The Very Low Albedo of an Extrasolar Planet: MOST Spacebased Photometry of HD 209458

TL;DR

This study uses space-based photometry to measure the optical albedo of exoplanet HD 209458b, finding it to be very low, which suggests a lack of reflective clouds and provides insights into its atmospheric properties.

Contribution

First precise optical albedo measurement of HD 209458b using MOST satellite data, establishing a low albedo limit and refining system parameters.

Findings

Planet's geometric albedo is less than 0.08 at 1σ confidence.

HD 209458b is significantly less reflective than Jupiter.

Low albedo indicates absence of bright reflective clouds in the atmosphere.

Abstract

Measuring the albedo of an extrasolar planet provides insights into its atmospheric composition and its global thermal properties, including heat dissipation and weather patterns. Such a measurement requires very precise photometry of a transiting system sampling fully many phases of the secondary eclipse. Spacebased optical photometry of the transiting system HD 209458 from the MOST (Microvariablity and Oscillations of STars) satellite, spanning 14 and 44 days in 2004 and 2005 respectively, allows us to set a sensitive limit on the optical eclipse of the hot exosolar giant planet in this system. Our best fit to the observations yields a flux ratio of the planet and star of 7 $\pm$ 9 ppm (parts per million), which corresponds to a geometric albedo through the MOST bandpass (400-700 nm) of $A_g$ = 0.038 $\pm$ 0.045. This gives a 1$\sigma$ upper limit of 0.08 for the geometric albedo and a 3$\sigma$ upper limit of 0.17. HD 209458b is significantly less reflective than Jupiter (for which $A_g$ would be about 0.5). This low geometric albedo rules out the presence of bright reflective clouds in this exoplanet's atmosphere. We determine refined parameters for the star and exoplanet in the HD 209458 system based on a model fit to the MOST light curve.