© COROT/Tautenburg Observatory/Klaudia EinhornThe small planet Exo-7b (lower centre) was discovered by the way it dimmed its host star's light when it passed between the star and Earth.
The planet, known as Exo-7b, lies about 390 light years away and orbits a star slightly smaller and cooler than the Sun. It was found by the French satellite COROT, which looks for the dimming caused when planets pass in front of, or transit, their parent stars.
The method revealed the world's tiny size, but could not pin down its mass precisely. To do that, researchers must search for the subtle wobbles the orbiting planet induces in its host star, a difficult task since the star's own roiling activity can mask the subtle gravitational tugs of a lightweight planet.
Nonetheless, it weighs in the neighborhood of several Earths, which puts it in the running for the lightest exoplanet known to orbit a normal star. (A less massive planet has been found orbiting a dead pulsar, but its physical size has not been measured.)
Small planet, tight orbit
COROT-Exo-7b orbits its star in only 20 hours and is heated to over 1000 °Celsius, making it much too hot to support life. But its discovery offers observational proof that small planets can exist at close distances to their host stars.
Initially, astronomers suspected that small, rocky worlds might get kicked out of their planetary systems altogether by giant planets that were pulled towards their host stars to become 'hot Jupiters'. Computer simulations later showed that many of the lightweight worlds could survive their neighbors' migrations, but this discovery confirms the models.
"People started to get a bit worried that COROT might not find these small planets in tight orbits, so the discovery of COROT-Exo-7b is reassuring news," says Rens Waters of the University of Amsterdam, who was not part of the discovery team.
Other Earths
Confirming the detection was time-consuming, since many phenomena - including sunspot-like 'starspots' - can mimic a planet's signal. But the clockwork-like transits ruled out starspots, which come and go. Indeed, the star boasts several periodic signals, suggesting it hosts more than this one planet.
Malcolm Fridlund, European Space Agency project scientist for the mission, says there are "hundreds of other interesting candidate signals" in the COROT data waiting for painstaking follow-up observations with ground-based telescopes.
The discovery, announced today at a COROT symposium in Paris, is good news for NASA's Kepler mission, which will hunt for Earth-like planets orbiting in the habitable zones of their stars. Kepler, which also searches for transits, is due to launch on 5 March.
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