A new gas giant exoplanet roughly the size of Jupiter orbiting around a star similar to our Sun has been found by the CoRoT space telescope operated by the European Space Agency. The planet, dubbed CoRoT-Exo-4b, was discovered through a transit method, appears to complete an orbit around its star in about 9.2 days and the two objects seem to be gravitationally locked, meaning that the star's spin around its axis coincides to that of the planet's orbital speed, the central object always showing the same face towards its companion.

The discovery also marks the finding of the coolest gas giant planet outside the solar system to date. "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 researcher Suzanne Aigrain of the University of Exeter.

Although the orbital speed of the planet is synchronous to that of the star's spin, measurements seem to indicate that the gas giant is located too far away from its sun to have such a powerful influence on it, not to mention that its mass is also significantly lower than that required to produce the gravitational forces necessary to generate the observed effects. "It would take longer than the age of the universe" to achieve a gradual synchronization, Aigrain said.

However, a similar synchronization could have occurred naturally as the system formed. "It could be a coincidence, but I don't like explanations based on coincidence," says Aigrain. Instead she suggested that the two objects might have become gravitationally locked as a consequence of the interaction between magnetic fields, but until now no evidence to support this theory has been found.

The CoRoT spacecraft began its mission in December 2006, and is scheduled to end it by mid-2009. Further investigations will most certainly discover other exoplanets in the galaxy, to provide information about the way stars and planets interact with each other. Maybe, some of these systems will also be able to indicate what makes the orbit of CoRoT-Exo-4b synchronous with its star.