrestrial planets beyond the solar system.

PASSING BY One way to find an extrasolar planet is to detect a drop in brightness when these bodies pass in front of their parent stars. The COROT satellite used this technique to find a hot superEarth, the smallest planet known to transit a star beyond the solar system. IMAGE CREDIT: CNES

Once the team can refine an estimate of the planet’s mass, “this could be a potentially huge discovery, one that we’ve been waiting for for a long time,” comments theorist Sara Seager of MIT. “I’m excited to see what more the team will find out about this prospective terrestrial planet.”

Classified as a hot superEarth, the planet, too small to be imaged, lies much too close to the blistering heat of its parent star to support life. Located about 450 light-years from Earth, the planet whips about its sunlike star in just 20 hours, has a surface temperature between 1,000 and 1,500 degreesCelsius and might be covered by lava or water vapor.

Although scientists have found a few extrasolar planets with even smaller masses, the newly found body is the tiniest known that periodically passes between its parent star and Earth, blocking a tiny fraction of starlight during each transit. These minieclipses yield the planet’s radius and, in combination with another planet-searching technique, provide a trove of information, including the exact mass, density and composition of the bodies, notes Daniel Rouan of the Observatory of Paris in Meudon, France. He and his colleagues reported the find February 3 at a symposium in Paris about the European Space Agency’s COROT satellite, the craft that detected the planet’s transits.

Since 1999, researchers have discovered more than 40 transiting planets, most of them heavier than Jupiter, which is about 318 times more massive than Earth. Over the past 14 years, astronomers have discovered about 330 extrasolar planets. The vast majority were found solely by the wobble their gravity induces in the motion of their parent stars.

Rouan and his collaborators first saw signs of the transiting planet, dubbed COROT-Exo-7b, about a year ago. After COROT recorded the minieclipses —which by themselves reveal only the size of the orbiting body, between 1.75 and two times Earth’s diameter — the team followed up with ground-based measurements of the parent star’s wobble in order to determine the planet’s mass. But because of uncertainties in the wobble measurements, confounded by activity of the parent star that can mimic a wobble signal, for now the team can only say the planet is no heavier than 11 Earths. (Before the new find, the smallest known transiting extrasolar planet was GL 436b, a body that was discovered in 2007 and weighs 23 Earth masses, about the mass of Neptune.)

“All the possible sources for this COROT signal have been excluded at a high level of confidence, except one: a planet with a radius slightly smaller than two Earth radii transiting in front of the star,” says team member Michael Gillon of the Observatory of Geneva in Sauverny, Switzerland. Velocity measurements of the star “firmly detect a signal compatible with this scenario,” he says, but the team can only conclude so far that the planet weighs between five and 11 Earth masses.

An analysis of recently acquired data could provide a more accurate estimate of the mass of COROT-Exo-7b in a few weeks, Rouan told Science News. That information would be immediately used to discern the density and likely composition of this hot superEarth. Depending on this density, the extrasolar planet might be rocky, making it more Earthlike than, for example, Neptune-like.

However, a firm mass estimate is required before anyone gets too excited about the discovery, says theorist Alan Boss of the Carnegie Institution for Science in Washington, D.C.

Seager notes that Neptune-like planets previously found by the transit method have huge hydrogen and helium envelopes that would crush any solid surface and make it extremely hot, entirely unlike a terrestrial planet. “So finding a rocky planet is really important,” she says.

COROT-Exo-7b is indeed the smallest extrasolar planet found to date that transits, but not the smallest extrasolar planet ever detected, despite claims in a press release circulated by the European Space Agency, says Boss. For instance, the best estimate for the mass of the extrasolar planet GL 876d is 7.5 Earth masses, he notes. But such non-transiting orbs don’t reveal their composition or density, he adds.

Although the planet’s exact composition is unknown, Gillon says, it’s likely to be a massive rocky planet or a water-rich planet, perhaps the remnant of a Neptune-mass planet that lost its gaseous hydrogen envelope after migrating too close to its parent star. “This discovery is important because it opens the era of the study of terrestrial exoplanets,” he says. “For the very first time in human history, we can study a solid planet located outside our solar system. This is a major step towards the detection and study of actual Earth twins orbiting other stars.”

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