  We report detection of strong infrared thermal emission from the nearby (d=19
pc) transiting extrasolar planet HD189733b, by measuring the flux decrement
during its prominent secondary eclipse. A 6-hour photometric sequence using
Spitzer's infrared spectrograph in peak-up imaging mode at 16-microns shows the
secondary eclipse depth to be 0.551 +/-0.030%, with accuracy limited by
instrumental baseline uncertainties, but with 32-sigma precision (0.017%) on
the detection. The 16-micron brightness temperature of this planet (1117+/-42K)
is very similar to the Spitzer detections of TrES-1 and HD209458b, but the
observed planetary flux (660 micro-Jy) is an order of magnitude greater. This
large signal will allow a detailed characterization of this planet in the
infrared. Our photometry has sufficient signal-to-noise (~400 per point) to
motivate a search for structure in the ingress/egress portions of the eclipse
curve, caused by putative thermal structure on the disk of the planet. We show
that by binning our 6-second sampling down to 6-minute resolution, we detect
the modulation in the intensity derivative during ingress/egress due to the
overall shape of the planet, but our sensitivity is not yet sufficient to
distinguish between realistic models of the temperature distribution across the
planet's disk. We point out the potential for extending Spitzer secondary
eclipse detections down to the regime of transiting hot Neptunes, if such
systems are discovered among nearby lower main sequence stars.
