A class of extrasolar giant planets - the so-called ‘ hot Jupiters ’ [ 1 ] - orbit within 0.05 AU of their primary stars .
These planets should be hot and so emit detectable infrared radiation [ 2 ] .
The planet HD 209458b [ 3 , 4 ] is an ideal candidate for the detection and characterization of this infrared light because it is eclipsed by the star .
This planet has an anomalously large radius ( 1.35 times that of Jupiter [ 5 ] ) , which may be the result of ongoing tidal dissipation [ 6 ] , but this explanation requires a non-zero orbital eccentricity ( \sim 0.03 ) [ 6 , 7 ] , maintained by interaction with a hypothetical second planet .
Here we report detection of infrared ( 24 \mu m ) radiation from HD 209458b , by observing the decrement in flux during secondary eclipse , when the planet passes behind the star .
The planet ’ s 24 \mu m flux is 55 \pm 10 \mu Jy ( 1 \sigma ) , with a brightness temperature of 1130 \pm 150 Kelvins , confirming the predicted heating by stellar irradiation [ 2 , 8 ] .
The secondary eclipse occurs at the midpoint between transits of the planet in front of the star ( to within \pm 7 min , 1 \sigma ) , which means that a dynamically significant orbital eccentricity is unlikely .