The hot Jupiter HDÂ 209458b is particularly amenable to detailed study as it is among the brightest transiting exoplanet systems currently known ( V-mag = 7.65 ; K-mag = 6.308 ) and has a large planet-to-star contrast ratio .
HDÂ 209458b is predicted to be in synchronous rotation about its host star with a hot spot that is shifted eastward of the substellar point by superrotating equatorial winds .
Here we present the first full-orbit observations of HDÂ 209458b , in which its 4.5Â \micron emission was recorded with Spitzer /IRAC .
Our study revises the previous 4.5Â \micron measurement of HDÂ 209458b ’ s secondary eclipse emission downward by \sim 35 % to 0.1391 \% ^ { +0.0072 \% } _ { -0.0069 \% } , changing our interpretation of the properties of its dayside atmosphere .
We find that the hot spot on the planet ’ s dayside is shifted eastward of the substellar point by 40.9 \arcdeg \pm { 6.0 \arcdeg } , in agreement with circulation models predicting equatorial superrotation .
HDÂ 209458b ’ s dayside ( T _ { bright } = 1499 \pm 15 K ) and nightside ( T _ { bright } = 972 \pm 44 K ) emission indicates a day-to-night brightness temperature contrast smaller than that observed for more highly irradiated exoplanets , suggesting that the day-to-night temperature contrast may be partially a function of the incident stellar radiation .
The observed phase curve shape deviates modestly from global circulation model predictions potentially due to disequilibrium chemistry or deficiencies in the current hot CH _ { 4 } line lists used in these models .
Observations of the phase curve at additional wavelengths are needed in order to determine the possible presence and spatial extent of a dayside temperature inversion , as well as to improve our overall understanding of this planet ’ s atmospheric circulation .