A Search for Water and Methane on a Neptune-Mass Transiting Planet

Details A Search for Water and Methane on a Neptune-Mass Transiting Planet A Search for Water and Methane on a Neptune-Mass Transiting Planet

Jul 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11622k&link_type=abstract

HST Proposal ID #11622. Cycle 17

Computer Science

Scientific paper

GJ 436b is the only known Neptune-mass transiting exoplanet. Like Neptune, more than 80% of the mass is ice and rock, surrounded by a thin H/He envelope of only 1-3 earth masses. The similarities end there, however, as GJ 436b orbits a mere 0.03 A.U. from its M dwarf primary and has a toasty 700 K atmosphere. Although it is much warmer than the gas giant planets in the solar system, GJ 436b is the coolest transiting planet discovered to date. As a result, we expect most of the carbon in GJ 436b's atmosphere to exist in the form of methane instead of the carbon monoxide found in the atmospheres of the hotter, more massive transiting planets. We propose to test this prediction by searching for the signature of water and methane absorption in this unusual planet's atmosphere as seen in the wavelength dependence of the transit depth from 1.4-2.5 micron. This wavelength range includes strong absorption bands from both water and methane, and will also allow us to place limits on the presence of clouds and atmospheric hazes if present. Using the technique of transit timing, these observations {which span four transits} will also allow us to search for additional planetary companions with masses as small as that of Mars. A second planet would provide a natural explanation for GJ 436b's ability to maintain a significant orbital eccentricity, despite the fact that the circularization time scale for this system is significantly shorter than its current age.;

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