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

Abstract
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 T_eff = 3600±100 K. The revised stellar parameters yield a planetary radius R_p = 4.83^+0.22_-0.21 R_⊕ that is 13% larger than the value previously reported in the literature. We find a planetary mass M_p = 13.9^+1.5_-1.4 M_⊕ that translates to a very low planetary density, ρ_p = 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.

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

The Astrophysical Journal
Volume 768, Page 154_1
May 2013

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