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A&A
Volume 472, Number 2, September III 2007
Page(s) L13 - L16
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20077799


A&A 472, L13-L16 (2007)
DOI: 10.1051/0004-6361:20077799

Letter

Detection of transits of the nearby hot Neptune GJ 436 b

M. Gillon1, 2, F. Pont1, B.-O. Demory1, 3, F. Mallmann3, M. Mayor1, T. Mazeh4, D. Queloz1, A. Shporer4, S. Udry1, and C. Vuissoz5

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 $M = 22.6 \pm 1.9~M_\oplus $. The shape and depth of the transit lightcurves show that it is crossing the host star disc near its limb (impact parameter $0.84 \pm 0.03$) and that the planet size is comparable to that of Uranus and Neptune, $R = 25\,200 \pm 2200$ km =  $3.95 \pm 0.35~R_\oplus$. 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.


Key words: stars: planetary systems -- stars: individual: GJ 436 -- techniques: photometric



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