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The albedo of an exoplanet: spacebased photometry of the transiting system HD 209458 Rowe, Jason

Abstract

Very precise photometry of transiting extrasolar planets can be used to learn about the physical structure of Jupiter-like planets in an exoplanetary systems. The fraction of light reflected from the planet (albedo) provides crucial insight into the chemical structure of atmospheres and global thermal properties of a planet, including heat dissipation and global weather patterns. Measuring the albedo of an exoplanet requires very precise photometry with high time sampling and nearly continuous time coverage spanning may orbits, which can be achieved at present only from space. We present space-based photometry of the transiting exoplanetary system HD 209458 obtained with the MOST (Microvariablity and Oscillations of STars) satellite during 2004 and 2005. The data span 14 and 44 days respectively, and have nearly complete time coverage for both spans. The HD 209458 photometry was obtained in MOST's Direct Imaging mode, not part of the original mission but implemented to make possible measurements of stars in the brightness range 6.5 < V < 13. The photometric reduction techniques developed for this thesis have become the standard pipeline for processing MOST Direct Imaging data, in particular the corrections for stray Earthshine reaching the MOST instrument focal plane. Our analysis of MOST photometry of HD 209458 sets a 1 sigma upper limit on the depth of the optical eclipse of the planet of 17 parts per million (ppm) = 17 micromagnitudes. This corresponds to a 1 sigma upper limit on the flux ratio of planet to star of 1.57 x 10^-5 and an upper limit on the geometric albedo in the MOST bandpass (400 to 700 nm) of 8%. This is the most sensitive measurement of an exoplanetary albdeo ever obtained. The limit on the albedo of the gas giant HD 209458b means it much less reflective as Jupiter. This result rules out the presence of reflective clouds in the atmosphere of HD 209458b and has already enabled theoretical modeling of far-infrared measurements to contrain the planet's equilibrium atmospheric temperature to be 1550 +/- 150 K. The MOST albedo analysis demonstrates the potential of spacebased photometry missions like CoRoT (launched in December 2006) and Kepler (due for launch in early 2009) and has already provided important lessons for both missions.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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