COMPOSITIONAL DIVERSITY IN THE ATMOSPHERES OF HOT NEPTUNES, WITH APPLICATION TO GJ 436b

Neptune-sized extrasolar planets that orbit relatively close to their host stars - often called "hot Neptunes" - are common within the known population of exoplanets and planetary candidates. Similar to our own Uranus and Neptune, inefficient accretion of nebular gas is expected produce hot Neptunes whose masses are dominated by elements heavier than hydrogen and helium. At high atmospheric metallicities of 10-10,000× solar, hot Neptunes will exhibit an interesting continuum of atmospheric compositions, ranging from more Neptune-like, H2-dominated atmospheres to more Venus-like, CO2-dominated atmospheres. We explore the predicted equilibrium and disequilibrium chemistry of generic hot Neptunes and find that the atmospheric composition varies strongly as a function of temperature and bulk atmospheric properties such as metallicity and the C/O ratio. Relatively exotic H2O, CO, CO2, and even O2-dominated atmospheres are possible for hot Neptunes. We apply our models to the case of GJ 436b, where we find that a CO-rich, CH4-poor atmosphere can be a natural consequence of a very high atmospheric metallicity. From comparisons of our results with Spitzer eclipse data for GJ 436b, we conclude that although the spectral fit from the high-metallicity forward models is not quite as good as the best fit obtained from pure retrieval methods, the atmospheric composition predicted by these forward models is more physically and chemically plausible in terms of the relative abundance of major constituents. High-metallicity atmospheres (orders of magnitude in excess of solar) should therefore be considered as a possibility for GJ 436b and other hot Neptunes.

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

[2]  Howard Isaacson,et al.  A CORRELATION BETWEEN STELLAR ACTIVITY AND HOT JUPITER EMISSION SPECTRA , 2010, 1004.2702.

[3]  Portugal,et al.  Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b , 2007, 0707.2261.

[4]  J. Beaulieu,et al.  METHANE IN THE ATMOSPHERE OF THE TRANSITING HOT NEPTUNE GJ436B? , 2010, 1007.0324.

[5]  B. Fegley,et al.  Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars. II. Sulfur and Phosphorus , 2005, astro-ph/0511136.

[6]  I. Pater,et al.  Constraining the origins of Neptune’s carbon monoxide abundance with CARMA millimeter-wave observations , 2012, 1301.1990.

[7]  Mercedes Lopez-Morales,et al.  New Observations and a Possible Detection of Parameter Variations in the Transits of Gliese 436b , 2008, 0809.1664.

[8]  Joanna K. Barstow,et al.  On the potential of the EChO mission to characterise gas giant atmospheres , 2012, 1212.5020.

[9]  J. Niemantsverdriet,et al.  Chemical kinetics and catalysis , 1995 .

[10]  D. Saumon,et al.  NEGLECTED CLOUDS IN T AND Y DWARF ATMOSPHERES , 2012, 1206.4313.

[11]  R. Lindzen.  Turbulence and stress owing to gravity wave and tidal breakdown , 1981 .

[12]  B. Fegley,et al.  The Origin of Carbon Monoxide in Neptune's Atmosphere , 1993 .

[13]  I. Baraffe,et al.  Structure and evolution of super-Earth to super-Jupiter exoplanets - I. Heavy element enrichment in the interior , 2008, 0802.1810.

[14]  J. Waite,et al.  Atmospheres in the solar system : comparative aeronomy , 2002 .

[15]  N. Iro,et al.  A TIME-DEPENDENT RADIATIVE MODEL FOR THE ATMOSPHERE OF THE ECCENTRIC EXOPLANETS , 2010, 1001.1171.

[16]  P. Drossart,et al.  Carbon Monoxide on Jupiter: Evidence for Both Internal and External Sources , 2002 .

[17]  Patrick G. J. Irwin,et al.  Optimal estimation retrievals of the atmospheric structure and composition of HD 189733b from secondary eclipse spectroscopy , 2011, 1110.2934.

[18]  Debra A. Fischer,et al.  A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436 , 2004 .

[19]  Nikku Madhusudhan,et al.  C/O RATIO AS A DIMENSION FOR CHARACTERIZING EXOPLANETARY ATMOSPHERES , 2012, 1209.2412.

[20]  S. Seager,et al.  HIGH METALLICITY AND NON-EQUILIBRIUM CHEMISTRY IN THE DAYSIDE ATMOSPHERE OF HOT-NEPTUNE GJ 436b , 2010, 1004.5121.

[21]  James A. Miller,et al.  Detailed balance in multiple-well chemical reactions. , 2009, Physical chemistry chemical physics : PCCP.

[22]  Edwin A. Bergin,et al.  THE EFFECTS OF SNOWLINES ON C/O IN PLANETARY ATMOSPHERES , 2011, 1110.5567.

[23]  James F. Kasting,et al.  A PHOTOCHEMICAL MODEL FOR THE CARBON-RICH PLANET WASP-12b , 2011, 1110.2793.

[24]  K. Lodders,et al.  ATMOSPHERIC SULFUR PHOTOCHEMISTRY ON HOT JUPITERS , 2009, 0903.1663.

[25]  R. Redmer,et al.  THERMAL EVOLUTION AND STRUCTURE MODELS OF THE TRANSITING SUPER-EARTH GJ 1214b , 2010, 1010.0277.

[26]  J. Fortney,et al.  THE NATURE OF THE ATMOSPHERE OF THE TRANSITING SUPER-EARTH GJ 1214b , 2010, 1001.0976.

[27]  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.

[28]  Kevin France,et al.  THE ULTRAVIOLET RADIATION ENVIRONMENT AROUND M DWARF EXOPLANET HOST STARS , 2012, 1212.4833.

[29]  Willy Benz,et al.  Models of giant planet formation with migration and disc evolution , 2004 .

[30]  Mark S. Marley,et al.  Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits , 2006 .

[31]  Christoph Mordasini,et al.  A FRAMEWORK FOR CHARACTERIZING THE ATMOSPHERES OF LOW-MASS LOW-DENSITY TRANSITING PLANETS , 2013, 1306.4329.

[32]  E. Observatory,et al.  High cadence near infrared timing observations of extrasolar planets , 2009, 0905.1728.

[33]  E. Gaidos,et al.  GEODYNAMICS AND RATE OF VOLCANISM ON MASSIVE EARTH-LIKE PLANETS , 2008, 0809.2305.

[34]  CHEMISTRY OF SILICATE ATMOSPHERES OF EVAPORATING SUPER-EARTHS , 2009, 0906.1204.

[35]  S. Hawley,et al.  The Palomar/MSU Nearby Star Spectroscopic Survey.II.The Southern M Dwarfs and Investigation of Magnetic Activity , 1996 .

[36]  K. Rice,et al.  Protostars and Planets V , 2005 .

[37]  Nikku Madhusudhan,et al.  NEBULAR WATER DEPLETION AS THE CAUSE OF JUPITER'S LOW OXYGEN ABUNDANCE , 2012, 1204.3887.

[38]  Yuk L. Yung,et al.  High-temperature Photochemistry in the Atmosphere of HD 189733b , 2010 .

[39]  J. Moses,et al.  QUENCHING OF CARBON MONOXIDE AND METHANE IN THE ATMOSPHERES OF COOL BROWN DWARFS AND HOT JUPITERS , 2011, 1106.3525.

[40]  W. Demore,et al.  Photochemistry of Planetary Atmospheres , 1998 .

[41]  C. Saffe,et al.  On the ages of exoplanet host stars , 2005, astro-ph/0510092.

[42]  G. Benedict,et al.  Metallicities of M Dwarf Planet Hosts from Spectral Synthesis , 2006, astro-ph/0611060.

[43]  Larry W. Esposito,et al.  Saturn from Cassini-Huygens , 2009 .

[44]  M. Holman,et al.  A SEARCH FOR ADDITIONAL PLANETS IN THE NASA EPOXI OBSERVATIONS OF THE EXOPLANET SYSTEM GJ 436 , 2009, 0909.2875.

[45]  R. Prinn,et al.  Carbon monoxide on jupiter and implications for atmospheric convection. , 1977, Science.

[46]  Peter H. Hauschildt,et al.  Irradiated planets , 2001, astro-ph/0104262.

[47]  Franck Selsis,et al.  Signature of life on exoplanets: Can Darwin produce false positive detections? , 2002 .

[48]  A. Pollock,et al.  A scenario of planet erosion by coronal radiation (Corrigendum) , 2010, 1002.1875.

[49]  David Charbonneau,et al.  ATMOSPHERIC CIRCULATION OF HOT JUPITERS: COUPLED RADIATIVE-DYNAMICAL GENERAL CIRCULATION MODEL SIMULATIONS OF HD 189733b and HD 209458b , 2008, 0809.2089.

[50]  S. Seager,et al.  Source of Atomic Hydrogen in the Atmosphere of HD 209458b , 2003, astro-ph/0307037.

[51]  D. Ehrenreich,et al.  HST/STIS Lyman-alpha observations of the quiet M dwarf GJ436: Predictions for the exospheric transit signature of the hot neptune GJ436b , 2011, 1103.0009.

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

[53]  L. Kaltenegger,et al.  COMPOSITIONS OF HOT SUPER-EARTH ATMOSPHERES: EXPLORING KEPLER CANDIDATES , 2011, 1110.2426.

[54]  T. Guillot.  On the radiative equilibrium of irradiated planetary atmospheres , 2010, 1006.4702.

[55]  Jonathan J. Fortney,et al.  THE HEAVY-ELEMENT MASSES OF EXTRASOLAR GIANT PLANETS, REVEALED , 2011, 1105.0024.

[56]  B. Fegley,et al.  Condensation chemistry of carbon stars , 1997 .

[57]  R. P. Butler,et al.  The M Dwarf GJ 436 and its Neptune‐Mass Planet , 2006, astro-ph/0608260.

[58]  J. Fortney,et al.  THE ATMOSPHERIC CHEMISTRY OF GJ 1214b: PHOTOCHEMISTRY AND CLOUDS , 2011, 1104.5477.

[59]  V. L. Orkin,et al.  Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies: Evaluation Number 18 , 2015 .

[60]  J. Fortney,et al.  New indication for a dichotomy in the interior structure of Uranus and Neptune from the application of modified shape and rotation data , 2012, 1207.2309.

[61]  S. Seager,et al.  Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres , 2007, 0710.4941.

[62]  B. Fegley,et al.  Chemical Models of the Deep Atmospheres of Jupiter and Saturn , 1994 .

[63]  Sara Seager,et al.  PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. I. PHOTOCHEMISTRY MODEL AND BENCHMARK CASES , 2012, 1210.6885.

[64]  M. Marley,et al.  ATMOSPHERIC CIRCULATION OF ECCENTRIC HOT NEPTUNE GJ436b , 2010, 1007.2942.

[65]  L. Schaefer,et al.  VAPORIZATION OF THE EARTH: APPLICATION TO EXOPLANET ATMOSPHERES , 2011, 1108.4660.

[66]  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 .

[67]  F. Fressin,et al.  THE FALSE POSITIVE RATE OF KEPLER AND THE OCCURRENCE OF PLANETS , 2013, 1301.0842.

[68]  Roda Bounaceur,et al.  A chemical model for the atmosphere of hot Jupiters , 2012, 1208.0560.

[69]  F. Selsis,et al.  High-temperature measurements of VUV-absorption cross sections of CO2 and their application to exoplanets , 2013, 1302.2432.

[70]  Caltech,et al.  THE GJ 436 SYSTEM: DIRECTLY DETERMINED ASTROPHYSICAL PARAMETERS OF AN M DWARF AND IMPLICATIONS FOR THE TRANSITING HOT NEPTUNE , 2012, 1202.0083.

[71]  AZ,et al.  Characterization of the hot Neptune GJ 436 b with Spitzer and ground-based observations , 2007, 0707.3809.

[72]  D. Sasselov,et al.  THE ATMOSPHERIC SIGNATURES OF SUPER-EARTHS: HOW TO DISTINGUISH BETWEEN HYDROGEN-RICH AND HYDROGEN-POOR ATMOSPHERES , 2008, 0808.1902.

[73]  I. Ribas,et al.  Estimation of the XUV radiation onto close planets and their evaporation , 2011, 1105.0550.

[74]  Jack J. Lissauer,et al.  Formation of the Giant Planets by Concurrent Accretion of Solids and Gas , 1995 .

[75]  John Southworth,et al.  Homogeneous studies of transiting extrasolar planets - III. Additional planets and stellar models , 2010, 1006.4443.

[76]  Nikku Madhusudhan,et al.  CARBON-RICH GIANT PLANETS: ATMOSPHERIC CHEMISTRY, THERMAL INVERSIONS, SPECTRA, AND FORMATION CONDITIONS , 2011, 1109.3183.

[77]  Drake Deming,et al.  Spitzer Transit and Secondary Eclipse Photometry of GJ 436b , 2007, 0707.2778.

[78]  B. Fegley,et al.  Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars: I. Carbon, Nitrogen, and Oxygen , 2002 .

[79]  R. Barnes.  Formation and evolution of exoplanets , 2010 .

[80]  M. Holman,et al.  Transit infrared spectroscopy of the hot Neptune around GJ 436 with the Hubble Space Telescope , 2008, 0810.5731.

[81]  G. Vasisht,et al.  THERMOCHEMICAL AND PHOTOCHEMICAL KINETICS IN COOLER HYDROGEN-DOMINATED EXTRASOLAR PLANETS: A METHANE-POOR GJ436b? , 2011, 1104.3183.

[82]  G. Wasserburg,et al.  Molecular equilibria and condensation temperatures in carbon-rich gases , 1995 .

[83]  Bernard H. Foing,et al.  Lunar and Planetary Science Conference , 2013 .

[84]  B. Fegley,et al.  ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. III. IRON, MAGNESIUM, AND SILICON , 2010, 1001.3639.

[85]  C. Visscher.  CHEMICAL TIMESCALES IN THE ATMOSPHERES OF HIGHLY ECCENTRIC EXOPLANETS , 2012, 1207.0722.

[86]  France.,et al.  Limits to the planet candidate GJ 436c , 2008, 0804.3030.

[87]  David R. Alexander,et al.  THE LIMITING EFFECTS OF DUST IN BROWN DWARF MODEL ATMOSPHERES , 2001 .

[88]  R. Freedman,et al.  CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b , 2012, The Astrophysical journal.

[89]  Drake Deming,et al.  A SPITZER TRANSMISSION SPECTRUM FOR THE EXOPLANET GJ 436b, EVIDENCE FOR STELLAR VARIABILITY, AND CONSTRAINTS ON DAYSIDE FLUX VARIATIONS , 2011, 1104.2901.

[90]  Thomas P. Greene,et al.  TRANSMISSION SPECTRA OF TRANSITING PLANET ATMOSPHERES: MODEL VALIDATION AND SIMULATIONS OF THE HOT NEPTUNE GJ 436b FOR THE JAMES WEBB SPACE TELESCOPE , 2010, 1010.2451.

[91]  Sara Seager,et al.  ATMOSPHERIC RETRIEVAL FOR SUPER-EARTHS: UNIQUELY CONSTRAINING THE ATMOSPHERIC COMPOSITION WITH TRANSMISSION SPECTROSCOPY , 2012, 1203.4018.

[92]  B. Hansen,et al.  MIGRATION THEN ASSEMBLY: FORMATION OF NEPTUNE-MASS PLANETS INSIDE 1 AU , 2011, 1105.2050.

[93]  Nikole K. Lewis,et al.  DISEQUILIBRIUM CARBON, OXYGEN, AND NITROGEN CHEMISTRY IN THE ATMOSPHERES OF HD 189733b AND HD 209458b , 2011, 1102.0063.

[94]  L. J. Richardson,et al.  On the Dayside Thermal Emission of Hot Jupiters , 2005 .

[95]  M. E. Mickelson,et al.  The Abundances of Methane and Ortho/Para Hydrogen on Uranus and Neptune: Implications of New Laboratory 4-0 H2 Quadrupole Line Parameters , 1995 .

[96]  Peter H. Hauschildt,et al.  Phase-dependent Properties of Extrasolar Planet Atmospheres , 2005 .

[97]  Todd J. Henry,et al.  A Standard Stellar Spectral Sequence in the Red/Near-Infrared: Classes K5 to M9 , 1991 .

[98]  Avi Shporer,et al.  PHOTOMETRIC FOLLOW-UP OBSERVATIONS OF THE TRANSITING NEPTUNE-MASS PLANET GJ 436b , 2008, 0805.3915.

[99]  M. Marley,et al.  QUANTITATIVELY ASSESSING THE ROLE OF CLOUDS IN THE TRANSMISSION SPECTRUM OF GJ 1214b , 2013, 1305.4124.

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

[101]  F. Selsis,et al.  The impact of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b , 2012, 1210.6627.

[102]  Roger Atkinson,et al.  Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reaxtions of Ox, HOx, NOx and SOx species , 2004 .

[103]  M. Marley,et al.  Comparative models of Uranus and Neptune , 1995 .

[104]  Yuk L. Yung,et al.  Vertical transport and photochemistry in the terrestrial mesosphere and lower thermosphere (50–120 km) , 1981 .

[105]  M. Marley,et al.  On the Luminosity of Young Jupiters , 2006, astro-ph/0609739.

[106]  Sara Seager,et al.  On the Insignificance of Photochemical Hydrocarbon Aerosols in the Atmospheres of Close-in Extrasolar Giant Planets , 2004 .

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

[108]  A. Muñoz,et al.  Physical and chemical aeronomy of HD 209458b , 2007 .

[109]  D. Saumon,et al.  Atmosphere, Interior, and Evolution of the Metal-rich Transiting Planet HD 149026b , 2006 .

[110]  Curtis S. Cooper,et al.  Dynamics and Disequilibrium Carbon Chemistry in Hot Jupiter Atmospheres, with Application to HD 209458b , 2006 .

[111]  S. Seager,et al.  A TEMPERATURE AND ABUNDANCE RETRIEVAL METHOD FOR EXOPLANET ATMOSPHERES , 2009, 0910.1347.

[112]  Yanqin Wu,et al.  DENSITY AND ECCENTRICITY OF KEPLER PLANETS , 2012, 1210.7810.

[113]  M. R. Line,et al.  INFORMATION CONTENT OF EXOPLANETARY TRANSIT SPECTRA: AN INITIAL LOOK , 2011, 1111.2612.

[114]  G. Benedict,et al.  A Hubble Space Telescope Transit Light Curve For GJ 436B , 2008, 0806.0851.

[115]  A. Burrows,et al.  Atomic and Molecular Opacities for Brown Dwarf and Giant Planet Atmospheres , 2006, astro-ph/0607211.

[116]  S. Seager,et al.  Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets , 2008 .

[117]  Howard Isaacson,et al.  THE MASS OF KOI-94d AND A RELATION FOR PLANET RADIUS, MASS, AND INCIDENT FLUX , 2013, 1303.2150.

[118]  M. Tomasko,et al.  The haze and methane distributions on Uranus from HST-STIS spectroscopy , 2009 .

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

[120]  Peter Bodenheimer,et al.  Calculations of the accretion and evolution of giant planets: The effects of solid cores , 1986 .

[121]  Katharina Lodders,et al.  Jupiter Formed with More Tar than Ice , 2004 .

[122]  R. Redmer,et al.  Interior structure models of GJ 436b , 2010, 1002.4447.

[123]  A. Burrows,et al.  MODELS OF NEPTUNE-MASS EXOPLANETS: EMERGENT FLUXES AND ALBEDOS , 2009, 0909.2043.

[124]  F. Fressin,et al.  CHARACTERISTICS OF PLANETARY CANDIDATES OBSERVED BY KEPLER. II. ANALYSIS OF THE FIRST FOUR MONTHS OF DATA , 2011, 1102.0541.

[125]  Formation and structure of the three Neptune-mass planets system around HD 69830 , 2006, astro-ph/0607175.

[126]  Y. Alibert,et al.  Characterization of exoplanets from their formation - II. The planetary mass-radius relationship , 2012, 1206.3303.

[127]  University of Exeter,et al.  A new look at NICMOS transmission spectroscopy of HD 189733, GJ-436 and XO-1: no conclusive evidence for molecular features , 2010, 1010.1753.

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

[129]  Y. Alibert,et al.  Characterization of exoplanets from their formation - I. Models of combined planet formation and evolution , 2012, 1206.6103.

[130]  Jonathan J. Fortney,et al.  The effect of condensates on the characterization of transiting planet atmospheres with transmission spectroscopy , 2005, astro-ph/0509292.

[131]  D. Cordier,et al.  ON THE VOLATILE ENRICHMENTS AND HEAVY ELEMENT CONTENT IN HD189733b , 2010, 1011.4168.

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

[133]  Christopher J. Campo,et al.  TWO NEARBY SUB-EARTH-SIZED EXOPLANET CANDIDATES IN THE GJ 436 SYSTEM , 2012, 1207.4245.

[134]  J. Moses,et al.  The deep water abundance on Jupiter: New constraints from thermochemical kinetics and diffusion modeling , 2010, 1003.6077.

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

[136]  W. C. Bowman,et al.  A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12b , 2010, Nature.