“ Hot Jupiter ” extrasolar planets are expected to be tidally locked because they are close ( < 0.05 astronomical units , where 1 AU is the average Sun-Earth distance ) to their parent stars , resulting in permanent daysides and nightsides .
By observing systems where the planet and star periodically eclipse each other , several groups have been able to estimate the temperatures of the daysides of these planets [ 1 , 2 , 3 ] .
A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside , which will determine the temperature at different points on the planet ’ s surface .
Here we report observations of HD 189733 , the closest of these eclipsing planetary systems [ 4 , 5 , 6 ] , over half an orbital period , from which we can construct a ’ map ’ of the distribution of temperatures .
We detected the increase in brightness as the dayside of the planet rotated into view .
We estimate a minimum brightness temperature of 973 \pm 33 K and a maximum brightness temperature of 1212 \pm 11 K at a wavelength of 8 \mu m , indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere , in contrast to a recent claim for another hot Jupiter [ 7 ] .
Our data indicate that the peak hemisphere-integrated brightness occurs 16 \pm 6 degrees before opposition , corresponding to a hot spot shifted east of the substellar point .
The secondary eclipse ( when the planet moves behind the star ) occurs 120 \pm 24 s later than predicted , which may indicate a slightly eccentric orbit .