Detection of transits of the nearby hot Neptune GJ 436 b

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 +/- 1.9 M-circle plus. 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 = 25 200 +/- 2200 km = 3.95 +/- 0.35 R-circle plus. 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.