CoRoT-7b is a planet known as a Super Earth – that is observations of the light it blocks from its host star suggest a diameter small enough to be a rocky planet. It lies 500 light years from us and was discovered by the French Convection Rotation Transit or CoRoT probe. The diameter was worked out to be 22,900 km, or 1.8 times Earth’s diameter. It goes round the star in 20.4 hours at a distance of 2.5 million kilometres, or about one 23rd the Sun-Mercury distance.
Follow up observations with the High Accuracy Radial Velocity Planet Searcher (Harps) instrument at the La Silla observatory of European Southern Observatory, Chile, showed that the pull of the planet’s gravity on the star suggested a mass five times that of the Earth, giving a density of around 85% that of our own planet, suggesting it is rocky.
HARPS was required to perform highly detailed spectral studies over 70 hours of observing time due to the small displacement of spectral lines by the planet and interference from star spots, which affect the spectrum of the star. During that time another super Earth, this time eight times the mass of Earth, was seen to be affecting the spectrum. This one (known as CoRoT-7c) doesn’t transit and so won’t get other properties measured in the same detail as 7b.
There are other suspected rocky planets about, and it is likely that the new Kepler probe will put out a few more candidates, but this is the first time spectroscopic and transit methods have been brought to bear on one to show it probably is rocky. However, one should always remember to take these things with a bit of salt – as is mentioned here, there is a difference between what we know (data – transit and spectroscopic) and what we guess (inferences) based on the data.