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

Demory, Brice-Olivier; Torres, Guillermo; Neves, Vasco; Rogers, Leslie; Gillon, Michaël; Horch, Elliott; Sullivan, Peter; Bonfils, Xavier; Delfosse, Xavier; Forveille, Thierry; Lovis, Christophe; Mayor, Michel; Santos, Nuno; Seager, Sara; Smalley, Barry; Udry, Stephane

Abstract

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 Teff = 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.

Citation

Demory, B., Torres, G., Neves, V., Rogers, L., Gillon, M., Horch, E., …Udry, S. SPITZEROBSERVATIONS OF GJ 3470 b: A VERY LOW-DENSITY NEPTUNE-SIZE PLANET ORBITING A METAL-RICH M DWARF. Astrophysical Journal, 768(2), 154. https://doi.org/10.1088/0004-637x/768/2/154

• Journal Article Type: Article
• Online Publication Date: Apr 25, 2013
• Deposit Date: Jun 16, 2023
• Journal: The Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 768
• Issue: 2
• Pages: 154
• DOI: https://doi.org/10.1088/0004-637x/768/2/154
• Keywords: Space and Planetary Science; Astronomy and Astrophysics