Star Distorts Planet's True Size

July 9, 2009 -- Astronomers observing exoplanets around other stars may be underestimating their size, according to a German study.

The researchers believe the error may be due to active stars adding 'noise' to the observation of exoplanets using the transit method.

Their study appears on the electronic preprint website arXiv.

The transit method detects exoplanets as they pass in front of their parent star, reducing the amount of light reaching telescopes on, and orbiting, Earth.

Although the transit method isn't the best method for detecting exoplanets, it provides a reliable estimate of its size and mass.

PhD student Stefan Czesla of the Hamburg Observatory in Germany, and colleagues, examined the giant exoplanet Corot-2b, using data from the French COROT satellite.

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Discovered in 2007, Corot-2b is three and a half times the mass of Jupiter and orbits its star in just 1.74 days.

After closely examining the light curves recorded by COROT, which involved splitting them into their red, green and blue components, the researchers determined that the exoplanet is 3 percent bigger than previously thought.

Czelsa and colleagues believe this discrepancy may be true for other exoplanets around active stars.

"For planets found around active stars the determination of their exact physical parameters is considerably complicated by stellar activity," says Czesla.

Bright and dark spots on the star can modify the transit light curves, something that isn't accounted for in models currently used to calculate an exoplanet's size, he says.