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.

function of the column density of metastable helium. Particles in the exosphere (c) are colored as in panel (a). Panel b shows the grids discretizing the stellar disk, the thermosphere, and the planetary disk.

Broad absorption signatures from alkali metals, such as the sodium (Na I) and potassium (K I) resonance doublets, have long been predicted in the optical atmospheric spectra of cloud-free irradiated gas giant exoplanets. However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles. Cloud and haze opacity at the day-night planetary terminator are considered to be responsible for obscuring the absorption-line wings, which hinders constraints on absolute atmospheric abundances. Here we report an optical transmission spectrum for the 'hot Saturn' exoplanet WASP-96b obtained with the Very Large Telescope, which exhibits the complete pressure-broadened profile of the sodium absorption feature. The spectrum is in excellent agreement with cloud-free, solar-abundance models assuming chemical equilibrium. We are able to measure a precise, absolute sodium abundance of logε = [Formula: see text], and use it as a proxy for the planet's atmospheric metallicity relative to the solar value (Z/Z = [Formula: see text]). This result is consistent with the mass-metallicity trend observed for Solar System planets and exoplanets.

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Context. Probing the evaporation of exoplanet atmospheres is key to understanding the formation and evolution of exoplanetary systems. The main tracer of evaporation in the UV is the Lyman-α transition, which can reveal extended exospheres of neutral hydrogen. Recently, the near-infrared (NIR) metastable helium triplet (10833 Å) revealed extended thermospheres in several exoplanets, opening a new window into evaporation. Aims. We aim at spectrally resolving the first helium absorption signature detected in the warm Saturn WASP-107b with HST/WFC3. Methods. We obtained one transit of WASP-107b with the high-resolution spectrograph CARMENES on the 3.5m telescope in Calar Alto. Results. We detect an excess helium absorption signature of 5.54±0.27 % (20σ) in the planet rest frame during the transit. The detection is in agreement with the previous detection done with HST/WFC3. The signature shows an excess absorption in the blue part of the lines suggesting that He i atoms are escaping from the atmosphere of WASP-107b. We interpret the time-series absorption spectra using the 3D EVE code. Our observations can be explained by combining an extended thermosphere filling half the Roche lobe and a large exospheric tail sustained by an escape rate of metastable helium on the order of 10 6 g·s −1 . In this scenario, however, the upper atmosphere needs to be subjected to a reduced photoionisation and radiation pressure from the star for the model to match the observations. Conclusions. We confirm the presence of helium in the atmosphere of WASP-107b at high-confidence. The helium feature is detected from space and the ground. The ground-based high-resolution signal brings detailed information about the spatial and dynamical structure of the upper atmosphere, and simulations suggest that the He i signature of WASP-107b probes both its thermosphere and exosphere establishing this signature as a robust probe of exoplanetary upper atmospheres. Surveys with NIR high-resolution spectrographs (e.g. CARMENES, SPIRou or NIRPS) will deliver a statistical understanding of exoplanet thermospheres and exospheres via the helium triplet.