[en] 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_â . 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_â . 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.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Gillon, Michaël ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astrophysique et traitement de l'image
Pont, F.; Observatoire de Genève, Université de Genève, 1290 Sauverny, Switzerland
Demory, B*-O; Observatoire de Genève, Université de Genève, 1290 Sauverny, Switzerland ; Observatoire François-Xavier Bagnoud - OFXB, 3961 Saint-Luc, Switzerland
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