2013ApJ...768..154D

We present Spitzer/IRAC 4.5 µm 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_–0.043^+0.047 M_☉ and a radius of R_* = 0.568_–0.031^+0.037 R_☉. We determine the host star of GJ 3470 b to be metal-rich, with a metallicity 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_–0.21^+0.22 R_⊕ that is 13% larger than the value previously reported in the literature. We find a planetary mass M_p = 13.9_–1.4^+1.5 M_⊕ that translates to a very low planetary density, {rho=_p = 0.72_–0.12^+0.13 g/cm3, 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.