  We present Spitzer/IRAC 4.5-micron transit photometry of GJ3470b, a
Neptune-size planet orbiting a 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_sun and a radius of
R_star = 0.568+0.037-0.031 R_sun. We determine the host star of GJ3470b to be
metal-rich, with a metallicity of [Fe/H] = +0.20 +/- 0.10 and an effective
temperature of Teff = 3600 +/- 100 K. The revised stellar parameters yield a
planetary radius R_pl = 4.83+0.22-0.21 R_Earth that is 13 percent larger than
the value previously reported in the literature. We find a planetary mass M_pl
= 13.9+1.5-1.4 M_Earth that translates to a very low planetary density, rho_pl
= 0.72+0.13-0.12 gcm-3, which is 33% smaller than the original value. With a
mean density half of that of GJ436b, GJ3470b is an example of a very
low-density low-mass planet, similar to Kepler-11d, Kepler-11e, and Kepler-18c
but orbiting a much brighter nearby star that is more conducive to follow-up
studies.
