An exoplanet, or extrasolar planet, is a planet orbiting a star other than the Sun. Since the first was discovered in 1995, more than 400 exoplanets were detected by different techniques. One of those techniques is using transits (like CoRoT) to measure the temporary and very low drop in the light emissions received from a star when a planet is passing between the star and us - just like when the moon is between the Sun and the Earth.
This transit method offers a huge benefit providing the planet diameter: the bigger the planet, the bigger the drop in received light is. Thanks to complementary spectroscopic measurements from the ground, it is thus possible to calculate its mass and density. Then, using our knowledge about the solar system's planets, scientists can guess its nature: rocky, liquid, gaseous, etc. It is even possible to provide a general idea of its structure. It is impossible to go this far in the exoplanet characterization if it does not transit.
But the transit method, like almost every others, can hardly detect exoplanets little larger than the Earth, called Super-Earths. That is why, before CoRoT-7b, the only transiting exoplanets discovered (around 70 planets) were gaseous giant planets such as Jupiter. A little disappointing when searching if, and where, life could have appeared. The major achievement of CoRoT is to be the first to discover a transiting Super-Earth: the received light emissions only dropped by 0.03%!
CoRoT-7b is approximately 1.7 times larger and 5 times more massive than Earth. Its Earthlike density confirms its telluric (or rocky) nature. Its structure could also be similar to the Earth's one. Very close to its host star, CoRoT-7b orbits in less than 21 hours - new record! The temperature on its day-face, constantly illuminated, can reach 2,500ºC, whereas it is freezing on its night-face at -240ºC.
CoRoT-7b is thus highly unlikely to host life. But it is the evidence of the existence of rocky planets in the universe. And the climate could be more hospitable on some of them... The record was broken with the Kepler-10b exoplanet.