More than a hundred extra solar planets have been found in recent years but until now all were discovered by the “radial velocity method”, which detects the gravitational pull a planet exerts on its parent star. Sasselov and co-workers have used an independent method, which measures the periodic dimming of starlight as a planet moves across -- or transits -- the line of sight between the observer and the star. Despite many searches, no transiting planets have been discovered this way before.

Sasselov and colleagues examined data from the Optical Gravitational Lensing Experiment and identified five stars that they thought could be transited by a planet. They used the high-resolution echelle spectrograph on the Keck telescope in Hawaii to obtain detailed spectra and precise velocities of these objects. The team found that one of these stars, known as OGLE-TR-56, showed a variation in brightness that was consistent with velocity changes measured using traditional radial velocity methods.

The results from the Keck study suggest that the planet is about 5000 light years from Earth -- making it the most distant extra solar planet to be found to date. Radial velocity measurements show that it is about 0.9 times as heavy as Jupiter and orbits its parent star every 29 hours at distance of only 3.5 million kilometres -- closer than any other known planet to its star.

The researchers say that the planet is the first to be found with an orbital radius much smaller than current models allow -- some theorists believe that the planet should have been consumed by its star. The results, which have yet to be confirmed by further observation, suggest that the planet may belong to a small population of objects known as “Class II planets”. These planets have lost a large amount of their mass to the parent star but have still managed to survive.