Direct Measure of Radiative and Dynamical Properties of an Exoplanet Atmosphere

Two decades after the discovery of 51Pegb, the formation processes and atmospheres of short-period gas giants remain poorly understood. Observations of eccentric systems provide key insights on those topics as they can illuminate how a planet’s atmosphere responds to changes in incident flux. We report here the analysis of multi-day multi-channel photometry of the eccentric (e 0.93 ~ ) hot Jupiter HD80606b obtained with the Spitzer Space Telescope. The planet’s extreme eccentricity combined with the long coverage and exquisite precision of new periastron-passage observations allow us to break the degeneracy between the radiative and dynamical timescales of HD80606b’s atmosphere and constrain its global thermal response. Our analysis reveals that the atmospheric layers probed heat rapidly (∼4 hr radiative timescale) from 500 < to 1400 K as they absorb 20% ~ of the incoming stellar flux during the periastron passage, while the planet’s rotation period is 93 35 85 + hr, which exceeds the predicted pseudo-synchronous period (40 hr).