  We present Spitzer Space Telescope infrared photometric time series of the
transiting extrasolar planet system TrES-1. The data span a predicted time of
secondary eclipse, corresponding to the passage of the planet behind the star.
In both bands of our observations, we detect a flux decrement with a timing,
amplitude, and duration as predicted by published parameters of the system.
This signal represents the first direct detection of (i.e. the observation of
photons emitted by) a planet orbiting another star. The observed eclipse depths
(in units of relative flux) are 0.00066 +/- 0.00013 at 4.5um and 0.00225 +/-
0.00036 at 8.0um. These estimates provide the first observational constraints
on models of the thermal emission of hot Jupiters. Assuming that the planet
emits as a blackbody, we estimate an effective temperature of T_p=1060 +/- 50
K. Under the additional assumptions that the planet is in thermal equilibrium
with the radiation from the star and emits isotropically, we find a Bond albedo
of A = 0.31 +/- 0.14. This would imply that the planet absorbs the majority of
stellar radiation incident upon it, a conclusion of significant impact to
atmospheric models of these objects. We compare our data to a
previously-published model of the planetary thermal emission, which predicts
prominent spectral features in our observational bands due to water and carbon
monoxide. Based on the time of secondary eclipse, we present an upper limit on
the orbital eccentricity that is sufficiently small that we conclude that tidal
dissipation is unlikely to provide a significant source of energy interior to
the planet.(abridged)
