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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.

The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed. Together with radial velocity measurements, this has led to a determination of the planet's radius and mass, confirming it to be a gas giant. But despite numerous searches for an atmospheric signature, only the dense lower atmosphere of HD209458b has been observed, through the detection of neutral sodium absorption. Here we report the detection of atomic hydrogen absorption in the stellar Lyman alpha line during three transits of HD209458b. An absorption of 15 +/- 4% (1sigma) is observed. Comparison with models shows that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.

Statistical analysis of over a thousand spectra of the star β Pictoris reveals that it has two kinds of exocomets circling it: old exhausted comets trapped in mean-motion resonance with a massive planet, and fragments of comets. Detection of β Pictoris exocomets The nearby star β Pictoris possesses a young planetary system that appears much like our own would have been few million years after its formation. This analysis of more a thousand archival spectra recorded between 2003 and 2011 reveals variable dust absorption signatures arising from transiting exocomets belonging to two distinct families of comets. First, an old volatile-exhausted population displaying signs of orbital evolution due to interactions with the host planet and second, a volatile-rich population presumably originating from the break-up of a few parent bodies. The young planetary system surrounding the star β Pictoris harbours active minor bodies 1 , 2 , 3 , 4 , 5 , 6 . These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System 7 . Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies 8 , 9 , 10 , 11 , that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.

Arising from: M. Holmström et al. Nature451, 970–972 (2008)10.1038/nature06600 ; Holmström et al. reply Using numerical simulation, Holmström et al. 1 proposed a plausible alternative explanation of the observed Lyman-α absorption that was seen during the transit of HD 209458b (ref. 2 ). They conclude that radiation pressure alone cannot explain the observations and that a peculiar stellar wind is needed. Here we show that radiation pressure alone can in fact produce the observed high-velocity hydrogen atoms. We also emphasize that even if the stellar wind is responsible for the observed hydrogen, to have a sufficient number of atoms for charge exchange with stellar wind, the energetic neutral atom (ENA) model also needs a significant escape from the planet atmosphere of similar amplitude as quoted in ref. 2 .

The star Beta Pic is widely known for harbouring a large population of exocomets, which create variable absorption signatures in the stellar spectrum as they transit the star. While the physical and chemical properties of these objects have long remained elusive, Vrignaud et al. (2024) recently introduced the exocomet curve of growth approach, enabling, for the first time, the estimate of exocometary column densities and excitation temperature using absorption measurements in many spectral lines. Using this new tool, we present a refined study of a Beta Pic exocomet observed on December 6, 1997 with the HST. We first show that the comet's signature in FeII lines is well explained by the transit of two gaseous components, with different covering factors and opacities. Then, we show that the studied comet is detected in the lines of other species, such as NiII and CrII. These species are shown to experience similar physical conditions than FeII (same radial velocity profile, same excitation temperature), hinting that they are well-mixed. Finally, using almost 100 FeII lines rising from energy levels between 0 and 33000 cm-1, we derive the complete excitation diagram of Fe+ in the comet. The transiting gas is found to be populated at an excitation temperature of 8190+-160 K, very close to the stellar effective temperature (8052 K). Using a model of radiative and collisional excitation, we show that the observed excitation diagram is compatible with a radiative regime, associated with a close transit distance (< 0.43 au) and a low electronic density (< 1e7 cm-3). In this regime, the excitation of Fe+ is controlled by the stellar flux, and do not depend on the local electronic temperature or density. These results allow us to derive the Ni+/Fe+ and Cr+/Fe+ ratios in the December 6, 1997 comet, at 8.5 +- 0.8 x 10-2 and 1.04 +- 0.15 x 10-2 respectively, close to solar abundances.

Planets with radii between that of the Earth and Neptune (hereafter referred to as ‘sub-Neptunes’) are found in close-in orbits around more than half of all Sun-like stars 1 , 2 . However, their composition, formation and evolution remain poorly understood 3 . The study of multiplanetary systems offers an opportunity to investigate the outcomes of planet formation and evolution while controlling for initial conditions and environment. Those in resonance (with their orbital periods related by a ratio of small integers) are particularly valuable because they imply a system architecture practically unchanged since its birth. Here we present the observations of six transiting planets around the bright nearby star HD 110067. We find that the planets follow a chain of resonant orbits. A dynamical study of the innermost planet triplet allowed the prediction and later confirmation of the orbits of the rest of the planets in the system. The six planets are found to be sub-Neptunes with radii ranging from 1.94 R ⊕ to 2.85 R ⊕ . Three of the planets have measured masses, yielding low bulk densities that suggest the presence of large hydrogen-dominated atmospheres. Observations of six transiting planets around the bright nearby star HD 110067 show that they follow a chain of resonant orbits, with three of the planets inferring the presence of large hydrogen-dominated atmospheres.

Molecular hydrogen (H2) is by far the most abundant material from which stars, protoplanetary disks and giant planets form, but it is difficult to detect directly. Infrared emission lines from H2 have recently been reported towards beta Pictoris, a star harbouring a young planetary system. This star is surrounded by a dusty 'debris disk' that is continuously replenished either by collisions between asteroidal objects or by evaporation of ices on Chiron-like objects. A gaseous disk has also been inferred from absorption lines in the stellar spectrum. Here we present the far-ultraviolet spectrum of beta Pictoris, in which H2 absorption lines are not seen. This allows us to set a very low upper limit on the column density of H2: N(H2) 6 x 10-4. As CO would be destroyed under ambient conditions in about 200 years (refs 9, 11), our result demonstrates that the CO in the disk arises from evaporation of planetesimals.

Beta Pic is a young, A5V star, known for harbouring a large number of exocomets, which frequently transit the star and produce absorption signatures. The physical and chemical properties of these exocomets can be probed by the recently introduced curve of growth approach, which enables column densities measurements in exocomets using observations in numerous spectral lines. Using this approach, we present a new study of archival spectra of Beta Pic obtained with the HST, the HARPS spectrograph, and at the Mont John University Observatory, aimed at constraining the abundance of refractory ions in Beta Pic exocomets. 29 individual objects are studied, all observed in FeII lines (used as a reference ion) and at least one other species (Ni II, Ca II, Cr II...). We find that the refractory composition of exocomets is overall stable, especially for singly ionised species, and consistent with solar abundances. This validates the use of the curve of growth approach to study exocometary composition. We also show that some ions, such as Ca II, are significantly depleted compared to solar abundances, allowing us to constrain the ionisation state in Beta Pic exocomets. We find that most refractory elements (Mg, Ni, Fe...) are split in similar fractions between their first and second ionisation states, with the exception of Ca, mostly ionized twice. A strong correlation between the Al III/Fe II ratio and radial velocity is also found, showing that the most redshifted exocomets tend to be more ionised. These results open the way for further modelling, in order to better understand the physical processes that influence the composition and shape of exocometary tails.

Transiting planets like HD209458b offer a unique opportunity to scrutinize their atmospheric composition and structure. Transit spectroscopy probes the transition region between the day and night sides, called the limb. We present a re-analysis of existing HST/STIS transmission spectra of HD209458b's atmosphere. From these observations we identify H2 Rayleigh scattering, derive the absolute Sodium abundance and quantify its depletion in the upper atmosphere, extract a stratospheric T-P profile and find a temperature inversion and explain broad band absorptions with the presence of TiO and VO molecules.

Exospheric atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. We present observations of this transiting planet's extended exosphere with the Advanced Camera for Surveys on board the Hubble Space Telescope. From the two transit light curves obtained at Lyman α, we find an in-transit absorption of (8.0±5.7)%, in good agreement with previous studies. These new constraints on the size of the exosphere strengthens the evaporation scenario. Full details are provided in Ehrenreich et al. (2008).