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.

The nearby extrasolar planet GJ 436b-which has been labelled as a 'hot Neptune'-reveals itself by the dimming of light as it crosses in front of and behind its parent star as seen from Earth. Respectively known as the primary transit and secondary eclipse, the former constrains the planet's radius and mass 1,2 , and the latter constrains the planet's temperature 3,4 and, with measurements at multiple wavelengths, its atmospheric composition. Previous work 5 using transmission spectroscopy failed to detect the 1.4-m water vapour band, leaving the planet's atmospheric composition poorly constrained. Here we report the detection of planetary thermal emission from the dayside of GJ 436b at multiple infrared wavelengths during the secondary eclipse.The best-fit compositional models contain a high CO abundance and a substantial methane (CH 4 ) deficiency relative to thermochemical equilibrium models 6 for the predicted hydrogen-dominated atmosphere 7,8 . Moreover, we report the presence of some H 2 O and traces of CO 2 . Because CH 4 is expected to be the dominant carbonbearing species, disequilibrium processes such as vertical mixing 9 and polymerization of methane 10 into substances such as ethylene may be required to explain the hot Neptune's small CH 4 -to-CO ratio, which is at least 10 5 times smaller than predicted 6 .Using the Spitzer Space Telescope 11 , the Spitzer Exoplanet Target of Opportunity program observed multiple secondary eclipses at wavelengths of 3. 6, 4.5, 5.8, 8.0 Figure 1 shows the observed secondary eclipses with best-fit models, and Table 1 presents the relevant eclipse parameters.The phase of secondary eclipse imposes a tight constraint on the planet"s eccentricity, e, and argument of periapsis, . Using the secondary eclipse times listed in Table 1, in addition to published transit 16 and radial-velocity data 17 , a single-planet Our broadband observations constrain a one-dimensional atmospheric model, using a new temperature and abundance retrieval method 18 . This method searches over a wide parameter space using a functional form for the pressure-temperature profile (based on prior "hot Jupiter" and Solar System studies), a grid of abundance combinations, and energy conservation. We calculated ~10 6 models, which considered both inversion and noninversion temperature profiles and abundances that varied over several orders of magnitude Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature09013Page 3 of 42per constituent. Figure 2 shows two representative models (the red and blue lines) that fit the data, and and, to a lesser extent, CO, and possibly CO 2 . In a reduced, hydrogen-dominated atmosphere at ~700 K, CH 4 is thermochemically favoured to be the main carbon-bearing molecule. Assuming solar abundances for the elements and the pressure-temperature profile shown in Supplementary Fig. 5, chemical equilibrium predicts 6 a CH 4 -to-H 2 mixing ratio of 7 × 10 4 and an H 2 O mixing ratio of 2 × 10 3 . However, the strong planetary emission at 3.6 m...

The carbon-to-oxygen ratio (C/O

The dayside of HD 149026b is near the edge of detectability by the Spitzer Space Telescope. We report on eleven secondary-eclipse events at 3.6, 4.5, 3 × 5.8, 4 × 8.0, and 2 × 16 µm plus three primary-transit events at 8.0 µm. The eclipse depths from jointly-fit models at each wavelength are 0.040 ± 0.003% at 3.6 µm, 0.034 ± 0.006% at 4.5 µm, 0.044 ± 0.010% at 5.8 µm, 0.052 ± 0.006% at 8.0 µm, and 0.085 ± 0.032% at 16 µm. Multiple observations at the longer wavelengths improved eclipse-depth signal-to-noise ratios by up to a factor of two and improved estimates of the planet-to-star radius ratio (R p /R ⋆ = 0.0518 ± 0.0006). We also identify no significant deviations from a circular orbit and, using this model, report an improved period of 2.8758916 ± 0.0000014 days. Chemical-equilibrium models find no indication of a temperature inversion in the dayside atmosphere of HD 149026b. Our best-fit model favors large amounts of CO and CO 2 , moderate heat redistribution ( f = 0.5), and a strongly enhanced metallicity. These analyses use BiLinearly-Interpolated Subpixel Sensitivity (BLISS) mapping, a new technique to model two position-dependent systematics (intrapixel variability and pixelation) by mapping the pixel surface at high resolution. BLISS mapping outperforms previous methods in both speed and goodness of fit. We also present an orthogonalization technique for linearly-correlated parameters that accelerates the convergence of Markov chains that employ the Metropolis random walk sampler. The electronic supplement contains light-curve files and supplementary figures.

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.