This paper presents a detailed hydrostatic model of the upper atmosphere of HD 189733b, with the goal of constraining its temperature, particle densities, and radiation field over the pressure range 10 −4 −10 µbar, where the observed Hα transmission spectrum is produced. The atomic hydrogen level population is computed including both collisional and radiative transition rates. The Lyα resonant scattering is computed using a Monte-Carlo simulation. The model transmission spectra are in broad agreement with the data. Excitation of the H(2 ) population is mainly by Lyα radiative excitation due to the large Lyα intensity. The density of H(2 ) is nearly flat over two decades in pressure, and is optically thick to Hα. Additional models computed for a range of the stellar Lyman continuum (LyC) flux suggest that the variability in Hα transit depth may be due to the variability in the stellar LyC. Since metal lines provide the dominant cooling of this part of the atmosphere, the atmosphere structure is sensitive to the density of species such as Mg and Na which may themselves be constrained by observations. Since the Hα and Na D lines have comparable absorption depths, we argue that the center of the Na D lines are also formed in the atomic layer where the Hα line is formed.

Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the Solar System, Io, we describe how sodium (Na I) and potassium (K I) gas could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼ a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal Q p 10 11 . This tidal energy is focused into the satellite driving a ∼ 10 5±2 higher mass loss rate than Io's supply to Jupiter's Na exosphere, based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are on average more than sufficient to provide the ∼ 10 10±1 Na cm −2 required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended (∼ 1000 R J ) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP 49-b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.

Ultra-hot Jupiters (UHJs) are highly irradiated giant exoplanets with extremely high day-side temperatures, which lead to thermal dissociation of most of the molecular species. It is expected that the neutral hydrogen atom is one of the main species in the upper atmospheres of ultra-hot Jupiters. Neutral hydrogen has been detected in several UHJs by observing its Balmer line absorption. Here, we report four transit observations of the ultra-hot Jupiter WASP-33b, performed with the CARMENES and HARPS-North spectrographs, and the detection of the Hα, Hβ, and Hγ lines in the planetary transmission spectrum. The combined Hα transmission spectrum of the four transits has an absorption depth of 0.99±0.05 %, which corresponds to an effective radius of 1.31±0.01 R p. The strong Hα absorption indicates that the line probes the high-altitude thermosphere. We further fitted the three Balmer lines using the PAWN model, assuming that the atmosphere is hydrodynamic and in LTE. We retrieved a thermosphere temperature 12200 +1300 −1000 K and a mass-loss rateṀ = 10 11.8 +0.6 −0.5 g s −1. The retrieved large mass-loss rate is compatible with the "Balmer-driven" atmospheric escape scenario, in which the stellar Balmer continua radiation in the near-ultraviolet is substantially absorbed by the excited hydrogen atoms in the planetary thermosphere.

Context. Several observational and theoretical results indicate that the atmospheric temperature of the ultra-hot Jupiter KELT-9b in the main line formation region is a few thousand degrees higher than predicted by self-consistent models. Aims. Our aim was to test whether non-local thermodynamic equilibrium (NLTE) effects are responsible for the presumably higher temperature. Methods. We employed the Cloudy NLTE radiative transfer code to self-consistently compute the upper atmospheric temperature-pressure (TP) profile of KELT-9b, assuming solar metallicity and accounting for Roche potential. In the lower atmosphere, we used an updated version of the HELIOS radiative-convective equilibrium code to constrain the Cloudy model. Results. The Cloudy NLTE TP profile is ≈2000 K hotter than that obtained with previous models assuming LTE. In particular, in the 1–10−7 bar range the temperature increases from ≈4000 to ≈8500 K, remaining roughly constant at lower pressures. We find that the high temperature in the upper atmosphere of KELT-9b is driven principally by NLTE effects modifying the Fe and Mg level populations, which strongly influence the atmospheric thermal balance. We employed Cloudy to compute LTE and NLTE synthetic transmission spectra on the basis of the TP profiles computed in LTE and NLTE, respectively, finding that the NLTE model generally produces stronger absorption lines, particularly in the ultraviolet, than the LTE model (up to 30%). We compared the NLTE synthetic transmission spectrum with the observed Hα and Hβ line profiles obtaining an excellent match, thus supporting our results. Conclusions. The NLTE synthetic transmission spectrum can be used to guide future observations aiming at detecting features in the KELT-9b transmission spectrum. Metals, such as Mg and Fe, and NLTE effects shape the upper atmospheric temperature structure of KELT-9b, and thus affect the mass-loss rates derived from it. Finally, our results call for checking whether this is the case also for cooler planets.