Photometric data of HD 209458b

HD 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 HD 209458b, in which its 4.5 {mu}m emission was recorded with Spitzer/IRAC.

Our study revises the previous 4.5 {mu}m measurement of HD 209458b's secondary eclipse emission downward by ~35% to 0.1391%_-0.0069%_^+0.0072^%, 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.{deg}9+/-6.{deg}0, in agreement with circulation models predicting equatorial superrotation. HD 209458b's dayside (T_bright_= 1499+/-15 K) and nightside (T_bright_= 972+/-44 K) emission indicate 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.