Detection of transits of the nearby hot Neptune GJ 436 b

Abstract

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⊕. 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⊕. 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. © ESO 2007.