London, July 26 : A team of European scientists working with COROT (Convection Rotation and planetary Transits) has discovered an exoplanet that seems to have a surprisingly powerful influence on its parent star, forcing the star to rotate at exactly the same rate as it orbits.
The planet's day is also the same length, so the pair are fixed in a face-to-face whirl.
According to a report in New Scientist, the puzzle is how this planet, called COROT-Exo-4b, could have so dominated the vastly larger star, which is bigger than our Sun.
The new planet was discovered by the European COROT satellite, launched in 2006, which searches for transits - the telltale dimming of stars caused by planets passing in front of them.
From the dimming effect on the light of its star, the COROT team worked out that that Exo-4b is roughly the size of Jupiter.
It is fairly close to its parent star, taking only 9.2 days to orbit, but that still puts it further out than most "hot Jupiters".
"The planet seems too distant to have such a strong influence on the star," said astronomer Suzanne Aigrain of the University of Exeter, UK.
Monitoring COROT-exo-4b continuously over several months, the team tracked variations in its brightness between transits. They derived its period of rotation by monitoring dark spots on its surface that rotated in and out of view. It is not known whether COROT-exo-4b and its star have always been rotating in sync since their formation about 1000 million years ago, or if the star's rotation synchronized later.
The planet's gravity will raise tides in the fluid body of the star, which would very gradually synchronise the planet's orbit and the star's rotation - but not within the billion-year lifetime of the system.
"It would take longer than the age of the universe," Aigrain told New Scientist.
Instead, the system might have started out that way.
Aigrain suggests that magnetic fields might have helped to lock the system together, but stresses that it is pure speculation at this point.
During its mission, COROT should discover many more planets.
Aigrain and her colleagues hope they will then have a better idea whether synchronised systems are common, and what causes the phenomenon.
Studying such systems with COROT will help scientists gain valuable insight into star-planet interactions.