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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 1 . Searches for helium, however, have hitherto been unsuccessful 2 . 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 3 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 10 to 3 × 10 11 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. A detection of helium absorption at 10,833 Å on the exoplanet WASP-107b reveals that its atmosphere is extended and eroding, and demonstrates a new way to study upper exoplanetary atmospheres.

Many gas giant exoplanets orbit so close to their host star that they are heated to high temperatures, causing atmospheric gases to escape. Gas giant atmospheres are mostly hydrogen and helium, which are difficult to observe. Two papers have now observed escaping helium in the near-infrared (see the Perspective by Brogi). Allart et al. observed helium in a Neptune-mass exoplanet and performed detailed simulations of its atmosphere, which put constraints on the escape rate. Nortmann et al. found that helium is escaping a Saturn-mass planet, trailing behind it in its orbit. They combined this with observations of several other exoplanets to show that atmospheres are being lost more quickly by exoplanets that are more strongly heated. Science , this issue p. 1384 , p. 1388 ; see also p. 1360 Helium is observed in the atmosphere of a warm Neptune-mass exoplanet, constraining the atmospheric loss rate. Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature—neutral hydrogen at ultraviolet wavelengths—is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b by using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08 ± 0.05%. Interpreting absorption spectra with three-dimensional simulations of the planet’s upper atmosphere suggests that it extends beyond 5 planetary radii, with a large-scale height and a helium mass loss rate of ≲3 × 10 5 grams per second. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 kilometers per second from day to night-side.

Aerosols have been found to be nearly ubiquitous in substellar atmospheres 1 – 3 . The precise temperature at which these aerosols begin to form in exoplanets has yet to be observationally constrained. Theoretical models and observations of muted spectral features indicate that silicate clouds play an important role in exoplanets between at least 950 and 2,100 K (ref. 4 ). Some giant planets, however, are thought to be hot enough to avoid condensation altogether 5 , 6 . Here we report the near-ultraviolet transmission spectrum of the ultra-hot Jupiter WASP-178b (approximately 2,450 K), which exhibits substantial absorption. Bayesian retrievals indicate the presence of gaseous refractory species containing silicon and magnesium, which are the precursors to condensate clouds at lower temperatures. SiO, in particular, has not previously, to our knowledge, been detected in exoplanets, but the presence of SiO in WASP-178b is consistent with theoretical expectations as the dominant Si-bearing species at high temperatures. These observations allow us to re-interpret previous observations of HAT-P-41b and WASP-121b that did not consider SiO, to suggest that silicate cloud formation begins on exoplanets with equilibrium temperatures between 1,950 and 2,450 K. Analysis is presented that indicates that there is strong evidence that SiO or Mg, the major constituents of silicate condensates, must be present in gaseous form in the atmosphere of ultra-hot Jupiter WASP-178b.

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 1 – 3 . However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles 4 – 6 . 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 7 – 9 . 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 log ε Na  =  6.9 - 0.4 + 0.6 , and use it as a proxy for the planet’s atmospheric metallicity relative to the solar value ( Z p / Z ʘ  =  2.3 - 1.7 + 8.9 ). This result is consistent with the mass–metallicity trend observed for Solar System planets and exoplanets 10 – 12 . The optical transmission spectrum for the ‘hot Saturn’ exoplanet WASP-96b reveals a clear atmosphere, an atmospheric sodium abundance and hence its metallicity, which is consistent with the metallicity trend observed in Solar System planets and exoplanets.

The temperature profile of a planetary atmosphere is a key diagnostic of radiative and dynamical processes governing the absorption, redistribution and emission of energy. Observations have revealed dayside stratospheres that either cool 1 , 2 or warm 3 , 4 with altitude for a small number of gas giant exoplanets, whereas other dayside stratospheres are consistent with constant temperatures 5 – 7 . Here we report spectroscopic phase curve measurements for the gas giant WASP-121b (ref. 8 ) that constrain stratospheric temperatures throughout the diurnal cycle. Variations measured for a water vapour spectral feature reveal a temperature profile that transitions from warming with altitude on the dayside hemisphere to cooling with altitude on the nightside hemisphere. The data are well explained by models assuming chemical equilibrium, with water molecules thermally dissociating at low pressures on the dayside and recombining on the nightside 9 , 10 . Nightside temperatures are low enough for perovskite (CaTiO 3 ) to condense, which could deplete titanium from the gas phase 11 , 12 and explain recent non-detections at the day–night terminator 13 – 16 . Nightside temperatures are also consistent with the condensation of refractory species such as magnesium, iron and vanadium. Detections 15 – 18 of these metals at the day–night terminator suggest, however, that if they do form nightside clouds, cold trapping does not efficiently remove them from the upper atmosphere. Horizontal winds and vertical mixing could keep these refractory condensates aloft in the upper atmosphere of the nightside hemisphere until they are recirculated to the hotter dayside hemisphere and vaporized. Ultrahot giant planet WASP-121b has a stratosphere that warms up with altitude during the day and cools down with altitude during the night. This trend is in agreement with predictions from circulation models in chemical equilibrium. Efficient vertical mixing hinders condensation of at least some refractory materials.

We report the discovery of a new transiting planet from the WASP survey. WASP-135b is a hot Jupiter with radius 1.30 0.09 RJup, mass 1.90 0.08 MJup, and an orbital period of 1.401 days. Its host is a Sun-like star, with a G5 spectral type and a mass and radius of 0.98 0.06 M☉ and 0.96 0.05 R☉, respectively. The proximity of the planet to its star means that WASP-135b receives high levels of insolation, which may be the cause of its inflated radius. Additionally, we report weak evidence of a transfer of angular momentum from the planet to its host star.

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, timeseries observations required for such investigations have unique technical challenges, and prior experience with Hubble, Spitzer, and other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper, we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. We also describe the simulations used to establish the program. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative data sets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the timeseries modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. The targets have been vetted with previous measurements, will be observable early in the mission, and have exceptional scientific merit. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, timeseries instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

Carbon dioxide (CO 2 ) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO 2 is an indicator of the metal enrichment (that is, elements heavier than helium, also called ‘metallicity’) 1 – 3 , and thus the formation processes of the primary atmospheres of hot gas giants 4 – 6 . It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets 7 – 9 . Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO 2 , but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification 10 – 12 . Here we present the detection of CO 2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme 13 , 14 . The data used in this study span 3.0–5.5 micrometres in wavelength and show a prominent CO 2 absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative–convective–thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO 2 , but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models. Transmission spectroscopy observations from the James Webb Space Telescope show the detection of carbon dioxide in the atmosphere of the gas giant exoplanet WASP-39b.

In traditional transit timing variations (TTVs) analysis of multi-planetary systems, the individual TTVs are first derived from transit fitting and later modelled using n-body dynamic simulations to constrain planetary masses. We show that fitting simultaneously the transit light curves with the system dynamics (photo-dynamical model) increases the precision of the TTV measurements and helps constrain the system architecture. We exemplify the advantages of applying this photo-dynamical model to a multi-planetary system found in K2 data very close to 3:2 mean motion resonance, K2-19. In this case the period of the larger TTV variations (libration period) is much longer (>1.5 years) than the duration of the K2 observations (80 days). However, our method allows to detect the short period TTVs produced by the orbital conjunctions between the planets that in turn permits to uniquely characterise the system. Therefore, our method can be used to constrain the masses of near-resonant systems even when the full libration curve is not observed.

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.sup.1. Searches for helium, however, have hitherto been unsuccessful.sup.2. 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.sup.3 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 [plus or minus] 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.sup.10 to 3 × 10.sup.11 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.