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Spitzer observations of GJ 3470 b: A very low-density neptune-size planet orbiting a metal-rich m dwarf

B.-O. Demory, G. Torres, V. Neves, L. Rogers, M. Gillon, E. P. Horch, P. Sullivan, X. Bonfils, X. Delfosse, T. Forveille, C. Lovis, M. Mayor, N. C. Santos, S. Seager, B. Smalley, S. Udry

We present Spitzer/IRAC 4.5µm transit photometry of GJ 3470 b, a Neptune-size planet orbiting an M1.5 dwarf star with a 3.3 day period recently discovered in the course of the HARPS M-dwarf survey. We refine the stellar parameters by employing purely empirical mass–luminosity and surface brightness relations constrained by our updated value for the mean stellar density, and additional information from new near-infrared spectroscopic observations.We derive a stellar mass of M* = 0.539+0.047-0.043 M and a radius of R* = 0.568+0.037-0.031 R. We determine the host star of GJ 3470 b to bemetal-rich, with ametallicity of [Fe/H] = +0.20±0.10 and an effective temperature of Teff = 3600±100 K. The revised stellar parameters yield a planetary radius Rp = 4.83+0.22-0.21 R that is 13% larger than the value previously reported in the literature. We find a planetary mass Mp = 13.9+1.5-1.4 M that translates to a very low planetary density, ρp = 0.72+0.13-0.12 g cm-3, which is 33% smaller than the original value. With a mean density half of that of GJ 436 b, GJ 3470 b is an example of a very low-density low-mass planet, similar to Kepler-11 d, Kepler-11 e, and Kepler-18 c, but orbiting a much brighter nearby star that is more conducive to follow-up studies.

planetary systems – stars: individual (GJ 3470) – techniques: photometric – techniques: spectroscopic

The Astrophysical Journal
Volume 768, Number 2, Page 154_1
2013 May

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia