Letter to the Editor

Detection of transits of the nearby hot Neptune GJ 436 b

¹ Observatoire de Genève, Université de Genève, 1290 Sauverny, Switzerland e-mail: michael.gillon@obs.unige.ch
² Institut d'Astrophysique et de Géophysique, Université de Liège, 4000 Liège, Belgium
³ Observatoire François-Xavier Bagnoud – OFXB, 3961 Saint-Luc, Switzerland
⁴ School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
⁵ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland

Received: 6 May 2007
Accepted: 16 May 2007

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 . The shape and depth of the transit lightcurves show that it is crossing the host star disc near its limb (impact parameter ) and that the planet size is comparable to that of Uranus and Neptune,  km = . 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.