We report detection of strong infrared thermal emission from the nearby ( d = 19 pc ) transiting extrasolar planet HD 189733b , 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 ~ { } \mu m shows the secondary eclipse depth to be 0.551 \pm 0.030 \% , with accuracy limited by instrumental baseline uncertainties , but with 32 \sigma precision ( \sigma = 0.017 \% ) on the detection .
The 16 ~ { } \mu m brightness temperature of this planet ( 1117 \pm 42 K ) is very similar to the Spitzer detections of TrES-1 and HD 209458b , but the observed planetary flux ( 660 ~ { } \mu 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 ( \sim 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 \sim 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 .