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_{\star } = 0.539^{ 0.047}_{-0.043} \,M_{\odot } and a radius of R_{\star } = 0.568^{ 0.037}_{-0.031}\,R_{\odot }. 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_{\oplus } 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_{\oplus } that translates to a very low planetary density, \rho _{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. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
