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There's now a second known transiting planet in the "super-Earth" mass range: GJ 1214b

Unlike CoRoT-7b, which has a density consistent with being mainly rocky, this one has a low density, so low that it probably has significant amounts of hydrogen in its atmosphere, more like a scaled-down version of Uranus or Neptune than a scaled-up terrestrial planet.

Nature paper

The planet could be a "hot Super-Pluto".

As we all know, Pluto is a world composed of 50% rock and 50% ice. The newly discovered planet would be a much larger version of Pluto in terms of composition. The planet could have formed well beyond the snow line, so a significant portion of the planet's mass is ice. If the planet underwent inward migration to where it is now, then the top icy layer would melt, then as the planet got even closer, the planet would build up an atmosphere of hot steam. Around a Sun-like star, the water molecules would be photolyzed by ultraviolet radiation. The host star, however, is not sufficiently hot to emit much ultraviolet radiation, so the steam just simply builds up over time. As it stands now, the planet receives 170 times the stellar insolation that Earth does, which would make the planet very hot. Having over five times the mass of Earth, the planet would be able to hold onto an atmosphere of hot steam almost indefinitely.

The diameter of the planet as measured from the transit is 34500 km (21400 miles). If the planet's compositon is similar to Pluto (which very well could be the case), the uncompressed density would be around 1.7 g/cm^3. Accounting for gravitational compression, the actual diameter of the planet itself could be around 30000 km (18600 miles). The difference between the two values would be due to the atmosphere, in this case, an atmosphere of super-heated steam that is around 2200 km (1400 miles) thick. Given that water vapor is a greenhouse gas, the planet's surface would be extremely hot. Not exactly a friendly place for life.