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Exoplanets orbiting host stars in reverse direction found

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An artistic impression of an exoplanet in a retrograde orbit. An ESO Photo

PARIS (BNS): In an intriguing finding that could pose a serious challenge to the existing theories of planet formation, astronomers have come across some transiting exoplanets that are orbiting in the opposite direction of their host stars.

“This is a real bomb we are dropping into the field of exoplanets,” said Amaury Triaud, a PhD student at the Geneva Observatory who leads a major part of the observational campaign.

The new findings emerged after the discovery of nine new transiting exoplanets which was announced on Tuesday at the RAS National Astronomy Meeting (NAM2010).

When the new results were combined with previous observations, astronomers found that six out of 27 exoplanets were revolving around their parent stars in “retrograde motion.” They also suggest that systems with exoplanets of the type known as hot Jupiters are unlikely to contain Earth-like planets.

Astronomers have long propounded the theory that planets form in a disc of gas and dust that encircles a star and such planets orbit their host stars more or less in the same direction as the stars’ own rotation path. Similar pattern is observed in our Solar System.

However, the newly discovered exoplanets do not follow this rule. While some hot Jupiter exoplanets (having a mass similar to or more than Jupiter) were found to be “misaligned” with the rotation axis of their parent stars, six other exoplanets were orbiting their parent stars in “wrong” direction.

The origin of hot Jupiters has always puzzled researchers. With a larger mass, these planets circle their suns from a very close distance. Their cores are thought to form from a mix of rock and ice particles found only in the cold outer reaches of planetary systems. Hot Jupiters must therefore form far from their star and subsequently migrate inwards to orbits much closer to the parent star.

Many astronomers believed this was due to gravitational interactions with the disc of dust from which they formed. This scenario takes place over a few million years and results in an orbit aligned with the rotation axis of the parent star. It would also allow Earth-like rocky planets to form subsequently, but unfortunately it cannot account for the new observations.

“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,” said Andrew Cameron of the University of St Andrews, who presented the new results at the RAS National Astronomy Meeting in Glasgow.

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