Detailed characterization of exoplanets has begun to yield measurements of their atmospheric properties that constrain the planets origins and evolution. For example, past observations of the dayside emission spectrum of the hot Jupiter WASP-12b indi
cated that its atmosphere has a high carbon-to-oxygen ratio (C/O $>$ 1), suggesting it had a different formation pathway than is commonly assumed for giant planets. Here we report a precise near-infrared transmission spectrum for WASP-12b based on six transit observations with the Hubble Space Telescope/Wide Field Camera 3. We bin the data in 13 spectrophotometric light curves from 0.84 - 1.67 $mu$m and measure the transit depths to a median precision of 51 ppm. We retrieve the atmospheric properties using the transmission spectrum and find strong evidence for water absorption (7$sigma$ confidence). This detection marks the first high-confidence, spectroscopic identification of a molecule in the atmosphere of WASP-12b. The retrieved 1$sigma$ water volume mixing ratio is between $10^{-5}-10^{-2}$, which is consistent with C/O $>$ 1 to within 2$sigma$. However, we also introduce a new retrieval parameterization that fits for C/O and metallicity under the assumption of chemical equilibrium. With this approach, we constrain C/O to $0.5^{+0.2}_{-0.3}$ at $1,sigma$ and rule out a carbon-rich atmosphere composition (C/O$>1$) at $>3sigma$ confidence. Further observations and modeling of the planets global thermal structure and dynamics would aid in resolving the tension between our inferred C/O and previous constraints. Our findings highlight the importance of obtaining high-precision data with multiple observing techniques in order to obtain robust constraints on the chemistry and physics of exoplanet atmospheres.
We present a HST WFC3 transmission spectrum for the transiting exoplanet HAT-P-12b. This warm (1000 K) sub-Saturn-mass planet has a smaller mass and a lower temperature than the hot-Jupiters that have been studied so far. We find that the planets mea
sured transmission spectrum lacks the expected water absorption feature for a hydrogen-dominated atmosphere, and is instead best-described by a model with high-altitude clouds. Using a frequentist hypothesis testing procedure, we can rule out a hydrogen-dominated cloud free atmosphere to 4.9$sigma$. When combined with other recent WFC3 studies, our observations suggest that clouds may be common in exo-planetary atmospheres.
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 accreti
on of nebular gas is expected produce hot Neptunes whose masses are dominated by elements heavier than hydrogen and helium. At high atmospheric metallicities of 10-10,000x solar, hot Neptunes will exhibit an interesting continuum of atmospheric compositions, ranging from more Neptune-like, H2-dominated atmospheres to more Venus-like, CO2-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 H2O, CO, CO2, and even O2-dominated atmospheres are possible for hot Neptunes. We apply our models to the case of GJ 436b, where we find that a CO-rich, CH4-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 fit obtained from pure retrieval methods, the atmospheric composition predicted by these forward models is more physically and chemically plausible. 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 introduce a new thermochemical kinetics and photochemical model. We use high-temperature bidirectional reaction rates for important H, C, O and N reactions (most importantly for CH$_4$ to CO interconversion), allowing us to attain thermochemical e
quilibrium, deep in an atmosphere, purely kinetically. This allows ab initio chemical modeling of an entire atmosphere, from deep-atmosphere thermochemical equilibrium to the photochemically dominated regime. We use our model to explore the atmospheric chemistry of cooler ($T_{eff} < 10^3$ K) extrasolar giant planets. In particular, we choose to model the nearby hot Neptune GJ436b, the only planet in this temperature regime for which spectroscopic measurements and estimates of chemical abundances now exist. Recent {it Spitzer} measurements with retrieval have shown that methane is driven strongly out of equilibrium and is deeply depleted on the dayside of GJ 436b, whereas quenched carbon monoxide is abundant. This is surprising because GJ 436b is cooler than many of the heavily irradiated hot Jovians and thermally favorable for CH$_4$, and thus requires an efficient mechanism for destroying it. We include realistic estimates of ultraviolet flux from the parent dM star GJ 436, to bound the direct photolysis and photosensitized depletion of CH$_4$. While our models indicate fairly rich disequilibrium conditions are likely in cooler exoplanets over a range of planetary metallicities, we are unable to generate the conditions for substantial CH$_4$ destruction. One possibility is an anomalous source of abundant H atoms between 0.01-1 bars (which attack CH$_4$), but we cannot as yet identify an efficient means to produce these hot atoms.
