We present here the analysis of 30 gaseous extrasolar planets, with temperatures between 600 and 2400 K and radii between 0.35 and 1.9 R Jup . The quality of the HST/WFC3 spatially scanned data combined with our specialized analysis tools allow us to study the largest and most self-consistent sample of exoplanetary transmission spectra to date and examine the collective behavior of warm and hot gaseous planets rather than isolated case-studies. We define a new metric, the Atmospheric Detectability Index (ADI) to evaluate the statistical significance of an atmospheric detection and find statistically significant atmospheres around 16 planets out of the 30 analysed. For most of the Jupiters in our sample, we find the detectability of their atmospheres to be dependent on the planetary radius but not on the planetary mass. This indicates that planetary gravity plays a secondary role in the state of gaseous planetary atmospheres. We detect the presence of water vapour in all of the statistically detectable atmospheres, and we cannot rule out its presence in the atmospheres of the others. In addition, TiO and/or VO signatures are detected with 4 σ confidence in WASP-76 b, and they are most likely present in WASP-121 b. We find no correlation between expected signal-to-noise and atmospheric detectability for most targets. This has important implications for future large-scale surveys.

In the past decade, observations from space and ground have found H 2 O to be the most abundant molecular species, after hydrogen, in the atmospheres of hot, gaseous, extrasolar planets 1-5 . Being the main molecular carrier of oxygen, H 2 O is a tracer of the origin and the evolution mechanisms of planets. For temperate, terrestrial planets, the presence of H 2 O is of great significance as an indicator of habitable conditions. Being small and relatively cold, these planets and their atmospheres are the most challenging to observe, and therefore no atmospheric spectral signatures have so far been detected 6 . Super-Earths -planets lighter than ten M ⊕ -around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations. Here we report the detection of an H 2 O spectroscopic signature in the atmosphere of K2-18 b -an eight M ⊕ planet in the habitable-zone of an M-dwarf 7 -with high statistical confidence (ADI 5 = 5.0, ∼3.6σ 8,9 ). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/WFC3 camera, and analysed with our dedicated, publicly avail-1 arXiv:1909.05218v1 [astro-ph.EP] 11 Sep 2019 able, algorithms 5, 9 . While the suitability of M-dwarfs to host habitable worlds is still under discussion 10-13 , K2-18 b offers an unprecedented opportunity to get insight into the composition and climate of habitable-zone planets. Atmospheric characterisation of super-Earths is currently within reach of the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST), combined with the recently implemented spatial scanning observational strategy 14 . The spectra of three hot transiting planets with radii less than 3.0 R ⊕ have been published so far: GJ-1214 b 15 , HD 97658 b 16 and 55 Cnc e 17 . The first two do not show any evident transit depth modulation with wavelength, suggesting an atmosphere covered by thick clouds or made of molecular species heavier than hydrogen, while only the spectrum of 55 Cnc e has revealed a light-weighted atmosphere, suggesting H/He still being present. In addition, transit observations of six temperate Earth-size planets around the ultra-cool dwarf TRAPPIST-1 -planets b, c, d, e, f 6 , and g 18 -have not shown any molecular signatures and have excluded the presence of cloud-free, H/He atmospheres around them. K2-18 b was discovered in 2015 by the Kepler spacecraft 7 , and it is orbiting around an M2.5 ([Fe/H] = 0.123 ± 0.157 dex, T eff = 3457 ± 39 K, M * = 0.359 ± 0.047 M , R * = 0.411±0.038 R ) 19 dwarf star, 34 pc away from the Earth. The star-planet distance of 0.1429 AU 19suggests a planet within the star's habitable zone (∼ 0.12 -0.25 AU) 20 , with effective temperature between 200 K and 320 K, depending on the albedo and the emissivity of its surface and/or its atmosphere. This crude estimate accounts for neither possible tidal energy sources 21 nor atmospheric heat ...

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

We report the analysis of two new spectroscopic observations in the near-infrared of the super-Earth 55 Cancri e, obtained with the WFC3 camera on board the Hubble Space Telescope. 55 Cancri e orbits so close to its parent star that temperatures much higher than 2000 K are expected on its surface. Given the brightness of 55 Cancri, the observations were obtained in scanning mode, adopting a very long scanning length and a very high scanning speed. We use our specialized pipeline to take into account systematics introduced by these observational parameters when coupled with the geometrical distortions of the instrument. We measure the transit depth per wavelength channel with an average relative uncertainty of 22 ppm per visit and find modulations that depart from a straight line model with a 6σ confidence level. These results suggest that 55 Cancri e is surrounded by an atmosphere, which is probably hydrogen-rich. Our fully Bayesian spectral retrieval code,  -REx, has identified HCN to be the most likely molecular candidate able to explain the features at 1.42 and 1.54 μm. While additional spectroscopic observations in a broader wavelength range in the infrared will be needed to confirm the HCN detection, we discuss here the implications of such a result. Our chemical model, developed with combustion specialists, indicates that relatively high mixing ratios of HCN may be caused by a high C/O ratio. This result suggests this super-Earth is a carbon-rich environment even more exotic than previously thought.

The Wide Field Camera 3 (WFC3) on Hubble Space Telescope (HST) is currently one of the most widely used instruments for observing exoplanetary atmospheres, especially with the use of the spatial scanning technique. An increasing number of exoplanets have been studied using this technique as it enables the observation of bright targets without saturating the sensitive detectors. In this work we present a new pipeline for analyzing the data obtained with the spatial scanning technique, starting from the raw data provided by the instrument. In addition to commonly used correction techniques, we take into account the geometric distortions of the instrument, whose impact may become important when combined to the scanning process. Our approach can improve the photometric precision for existing data and also push further the limits of the spatial scanning technique, as it allows the analysis of even longer spatial scans. As an application of our method and pipeline, we present the results from a reanalysis of the spatially scanned transit spectrum of HD 209458 b. We calculate the transit depth per wavelength channel with an average relative uncertainty of 40 ppm. We interpret the final spectrum with T -REx, our fully Bayesian spectral retrieval code, which confirms the presence of water vapor and clouds in the atmosphere of HD 209458 b. The narrow wavelength range limits our ability to disentangle the degeneracies between the fitted atmospheric parameters. Additional data over a broader spectral range are needed to address this issue.

Bean et al.

This paper presents the atmospheric characterization of three large, gaseous planets: WASP-127 b, WASP-79 b, and WASP-62 b. We analyzed spectroscopic data obtained with the G141 grism (1.088-1.68 μm) of the Wide Field Camera 3 on board the Hubble Space Telescope using the Iraclis pipeline and the TauREx3 retrieval code, both of which are publicly available. For WASP-127 b, which is the least dense planet discovered so far and is located in the shortperiod Neptune desert, our retrieval results found strong water absorption corresponding to an abundance of log(H 2 O)=−2.71-+ 1.05 0.78 and absorption compatible with an iron hydride abundance of log(FeH)=-+ 5.25 1.10 0.88 , with an extended cloudy atmosphere. We also detected water vapor in the atmospheres of WASP-79 b and WASP-62 b, with best-fit models indicating the presence of iron hydride, too. We used the Atmospheric Detectability Index as well as Bayesian log evidence to quantify the strength of the detection and compared our results to the hot Jupiter population study by Tsiaras et al. While all the planets studied here are suitable targets for characterization with upcoming facilities such as the James Webb Space Telescope and Ariel, WASP-127 b is of particular interest due to its low density, and a thorough atmospheric study would develop our understanding of planet formation and migration. Unified Astronomy Thesaurus concepts: Exoplanet astronomy (486); Exoplanet atmospheres (487); Astronomy data analysis (1858); Hubble Space Telescope (761)

The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atmospheric properties using transmission spectroscopy1–4. However, these efforts have been hampered by modelling degeneracies between composition and cloud properties that are caused by limited data quality5–9. Here we present the transmission spectrum of WASP-39b obtained using the Single-Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the JWST. This spectrum spans 0.6–2.8 μm in wavelength and shows several water-absorption bands, the potassium resonance doublet and signatures of clouds. The precision and broad wavelength coverage of NIRISS/SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favouring a heavy-element enhancement (‘metallicity’) of about 10–30 times the solar value, a sub-solar carbon-to-oxygen (C/O) ratio and a solar-to-super-solar potassium-to-oxygen (K/O) ratio. The observations are also best explained by wavelength-dependent, non-grey clouds with inhomogeneous coverageof the planet’s terminator.