COROT discovery stirs exoplanet classification rethink
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 06, 2008

COROT scientists have discovered the most massive planet-sized object closely orbiting its parent star yet, but there remains one question: is it really a planet or is it a failed star?

COROT-exo-3b is about the size of Jupiter but 20 times more massive and takes just 4 days and 6 hours to orbit its parent star, which is slightly larger than the Sun. The exotic body was detected by the COROT space telescope as the object transited in front of its parent star, forcing a drop in the star’s brightness.

"We were taken by surprise when we found this massive object orbiting so close to its parent star,” says Dr Magali Deleuil from the Laboratoire d'Astrophysique de Marseille and leader of the team that made the discovery. "COROT-exo-3b is really unique - we're still debating its nature."

An artist§s impression of the relative sizes of the Sun, Jupiter and COROT-exo-3b. Image: OAMP.

Planet-hunters have been searching for planets which closely orbit their parent stars with periods of less than 10 days for almost 15 years. During this time, scientists have encountered planets with masses 12 times that of Jupiter, and stars with 70 times as massive as Jupiter, but none in between, which adds to the surprise of discovering 20-Jupiter-mass COROT-exo-3b. And it is even more of a surprise since it does not fall into either conventional category of planets or brown dwarfs. A brown dwarf is a 'failed star', one that is not massive enough to maintain nuclear fusion at its core, but which still displays some stellar characteristics.

"COROT-exo-3b might turn out to be a rare object found by sheer luck,” says team member Dr Francois Bouchy. "But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our Sun. We're now beginning to think that the more massive the star, the more massive the planet.”

The perplexing discovery could require a rethink as to where to draw the line between planets and brown dwarfs. As a planet, COROT-exo-3b would be the most massive and the densest found to date - more than twice as dense as lead. Further study of the system will help astronomers better understand how to categorise such objects, and to learn how such a massive object formed so close to its parent star.

In addition to the initial discovery by COROT, the observations were supported from the ground to further study the object's mass, orbit and stellar properties. The international network of telescopes included the telescope of Observatoire de Haute Provence in France, the European Southern Observatory telescopes at Paranal and La Silla in Chile, the Thuringia State Observatory in Tautenburg, Germany, the Canada-France-Hawaii Telescope on Mauna Kea, the Swiss Euler Telescope at La Silla, Chile, the Wise Observatory in Israel, the ESA telescope on Mt. Teide, Tenerife, and the telescope of the Astrophysical Institute of the Canary Islands.

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