Through the observation of phenomena know as transits, astronomers believe that CoRoT will be able to discover a few dozen of telluric extrasolar planets ? i.e. bodies with characteristics similar to those of the rocky planets existing in our own solar system, orbiting around stars other than the Sun.

The existence of planetary systems beyond our own has been repeatedly confirmed over the past decade. In fact, over 200 planets have already been discovered orbiting stars other than the Sun. The great majority of these planets are giant gas planets, similar to Jupiter in our solar system, many of them in very close orbits around their parent star and hence known as ?hot Jupiters?. They have been detected through the effect they produce on the motion of the parent star. The detection of small rocky planets orbiting distant stars, on the other hand, remains extremely challenging.

The most promising method for detecting 'small worlds' consists in looking for the drop in brightness observed when they pass in front of their parent star. Such a celestial alignment is known as a transit. From Earth, both Mercury and Venus occasionally pass across the disk of the Sun. When they do, they look like tiny black dots transiting the bright solar surface. Such transits block a tiny fraction of the parent star light. When a distant star is transited by the equivalent of Jupiter, about 1% of its light is blocked from view. In the case of a transit of an Earth-like planet, the attenuation in the star light would be only one part in ten thousand.

During the course of the mission, CoRoT will observe many thousands of stars in search for exo-planetary systems. Astronomers expect that a large number of giants (hot Jupiters), and a few dozen of smaller rocky planets will be discovered, leading to an unprecedented step in our understanding of the formation of planetary systems around distant stars.