Detection of an Extrasolar Planet Atmosphere

We report high-precision spectrophotometric observations of four planetary transits of HD 209458, in the region of the sodium resonance doublet at 589.3 nm. We find that the photometric dimming during transit in a bandpass centered on the sodium feature is deeper by (2.32 ± 0.57) × 10-4 relative to simultaneous observations of the transit in adjacent bands. We interpret this additional dimming as absorption from sodium in the planetary atmosphere, as recently predicted from several theoretical modeling efforts. Our model for a cloudless planetary atmosphere with a solar abundance of sodium in atomic form predicts more sodium absorption than we observe. There are several possibilities that may account for this reduced amplitude, including reaction of atomic sodium into molecular gases and/or condensates, photoionization of sodium by the stellar flux, a low primordial abundance of sodium, and the presence of clouds high in the atmosphere.

[1]  F. Allard,et al.  Brown Dwarfs and Extrasolar Planets , 2003 .

[2]  Athena Coustenis,et al.  Search for spectroscopical signatures of transiting HD 209458b's exosphere , 2001 .

[3]  Zoran Ninkov,et al.  The Vulcan Photometer: A Dedicated Photometer for Extrasolar Planet Searches , 2001 .

[4]  Andrew S. Ackerman,et al.  Precipitating Condensation Clouds in Substellar Atmospheres , 2001, astro-ph/0103423.

[5]  A. Burrows,et al.  Hubble Space Telescope Time-Series Photometry of the Transiting Planet of HD 209458 , 2001, astro-ph/0101336.

[6]  T. Brown.  Transmission Spectra as Diagnostics of Extrasolar Giant Planet Atmospheres , 2001, astro-ph/0101307.

[7]  A. Burrows,et al.  Theory of Extrasolar Giant Planet Transits , 2001, astro-ph/0101024.

[8]  A. Claret,et al.  Probing the stellar surface of HD 209458 from multicolor transit observations , 2000, astro-ph/0012435.

[9]  Drake Deming,et al.  A Sensitive Search for Methane in the Infrared Spectrum of τ Bootis , 2000, astro-ph/0007216.

[10]  David Charbonneau,et al.  A Lack of Planets in 47 Tucanae from a Hubble Space Telescope Search , 2000 .

[11]  R. P. Butler,et al.  Planetary Companions to the Metal-rich Stars BD –10°3166 and HD 52265 , 2000 .

[12]  G. Marcy,et al.  A Search for Transit Effects in Spectra of 51 Pegasi and HD 209458 , 2000 .

[13]  A. Cox,et al.  Allen's astrophysical quantities , 2000 .

[14]  S. Jha,et al.  Multicolor Observations of a Planetary Transit of HD 209458 , 2000, astro-ph/0007245.

[15]  T. Mahoney,et al.  Disks, Planetesimals and Planets , 2000 .

[16]  B. Whitney,et al.  Photometric Light Curves and Polarization of Close-in Extrasolar Giant Planets , 2000, astro-ph/0004001.

[17]  R. P. Butler,et al.  A Transiting “51 Peg-like” Planet , 2000, The Astrophysical journal.

[18]  J. B. Laird,et al.  The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458 , 2000, The Astrophysical journal.

[19]  T. Brown,et al.  Detection of Planetary Transits Across a Sun-like Star , 1999, The Astrophysical journal.

[20]  A. Burrows,et al.  The Near-Infrared and Optical Spectra of Methane Dwarfs and Brown Dwarfs , 1999, astro-ph/9908078.

[21]  Princeton,et al.  Theoretical Transmission Spectra during Extrasolar Giant Planet Transits , 1999, astro-ph/9912241.

[22]  D. James,et al.  Probable detection of starlight reflected from the giant planet orbiting τ Boötis , 1999, Nature.

[23]  A. Burrows,et al.  Albedo and Reflection Spectra of Extrasolar Giant Planets , 1999, astro-ph/9910504.

[24]  David G. Monet,et al.  � 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. DISCOVERY OF FOUR FIELD METHANE (T-TYPE) DWARFS WITH THE TWO MICRON ALL-SKY SURVEY 1 , 1999 .

[25]  Randy A. Kimble,et al.  Linearity and High Signal‐to‐Noise Performance of the STIS CCD , 1999 .

[26]  David Charbonneau,et al.  An Upper Limit on the Reflected Light from the Planet Orbiting the Star τ Bootis , 1999, astro-ph/9907195.

[27]  K. Lodders.  Alkali Element Chemistry in Cool Dwarf Atmospheres , 1999 .

[28]  David G. Monet,et al.  Dwarfs Cooler than “M”: The Definition of Spectral Type “L” Using Discoveries from the 2-Micron All-Sky Survey (2MASS) , 1999 .

[29]  A. Burrows,et al.  Chemical Equilibrium Abundances in Brown Dwarf and Extrasolar Giant Planet Atmospheres , 1998, astro-ph/9807055.

[30]  J. Mariotti,et al.  Planets outside the solar system : theory and observations , 1999 .

[31]  J. Lunine,et al.  Reflected Spectra and Albedos of Extrasolar Giant Planets. I. Clear and Cloudy Atmospheres , 1998, astro-ph/9810073.

[32]  E. Lellouch,et al.  Radiative Equilibrium Model of 51 Peg B , 1998 .

[33]  James E. Larkin,et al.  Design and development of NIRSPEC: a near-infrared echelle spectrograph for the Keck II telescope , 1998, Astronomical Telescopes and Instrumentation.

[34]  D. Sasselov,et al.  Extrasolar Giant Planets under Strong Stellar Irradiation , 1998, astro-ph/9805335.

[35]  B. Oppenheimer,et al.  The Spectrum of the Brown Dwarf Gliese 229B , 1997, astro-ph/9802299.

[36]  G. González.  The stellar metallicity—giant planet connection , 1997 .

[37]  K. Lodders,et al.  Atmospheric Chemistry of the Brown Dwarf Gliese 229B: Thermochemical Equilibrium Predictions , 1996 .

[38]  E. Karkoschka.  Spectrophotometry of the Jovian Planets and Titan at 300- to 1000-nm Wavelength: The Methane Spectrum , 1994 .

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