Capitalizing on the observational advantage offered by its tiny M dwarf host, we present HST/WFC3 grism measurements of the transmission spectrum of the super-Earth exoplanet GJ1214b. These are the first published WFC3 observations of a transiting exoplanet atmosphere. After correcting for a ramp-like instrumental systematic, we achieve nearly photon-limited precision in these observations, finding the transmission spectrum of GJ1214b to be flat between 1.1 and 1.7 µm. Inconsistent with a cloud-free solar composition atmosphere at 8.2σ, the measured achromatic transit depth most likely implies a large mean molecular weight for GJ1214b's outer envelope. A dense atmosphere rules out bulk compositions for GJ1214b that explain its large radius by the presence of a very low density gas layer surrounding the planet. High-altitude clouds can alternatively explain the flat transmission spectrum, but they would need to be optically thick up to 10 mbar or consist of particles with a range of sizes approaching 1 µm in diameter.

In contrast to planets with masses similar to that of Jupiter and higher, the bulk compositions of planets in the so-called super-Earth regime (M p = 2 -10 M ⊕ ) cannot be uniquely determined from a mass and radius measurement alone. For these planets, there is a degeneracy between the mass and composition of the interior and a possible atmosphere in theoretical models 1, 2 . The recently discovered transiting super-Earth GJ 1214b is one example of this problem 3 . Three distinct models for the planet that are consistent with its mass and radius have been suggested 4 , and breaking the degeneracy between these models requires obtaining constraints on the planet's atmospheric composition 5,6 . Here we report a ground-based measurement of the transmission spectrum of GJ 1214b between 780 and 1000 nm. The lack of features in this spectrum rules out cloud-free atmospheres composed primarily of hydrogen at 4.9 σ confidence. If the planet's atmosphere is hydrogen-dominated, then it must contain clouds or hazes that are optically thick at the observed wavelengths at pressures less than 200 mbar. Alternatively, the featureless transmission spectrum is also consistent with the presence of a dense water vapor atmosphere.We observed transits of the planet GJ 1214b in front of its host star on UT dates 29 April and 6 June 2010 using the FORS2 instrument on the UT1 telescope of the Very Large Telescope facility. The instrument was configured for multi-object spectroscopy using a mask with slits positioned on GJ 1214 and six other nearby reference stars of similar brightness. The slits had lengths of 30 arc seconds and widths of 12.0 arc seconds to eliminate possible differential losses due to variations in the telescope guiding and seeing 7 . Complete spectra from 780 -1000 nm with a resolution of approximately 1 nm were obtained for all the stars in each exposure. A total of 197 exposures were obtained during the two observing runs, 88 of which were during a transit.We extracted both "white" and eleven 20 nm channel width spectrophotometric light curves for GJ 1214 and the reference stars by summing the obtained spectra over wavelength. We corrected the transit light curves by combining the fluxes for five of the reference stars and dividing them from the flux of GJ 1214. After this reduction, the light curves for GJ 1214 exhibit the expected transit morphology superimposed on a curvature that is well-matched by a second order polynomial as a function of time. We modeled this trend for each time series simultaneously with 1 the transit modeling described below. Normalized and corrected light curves for the spectrophotometric data are shown in Figure 1. The photon-limited uncertainties in the measurements after reduction and correction are 350 -710 ppm. We found that these estimates potentially underestimate the true uncertainties in the data, and we therefore revised the uncertainties upward by 25 -78% to yield reduced χ 2 values of unity for the light curve model fits. See the Supplementary Information for more details on ...

A key legacy of the recently launched TESS mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. JWST, although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as ARIEL require the discovery and confirmation of several hundred additional sub-Jovian size planets (R p < 10 R ⊕ ) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based ELTs will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al. (2015), we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of ∼ 300 high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds.

Ever since the discovery of the first exoplanet, astronomers have made steady progress towards finding and probing planets in the habitable zone of their host stars, where the conditions could be right for liquid water to form and life to sprawl. Results from the Kepler mission indicate that the occurrence rate of habitable-zone Earths and super-Earths may be as high as 5-20%. Despite this abundance, probing the conditions and atmospheric properties on any of these habitable-zone planets is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid water clouds in the atmosphere of the 8.6 M ⊕ habitable-zone planet K2-18b. With a 33 day orbit around a cool M3 dwarf, K2-18b receives virtually the same amount of total radiation from its host star (1441 ± 80 W/m 2 ) as the Earth receives from the Sun (1370 W/m 2 ), making it a good candidate to host liquid water clouds. In this study we observed eight transits using HST/WFC3 in order to achieve the necessary sensitivity to detect water vapor. While the thick gaseous envelope of K2-18b means that it is not a true Earth analogue, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.