We use the Spitzer Space Telescope to estimate the dayside thermal emission of the exoplanet TrES-3 integrated in the 3.6 , 4.5 , 5.8 , and 8.0 µm bandpasses of the Infrared Array Camera ( IRAC ) instrument .
We observe two secondary eclipses and find relative eclipse depths of 0.00346 \pm 0.00035 , 0.00372 \pm 0.00054 , 0.00449 \pm 0.00097 , and 0.00475 \pm 0.00046 , respectively in the 4 IRAC bandpasses .
We combine our results with the earlier K band measurement of De Mooij et al .
( 2009 ) , and compare them with models of the planetary emission .
We find that the planet does not require the presence of an inversion layer in the high atmosphere .
This is the first very strongly irradiated planet that does not have a temperature inversion , which indicates that stellar or planetary characteristics other than temperature have an important impact on temperature inversion .
De Mooij & Snellen ( 2009 ) also detected a possible slight offset in the timing of the secondary eclipse in K band .
However , based on our 4 Spitzer channels , we place a 3 \sigma upper limit of |ecos ( \omega ) | < 0.0056 where e is the planet’s orbital eccentricity and \omega is the longitude of the periastron .
This result strongly indicates that the orbit is circular , as expected from tidal circularization theory .