NASA's Spitzer Space Telescope has detected the crossing of a solid planet in front of a star located at only 42 light-years in the constellation Cancer. Thanks to this detection, astronomers know that this "super-Earth" measures 2.1 times the size of our Earth. This is the smallest exoplanet detected in the neighborhood of our Sun.

The discovery is based on data acquired by the Spitzer spacecraft last January. The data allowed astronomers to detect the "transit" of the planet, i.e. the tiny decrease of the star's brightness occurring when the planet passes in front of it.

"So far, the exquisite capabilities of Spitzer have been extensively used to study known transiting exoplanets, all of them being giant planets similar to Jupiter or Neptune. For the first time, Spitzer is used to detect the transit of a super-Earth, a solid planet not much larger than our own Earth," says Michaël Gillon from University of Liège (Belgium), the leader of the team that made this detection. "Thanks to the high-precision of Spitzer, we now know the nature of this planet, and, interestingly, it is very different from all the planets of our solar System."

The planet name is 55 Cancri e. With 8 times the mass of the Earth, for a size 2.1 larger, it is simply too big to be purely rocky, meaning that it must have a significant fraction of ice, the favored composition being a massive shell of water ice on top of a rocky nucleus. "We could call this planet a 'naked Neptune', as it is very similar to Neptune except that it has no gazeous envelope of hydrogen above its ice shell," says Brice-Olivier Demory from the Massachusets Institute of Technology (MIT), the first author of the paper describing the discovery. "But the analogy with Neptune stops here: 55 Cancri e is more than 1900 times closer to its star than Neptune to the Sun. At such a short distance, the planet is literally baked by the intense stellar radiation."

The host star itself has a particularity that makes this planet very interesting for astronomers. It is not a dim star that can only be seen with a telescope, but a very bright star of the solar neighborhood (42 light-years), visible even for the human eye. "The brightness of the host star will make possible the first throrough characterization of a solid planet orbiting a star similar to our Sun," says Gillon. "If this planet has an atmosphere, we will be able to study it details, unlike the other solid planets found so far around farer and dimmer stars."

Interestingly, the study of the atmosphere of 55 Cancri e has maybe already begun. Indeed, another team of astronomers indepently announced April 29, the detection of the transits of the same planet with another space telescope, MOST. This team observed at a different wavelength than Spitzer, and their measured planet radius is 1.3 times smaller. "A possible explanation for this discrepancy would be the presence of a huge molecular enveloppe outgassed by the planet, making the planet appear larger for Spitzer," says Demory. "Only new measurements will tell us."