Scientists have discovered a new Jupiter-sized planet orbiting around a distant sun-like star.

This planet takes 9.2 days to orbit its star, the longest period for any transiting exoplanet ever found.

The planet, called CoRoT-Exo-4b, was discovered by the European Space Agency's CoRoT space telescope, which was designed to search for extrasolar planets by looking for transits, or tiny dips in the light output from a star when a planet passes in front of it.

By tracking the time between transits, a team of scientists led by the French space agency CNES measured how long the planet takes to revolve around its star, and found that it is the same period of time its star, which is slightly larger than our sun, takes to rotate 360 degrees.

They were able to derive the star's period of rotation by monitoring dark spots on its surface that rotated in and out of view.

"We don't know if CoRoT-Exo-4b and its star have always been rotating in sync since their formation about 1 billion years ago, or if the star became synchronized later," said University of Exeter researcher Suzanne Aigrain.

The researchers said the finding is surprising because the planet is thought to be too low in mass and too distant from its star to have a strong enough gravitational pull to influence its rotation.

This is the first transiting exoplanet found with such a peculiar combination of mass and period of rotation.

The scientists say they hope CoRoT, which launched in December 2006, will help them discover the special circumstances of the planet's formation and evolution.

"CoRoT will no doubt find many more transiting planets, and by systematically measuring their host stars' rotation periods we will gain valuable insight into how stars interact with their planets," Aigrain said.

The research team presented the CoRoT-Exo-4b findings today at the Cool Stars 15 meeting at St Andrews University in Scotland.

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