SPITZER OBSERVATIONS OF GJ 3470 b: A VERY LOW-DENSITY NEPTUNE-SIZE PLANET ORBITING A METAL-RICH M DWARF

• Main Authors: Demory, Brice-Olivier (Contributor), Torres, Guillermo (Author), Neves, Vasco (Author), Rogers, Leslie (Author), Gillon, M. (Author), Horch, Elliott P. (Author), Bonfils, Xavier (Author), Delfosse, Xavier (Author), Forveille, Thierry (Author), Lovis, Christophe (Author), Mayor, Michel (Author), Santos, Nuno (Author), Seager, Sara (Contributor), Smalley, Barry (Author), Udry, Stephane (Author), Sullivan, Peter W. (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences (Contributor), Massachusetts Institute of Technology. Department of Physics (Contributor), MIT Kavli Institute for Astrophysics and Space Research (Contributor), Sullivan, Peter William (Contributor)
• Format: Article
• Language: English
• Published: IOP Publishing, 2014-04-11T16:32:44Z.
• Subjects: Article
• Online Access: Get fulltext

Author's final manuscript: March 7, 2013
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[subscript *] = 0.539[+0.047 over -0.043] M[subscript ʘ] and a radius of R[subscript *] = 0.568[+0.037 over -0.031] R[subscript ʘ]. 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 [subscript eff] = 3600 ± 100 K. The revised stellar parameters yield a planetary radius Rsubscript p] = 4.83[-0.21 over +0.22] R[subscript ⊕] that is 13% larger than the value previously reported in the literature. We find a planetary mass M[subscript p] = 13.9[+1.5 over -1.4] M[subscript ⊕] that translates to a very low planetary density, P[subscript p] = 0.72[+0.13 over -0.12] g cm[superscript -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.