Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres . Searches for helium, however, have hitherto been unsuccessful . Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near-infrared transmission spectrum of the warm gas giant WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 ± 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 10 to 3 × 10 grams per second (0.1-4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure.

We present HST near-ultraviolet (NUV) transits of the hot Jupiter WASP-121b, acquired as part of the PanCET program. Time series spectra during two transit events were used to measure the transmission spectra between 2280 and 3070Å at a resolution of 30,000. Using HST data from 61 STIS visits, we show that data from HST's Pointing Control System can be used to decorrelate the instrument systematic errors (Jitter Decorrelation), which we used to fit the WASP-121b light curves. The NUV spectrum show very strong absorption features, with the NUV white light curve found to be larger than the average optical and near-infrared value at 6-σ confidence. We identify and spectrally resolve absorption from the Mg ii doublet in the planetary exosphere at a 5.9-σ confidence level. The Mg ii doublet is observed to reach altitudes of R pl /R star = 0.284 ± 0.037 for the 2796Å line and 0.242 ± 0.0431 in the 2804Å line, which exceeds the Roche lobe size as viewed in transit geometry (R eqRL /R star = 0.158). We also detect and resolve strong features of the Fe ii UV1 and UV2 multiplets, and observe the lines reaching altitudes of R pl /R star ≈ 0.3. At these high altitudes, the atmospheric Mg ii and Fe ii gas is not gravitationally bound to the planet, and these ionized species may be hydrodynamically escaping or could be magnetically confined. Refractory Mg and Fe atoms at 2 Sing et al.high altitudes also indicates that these species are not trapped into condensate clouds at depth, which places constraints on the deep interior temperature.

We present a primary transit observation for the ultra hot (T eq ∼ 2400 K) gas giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field Camera 3 in spectroscopic mode across the 1.12-1.64µm wavelength range. The 1.4µm water absorption band is detected at high confidence (5.4σ) in the planetary atmosphere. We also reanalyze ground-based photometric lightcurves taken in the B, r , and z filters. Significantly deeper transits are measured in these optical bandpasses relative to the near-infrared wavelengths. We conclude that scattering by high-altitude haze alone is unlikely to account for this difference, and instead interpret it as evidence for titanium oxide and vanadium oxide absorption. Enhanced opacity is also inferred across the 1.12-1.3µm wavelength range, possibly due to iron hydride absorption. If confirmed, WASP-121b will be the first exoplanet with titanium oxide, vanadium oxide, and iron hydride detected in transmission. The latter are important species in M/L dwarfs, and their presence is likely to have a significant effect on the overall physics and chemistry of the atmosphere, including the production of a strong thermal inversion. 8 We quote equilibrium temperature Teq as the blackbody temperature required for planetary thermal emission to balance the stellar irradiation, assuming zero Bond albedo and uniform daynight recirculation. Due to the approximate nature of equilibrium temperature, we round values to the nearest 100 K in this Letter.

High-resolution Doppler-resolved spectroscopy has opened up a new window into the atmospheres of both transiting and non-transiting exoplanets. Here, we present VLT/UVES observations of a transit of WASP-121b, an 'ultra-hot' Jupiter previously found to exhibit a temperature inversion and detections of multiple species at optical wavelengths. We present initial results using the blue arm of UVES (≈3700 -5000Å), recovering a clear signal of neutral Fe in the planet's atmosphere at >8 σ, which could contribute to (or even fully explain) the temperature inversion in the stratosphere. However, using standard cross-correlation methods, it is difficult to extract physical parameters such as temperature and abundances. Recent pioneering efforts have sought to develop likelihood 'mappings' that can be used to directly fit models to high-resolution datasets. We introduce a new framework that directly computes the likelihood of the model fit to the data, and can be used to explore the posterior distribution of parameterised model atmospheres via MCMC techniques. Our method also recovers the physical extent of the atmosphere, as well as account for time-and wavelength-dependent uncertainties. We measure a temperature of 3710 +490 −510 K, indicating a higher temperature in the upper atmosphere when compared to low-resolution observations. We also show that the Fe i signal is physically separated from the exospheric Fe ii. However, the temperature measurements are highly degenerate with aerosol properties; detection of additional species, using more sophisticated atmospheric models, or combining these methods with low-resolution spectra should help break these degeneracies.