2007A&A...472L..13G

This Letter reports on the photometric detection of transits of the Neptune-mass planet orbiting the nearby M-dwarf star GJ 436. It is by far the closest, smallest, and least massive transiting planet detected so far. Its mass is slightly larger than Neptune's at M=22.6±1.9M_⊕ . The shape and depth of the transit lightcurves show that it is crossing the host star disc near its limb (impact parameter 0.84±0.03) and that the planet size is comparable to that of Uranus and Neptune, R=25200±2200km=3.95±0.35R_⊕. Its main constituant is therefore very likely to be water ice. If the current planet structure models are correct, an outer layer of H/He constituting up to ten percent in mass is probably needed on top of the ice to account for the observed radius.