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 (∼0.03) 6, 7 , maintained by interaction with a hypothetical second planet. Here we report detection of infrared (24 µm) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24 µm flux is 55±10 µJy (1σ), with a brightness temperature of 1130±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 ±7 min, 1σ), which means that a dynamically significant orbital eccentricity is unlikely.

We report detection of strong infrared thermal emission from the nearby (d ¼ 19 pc) transiting extrasolar planet HD 189733b by measuring the flux decrement during its prominent secondary eclipse. A 6 hr photometric sequence using Spitzer's infrared spectrograph in peak-up imaging mode at 16 m shows the secondary eclipse depth to be 0:551% AE 0:030%, with accuracy limited by instrumental baseline uncertainties, but with 32 precision ( ¼ 0:017%) on the detection. The 16 m brightness temperature of this planet (1117 AE 42 K) is very similar to the Spitzer detections of TrES-1 and HD 209458b, but the observed planetary flux (660 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 ratio ($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 s 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.

We discuss atmosphere models of HD209458b in light of the recent day-side flux measurement of HD209458b's secondary eclipse by Spitzer-MIPS at 24 microns. In addition, we present a revised secondary eclipse IRTF upper limit at 2.2 microns which places a stringent constraint on the adjacent H2O absorption band depths. These two measurements are complementary because they are both shaped by H2O absorption and because the former is on the Wien tail of the planet's thermal emission spectrum and the latter is near the thermal emission peak. A wide range of models fit the observational data, confirming our basic understanding of hot Jupiter atmospheric physics. Although a range of models are viable, some models at the hot and cold end of the plausible temperature range can be ruled out. One class of previously unconsidered hot Jupiter atmospheric models that fit the data are those with C/O >~ 1 (as Jupiter may have), which have a significant paucity of H2O compared to solar abundance models with C/O = 0.5. The models indicate that HD209458b is in a situation intermediate between pure in situ reradiation and very efficient redistribution of heat; one which will require a careful treatment of atmospheric circulation. We discuss how future wavelength-dependent and phase-dependent observations will further constrain the atmospheric circulation regime. In the shorter term, additional planned measurements for HD209458b, especially Spitzer IRAC photometry, should lift many of the model degeneracies. Multiwavelength IR observations constrain the atmospheric structure and circulation properties of hot Jupiters and thus open a new chapter in quantitative extrasolar planetology.Comment: 12 pages, 4 figures, submitted to Ap

The star upsilon Andromedae is orbited by three known planets, the innermost of which has an orbital period of 4.617 days and a mass at least 0.69 that of Jupiter. This planet is close enough to its host star that the radiation it absorbs overwhelms its internal heat losses. Here, we present the 24-micrometer light curve of this system, obtained with the Spitzer Space Telescope. It shows a variation in phase with the orbital motion of the innermost planet, demonstrating that such planets possess distinct hot substellar (day) and cold antistellar (night) faces.