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 the results of infrared (8 mm) transit and secondary eclipse photometry of the hot Neptune exoplanet, GJ 436b using Spitzer. The nearly photon-limited precision of these data allows us to measure an improved radius for the planet and to detect the secondary eclipse. The transit (centered at HJD p 2454280.78149 ‫ע‬ ) shows the flat-bottomed shape typical of infrared transits, and it precisely defines the planet-to-star 0.00016 radius ratio ( ), independent of the stellar properties. However, we obtain the planetary radius, 0.0839 ‫ע‬ 0.0005 as well as the stellar mass and radius, by fitting to the transit curve simultaneously with an empirical mass-radius relation for M dwarfs ( ). We find in solar units, and km M p R R p M p 0.47 ‫ע‬ 0.02 R p 27,600 ‫ע‬ 1170 * * p ( ). This radius significantly exceeds the radius of a naked ocean planet and requires a gaseous 4.33 ‫ע‬ 0.18 R hydrogen-helium envelope. The secondary eclipse occurs at phase , proving a significant orbital 0.587 ‫ע‬ 0.005 eccentricity ( ). The amplitude of the eclipse [ ] indicates a brightness tem-Ϫ4 e p 0.150 ‫ע‬ 0.012 (5.7 ‫ע‬ 0.8) # 10 perature for the planet of K. If this is indicative of the planet's physical temperature, it suggests T p 712 ‫ע‬ 36 the occurrence of tidal heating in the planet. An uncharacterized second planet likely provides ongoing gravitational perturbations that maintain GJ 436b's orbit eccentricity over long timescales.

We present new transit and occultation times for the hot Jupiter WASP-12b. The data are compatible with a constant period derivative: ˙ P = −29 ± 3 ms yr −1 and P/ ˙ P = 3.2 Myr. However, it is difficult to tell whether we have observed orbital decay or a portion of a 14-year apsidal precession cycle. If interpreted as decay, the star's tidal quality parameter Q ⋆ is about 2 × 10 5. If interpreted as precession, the planet's Love number is 0.44 ± 0.10. Orbital decay appears to be the more parsimonious model: it is favored by ∆χ 2 = 5.5 despite having two fewer free parameters than the precession model. The decay model implies that WASP-12 was discovered within the final ∼0.2% of its existence, which is an unlikely coincidence but harmonizes with independent evidence that the planet is nearing disruption. Precession does not invoke any temporal coincidence, but it does require some mechanism to maintain an eccentricity of ≈0.002 in the face of rapid tidal circularization. To distinguish unequivocally between decay and precession will probably require a few more years of monitoring. Particularly helpful will be occultation timing in 2019 and thereafter.