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

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 and a radius of . 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 that is 13% larger than the value previously reported in the literature. We find a planetary mass that translates to a very low planetary density, 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.

[1]  N. Santos,et al.  Metallicity of M dwarfs - III. Planet-metallicity and planet-stellar mass correlations of the HARPS GTO M dwarf sample , 2012, 1212.3372.

[2]  Steve B. Howell,et al.  OBSERVATIONS OF BINARY STARS WITH THE DIFFERENTIAL SPECKLE SURVEY INSTRUMENT. IV. OBSERVATIONS OF KEPLER, CoRoT, AND HIPPARCOS STARS FROM THE GEMINI NORTH TELESCOPE , 2012 .

[3]  J. Brewer,et al.  PROSPECTING IN LATE-TYPE DWARFS: A CALIBRATION OF INFRARED AND VISIBLE SPECTROSCOPIC METALLICITIES OF LATE K AND M DWARFS SPANNING 1.5 dex , 2012, 1211.4630.

[4]  Russel J. White,et al.  STELLAR DIAMETERS AND TEMPERATURES. II. MAIN-SEQUENCE K- AND M-STARS , 2012, 1208.2431.

[5]  Drake Deming,et al.  3.6 AND 4.5 μm PHASE CURVES AND EVIDENCE FOR NON-EQUILIBRIUM CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HD 189733b , 2012, 1206.6887.

[6]  David J Armstrong,et al.  A hot Uranus transiting the nearby M dwarf GJ 3470 - Detected with HARPS velocimetry. Captured in transit with TRAPPIST photometry , 2012, 1206.5307.

[7]  S. Seager,et al.  DETECTION OF THERMAL EMISSION FROM A SUPER-EARTH , 2012, 1205.1766.

[8]  Las Cumbres Observatory Global Telescope Network,et al.  PLANETARY CANDIDATES OBSERVED BY KEPLER. III. ANALYSIS OF THE FIRST 16 MONTHS OF DATA , 2012, 1202.5852.

[9]  L. Ramsey,et al.  AN H-BAND SPECTROSCOPIC METALLICITY CALIBRATION FOR M DWARFS , 2012, 1202.1800.

[10]  James P. Lloyd,et al.  METALLICITY AND TEMPERATURE INDICATORS IN M DWARF K-BAND SPECTRA: TESTING NEW AND UPDATED CALIBRATIONS WITH OBSERVATIONS OF 133 SOLAR NEIGHBORHOOD M DWARFS , 2011, 1112.4567.

[11]  F. Bouchy,et al.  The HARPS search for southern extra-solar planets - XXXI. The M-dwarf sample , 2011, 1111.5019.

[12]  C. S. Fernandes,et al.  Metallicity of M dwarfs II. A comparative study of photometric metallicity scales , 2011, 1110.2694.

[13]  Howard Isaacson,et al.  KEPLER-18b, c, AND d: A SYSTEM OF THREE PLANETS CONFIRMED BY TRANSIT TIMING VARIATIONS, LIGHT CURVE VALIDATION, WARM-SPITZER PHOTOMETRY, AND RADIAL VELOCITY MEASUREMENTS , 2011, 1110.0820.

[14]  P. Bodenheimer,et al.  FORMATION AND STRUCTURE OF LOW-DENSITY EXO-NEPTUNES , 2011, 1106.2807.

[15]  S. Bloemen,et al.  Gravity and limb-darkening coefficients for the Kepler, CoRoT, Spitzer, uvby, UBVRIJHK, and Sloan photometric systems , 2011 .

[16]  R. P. Butler,et al.  THE FREQUENCY OF LOW-MASS EXOPLANETS. III. TOWARD η⊕ AT SHORT PERIODS , 2011, 1103.4186.

[17]  M. R. Haas,et al.  PLANET OCCURRENCE WITHIN 0.25 AU OF SOLAR-TYPE STARS FROM KEPLER , 2011, 1103.2541.

[18]  F. Fressin,et al.  A closely packed system of low-mass, low-density planets transiting Kepler-11 , 2011, Nature.

[19]  Jacob L. Bean,et al.  A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b , 2010, Nature.

[20]  Howard Isaacson,et al.  The Occurrence and Mass Distribution of Close-in Super-Earths, Neptunes, and Jupiters , 2010, Science.

[21]  M. Everett,et al.  HAT-P-26b: A LOW-DENSITY NEPTUNE-MASS PLANET TRANSITING A K STAR , 2010, 1010.1008.

[22]  B. Scott Gaudi,et al.  Achieving Better Than 1 Minute Accuracy in the Heliocentric and Barycentric Julian Dates , 2010, 1005.4415.

[23]  Drake Deming,et al.  Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b , 2010, Nature.

[24]  S. Seager,et al.  THREE POSSIBLE ORIGINS FOR THE GAS LAYER ON GJ 1214b , 2009, 0912.3243.

[25]  S. Seager,et al.  A FRAMEWORK FOR QUANTIFYING THE DEGENERACIES OF EXOPLANET INTERIOR COMPOSITIONS , 2009, 0912.3288.

[26]  M. Holman,et al.  A super-Earth transiting a nearby low-mass star , 2009, Nature.

[27]  A. Collier Cameron,et al.  The thermal emission of the young and massive planet CoRoT-2b at 4.5 and 8 μm , 2009, 0911.5087.

[28]  R. P. Butler,et al.  THE FREQUENCY OF LOW-MASS EXOPLANETS , 2009, 0906.4619.

[29]  Roberto Baena Gallé,et al.  OBSERVATIONS OF BINARY STARS WITH THE DIFFERENTIAL SPECKLE SURVEY INSTRUMENT. I. INSTRUMENT DESCRIPTION AND FIRST RESULTS , 2009 .

[30]  Y. Alibert,et al.  Bulk composition of the transiting hot Neptune around GJ 436 , 2009, 0904.2979.

[31]  S. Seager,et al.  A Computational Tool to Interpret the Bulk Composition of Solid Exoplanets based on Mass and Radius Measurements , 2008, 0808.1916.

[32]  Rebecca A. Bernstein,et al.  FIRE: a near-infrared cross-dispersed echellette spectrometer for the Magellan telescopes , 2008, Astronomical Telescopes + Instrumentation.

[33]  L. Casagrande,et al.  M dwarfs: effective temperatures, radii and metallicities , 2008, 0806.2471.

[34]  Darko Jevremovic,et al.  The Dartmouth Stellar Evolution Database , 2008, 0804.4473.

[35]  David Charbonneau,et al.  Design Considerations for a Ground-Based Transit Search for Habitable Planets Orbiting M Dwarfs , 2007, 0709.2879.

[36]  S. Kenyon,et al.  Planet formation around M-dwarfs: the moving snow line and super-Earths , 2007 .

[37]  S. Seager,et al.  Mass-Radius Relationships for Solid Exoplanets , 2007, 0707.2895.

[38]  D. Queloz,et al.  Detection of transits of the nearby hot Neptune GJ 436 b , 2007, Astronomy & Astrophysics.

[39]  Diana Valencia,et al.  Detailed Models of Super-Earths: How Well Can We Infer Bulk Properties? , 2007, 0704.3454.

[40]  J. Lissauer.  Planets Formed in Habitable Zones of M Dwarf Stars Probably Are Deficient in Volatiles , 2007, astro-ph/0703576.

[41]  M. Holman,et al.  The Transit Light Curve Project. I. Four Consecutive Transits of the Exoplanet XO-1b , 2006, astro-ph/0607571.

[42]  R. P. Butler,et al.  A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436 , 2004, astro-ph/0408587.

[43]  Gary J. Melnick,et al.  In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope , 2004, SPIE Astronomical Telescopes + Instrumentation.

[44]  F. Thevenin,et al.  The angular sizes of dwarf stars and subgiants Surface brightness relations calibrated by interferometry , 2004, astro-ph/0404180.

[45]  H Germany,et al.  A Method of Correcting Near‐Infrared Spectra for Telluric Absorption , 2002, astro-ph/0211255.

[46]  L. Helmer,et al.  The Carlsberg Meridian Telescope CCD drift scan survey , 2002, astro-ph/0209184.

[47]  S. Seager,et al.  A Unique Solution of Planet and Star Parameters from an Extrasolar Planet Transit Light Curve , 2002, astro-ph/0206228.

[48]  J. Carpenter.  Color Transformations for the 2MASS Second Incremental Data Release , 2001, astro-ph/0101463.

[49]  E. Weis.  Photometric Parallaxes for Selected Stars of Color Class M from the NLTT Catalog. III. The Declination Zone + 20 Degrees to +45 Degrees , 1986 .

[50]  M. Holman,et al.  Accepted for publication in The Astrophysical Journal Preprint typeset using L ATEX style emulateapj v. 10/09/06 IMPROVED PARAMETERS FOR EXTRASOLAR TRANSITING PLANETS , 2008 .