Exoplanet atmosphere spectroscopy enables us to improve our understanding of exoplanets just as remote sensing in our own solar system has increased our understanding of the solar system bodies. The challenge is to quantitatively determine the range of temperatures and molecular abundances allowed by the data which is often difficult given the low information content of most exoplanet spectra which commonly leading to degeneracies in the interpretation. A variety of spectral retrieval approaches have been applied to exoplanet spectra, but no previous investigations have sought to compare these approaches. We compare three different retrieval methods: optimal estimation, differential evolution Markov chain Monte Carlo, and bootstrap Monte Carlo on a synthetic water dominated hot-Jupiter. We discuss expectations of uncertainties in abundances and temperatures given current and potential future observations. In general we find that the three approaches agree for high spectral resolution, high signal-to-noise data expected to come from potential future spaceborne missions, but disagree for low resolution, low signal-to-noise spectra representative of current observations. We also compare the results from a parameterized temperature profile versus a full classical Level-by-Level approach and discriminate in which situations each of these approaches is applicable. Furthermore, we discuss the implications of our models for the inferred C to O ratios of exoplanetary atmospheres. Specifically we show that in the observational limit of a few photometric points, the retrieved C/O is biased towards values near solar and near one simply due to the assumption of uninformative priors.

We have measured the dayside spectrum of HD 189733b between 1.5 and 2.5 µm using the NICMOS instrument on the Hubble Space Telescope. The emergent spectrum contains significant modulation, which we attribute to the presence of molecular bands seen in absorption. We find that water (H 2 O), carbon monoxide (CO), and carbon dioxide (CO 2 ) are needed to explain the observations, and we are able to estimate the mixing ratios for these molecules. We also find temperature decreases with altitude in the ∼ 0.01 < P < ∼ 1 bar region of the dayside near-infrared photosphere and set an upper limit to the dayside abundance of methane (CH 4 ) at these pressures.

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet's birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25-7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and welldefined planet sample within its 4-year mission lifetime. Transit, eclipse and phasecurve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10-100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H 2 O, CO 2 , CH 4 NH 3 , HCN, H 2 S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performedusing conservative estimates of mission performance and a

Using the NICMOS instrument on the Hubble Space Telescope, we have measured the dayside spectrum of HD 209458b between 1.5-2.5 µm. The emergent spectrum is dominated by features due to the presence of methane (CH 4 ) and water vapor (H 2 O), with smaller contributions from carbon dioxide (CO 2 ). Combining this near-infrared spectrum with existing mid-infrared measurements shows the existence of a temperature inversion and confirms the interpretation of previous photometry measurements. We find a family of plausible solutions for the molecular abundance and detailed temperature profile. Observationally resolving the ambiguity between abundance and temperature requires either (1) improved wavelength coverage or spectral resolution of the dayside emission spectrum, or (2) a transmission spectrum where abundance determinations are less sensitive to the temperature structure.

9down to "super-Earths" with diameters less than three times that of the Earth ).JWST will revolutionize our knowledge of the physical properties of dozens to possibly hundreds of exoplanets by making a variety of different types of observations. Here we focus on transits and phase curves; direct detection via coronagraphy was considered in a 2007 white paper (for all JWST white papers see http://www.stsci.edu/jwst/doc-archive/whitepapers) and will be revisited in the near future.JWST 's unique combination of high sensitivity and broad wavelength coverage enables the accurate measurement of transit and orbital parameters with high signal-to-noise (SNR). Most importantly, JWST will investigate planetary atmospheres, determine atomic and molecular compositions, probe vertical and horizontal structure, and follow dynamical evolution (i.e. exoplanet weather). It will do this for a diverse population of planets of varying masses and densities, in a wide variety of environments characterized by a range of host star masses and metallicities, orbital semi-major axes and eccentricities. 3The sensitivity of JWST over its wavelength range of 0.6 to 28 microns compared to other missions and ground-based facilities has been amply documented (http://www.stsci.edu/jwst/science/sensitivity) and JWST 's halo orbit around the Earth-Sun L2 point provides long, highly stable, uninterrupted observing sequences 3 Of particular interest for JWST will be small planets (R< 2−4R ⊕ ) located at a distance from their host stars such that their equilibrium temperatures could be comparable to that of our Earth. The range of the so-called "Habitable Zone" has been argued over by many authors since its original definition (Kasting et al. 1993). We take an agnostic approach to this question, referring loosely to planets whose stellar insolation is comparable to that of our own.-10compared with the ground or HST. JWST 's detectors are capable of much better than 100 parts per million (ppm) precision over time periods from hours to days. Its suite of four instruments and multiple operating modes provides a large range of choices in trading off spectral resolution (R between 4 -3000), photometric sensitivity, and observing time. Taken together, these characteristics will make JWST's transit and eclipse observations the best method for characterizing exoplanet atmospheres in the foreseeable future.

We report near-infrared measurements of the terminator region transmission spectrum and dayside emission spectrum of the exoplanet WASP-12b obtained using the HST WFC3 instrument. The disk-average dayside brightness temperature averages about 2900 K, peaking to 3200 K around 1.46 microns. We modeled a range of atmospheric cases for both the emission and transmission spectrum and confirm the recent finding by Crossfield et al. (2012b) that there is no evidence for C/O >1 in the atmosphere of WASP-12b. Assuming a physically plausible atmosphere, we find evidence that the presence of a number of molecules is consistent with the data, but the justification for inclusion of these opacity sources based on the Bayesian Information Criterion (BIC) is marginal. We also find the near-infrared primary eclipse light curve is consistent with small amounts of prolate distortion. As part of the calibration effort for these data, we conducted a detailed study of instrument systematics using 65 orbits of WFC3-IR grims observations. The instrument systematics are dominated by detector-related affects, which vary significantly depending on the detector readout mode. The 256x256 subarray observations of WASP 12 produced spectral measurements within 15% of the photon-noise limit using a simple calibration approach. Residual systematics are estimated to be less than 70 parts per million.Comment: Accepted for publication in Icaru

and is difficult to explain with models that assume local thermodynamic equilibrium

We report here the first infrared spectrum of the hot-Jupiter XO-1b.The observations were obtained with NICMOS instrument onboard the Hubble Space Telescope during a primary eclipse of the XO-1 system. Near photon-noise-limited spectroscopy between 1.2 and 1.8 µm allows us to determine the main composition of this hot-Jupiter's planetary atmosphere with good precision. This is the third hot-Jupiter's atmosphere for which spectroscopic data are available in the near IR. The spectrum shows the presence of water vapor (H 2 O), methane (CH 4 ) and carbon dioxide (CO 2 ), and suggests the possible presence of carbon monoxide (CO). We show that the published IRAC secondary transit emission photometric data are compatible with the atmospheric composition at the terminator determined from the NICMOS spectrum, with a range of possible mixing-ratios and thermal profiles; additional emission spectroscopy data are needed to reduce the degeneracy of the possible solutions. Finally, we note the similarity between the 1.2-1.8 µm transmission spectra of XO-1b and HD 209458b, suggesting that in addition to having similar stellar/orbital and planetary parameters the two systems may also have a similar exoplanetary atmospheric composition.