Recent surveys have revealed that planets intermediate in size between Earth and Neptune ('super-Earths') are among the most common planets in the Galaxy. Atmospheric studies are the next step towards developing a comprehensive understanding of this new class of object. Much effort has been focused on using transmission spectroscopy to characterize the atmosphere of the super-Earth archetype GJ 1214b (refs 7 - 17), but previous observations did not have sufficient precision to distinguish between two interpretations for the atmosphere. The planet's atmosphere could be dominated by relatively heavy molecules, such as water (for example, a 100 per cent water vapour composition), or it could contain high-altitude clouds that obscure its lower layers. Here we report a measurement of the transmission spectrum of GJ 1214b at near-infrared wavelengths that definitively resolves this ambiguity. The data, obtained with the Hubble Space Telescope, are sufficiently precise to detect absorption features from a high mean-molecular-mass atmosphere. The observed spectrum, however, is featureless. We rule out cloud-free atmospheric models with compositions dominated by water, methane, carbon monoxide, nitrogen or carbon dioxide at greater than 5σ confidence. The planet's atmosphere must contain clouds to be consistent with the data.

The water abundance in a planetary atmosphere provides a key constraint on the planet's primordial origins because water ice is expected to play an important role in the core accretion model of planet formation. However, the water content of the solar system giant planets is not well known because water is sequestered in clouds deep in their atmospheres. By contrast, short-period exoplanets have such high temperatures that their atmospheres have water in the gas phase, making it possible to measure the water abundance for these objects. We present a precise determination of the water abundance in the atmosphere of the 2 M Jup short-period exoplanet WASP-43b based on thermal emission and transmission spectroscopy measurements obtained with the Hubble Space Telescope. We find the water content is consistent with the value expected in a solar composition gas at planetary temperatures (0.4-3.5× solar at 1σ confidence). The metallicity of WASP-43b's atmosphere suggested by this result extends the trend observed in the solar system of lower metal enrichment for higher planet masses.

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...

Neptune-sized extrasolar planets that orbit relatively close to their host stars-often called "hot Neptunes"-are common within the known population of exoplanets and planetary candidates. Similar to our own Uranus and Neptune, inefficient accretion of nebular gas is expected produce hot Neptunes whose masses are dominated by elements heavier than hydrogen and helium. At high atmospheric metallicities of 10-10,000 times solar, hot Neptunes will exhibit an interesting continuum of atmospheric compositions, ranging from more Neptune-like, H 2 -dominated atmospheres to more Venus-like, CO 2 -dominated atmospheres. We explore the predicted equilibrium and disequilibrium chemistry of generic hot Neptunes and find that the atmospheric composition varies strongly as a function of temperature and bulk atmospheric properties such as metallicity and the C/O ratio. Relatively exotic H 2 O, CO, CO 2 , and even O 2 -dominated atmospheres are possible for hot Neptunes. We apply our models to the case of GJ 436b, where we find that a CO-rich, CH 4 -poor atmosphere can be a natural consequence of a very high atmospheric metallicity. From comparisons of our results with Spitzer eclipse data for GJ 436b, we conclude that although the spectral fit from the high-metallicity forward models is not quite as good as the best fit obtained from pure retrieval methods, the atmospheric composition predicted by these forward models is more physically and chemically plausible in terms of the relative abundance of major constituents. High-metallicity atmospheres (orders of magnitude in excess of solar) should therefore be considered as a possibility for GJ 436b and other hot Neptunes.

We present one of the most precise emission spectra of an exoplanet observed so far. We combine five secondary eclipses of the hot Jupiter WASP-18 b (T day ∼ 2900 K) that we secured between 1.1 and 1.7 µm with the WFC3 instrument aboard the Hubble Space Telescope. Our extracted spectrum (S/N=50, R∼40) does not exhibit clearly identifiable molecular features but is poorly matched by a blackbody spectrum. We complement this data with previously published Spitzer/IRAC observations of this target and interpret the combined spectrum by computing a grid of self-consistent, 1D forward models, varying the composition and energy budget. At these high temperatures, we find there are important contributions to the overall opacity from H − ions, as well as the removal of major molecules by thermal dissociation (including water), and thermal ionization of metals. These effects were omitted in previous spectral retrievals for very hot gas giants, and we argue that they must be included to properly interpret the spectra of these objects. We infer a new metallicity and C/O ratio for WASP-18 b, and find them well constrained to be solar ([M/H]= −0.01 ± 0.35, C/O < 0.85 at 3σ confidence level), unlike previous work but in line with expectations for giant planets. The best fitting selfconsistent temperature-pressure profiles are inverted, resulting in an emission feature at 4.5 µm seen in the Spitzer photometry. These results further strengthen the evidence that the family of very hot gas giant exoplanets commonly exhibit thermal inversions.