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We simulate with a global 3D aeronomy code two warm Neptunes in the TOI-421 system and show that both planets experience significant escape of their upper atmospheres. The double shock structures, generated around the planets in course of their interaction with the stellar wind (SW) plasma flow are revealed. The calculations of stellar Ly α transit absorption by the planets reveal that it reaches a detectable level only for a moderate or strong SW, with a sufficiently high density. In this case, the energetic neutral atoms provide significant absorption at the high velocity blue wing of the Ly α line, whereas the corresponding transit light curves exhibit an early ingress and extended egress features. With the same code, we also modelled the absorption at the position of the 10 830 Å line of the metastable helium, showing that it can be detected only for the farthest planet of the considered two, if the helium abundance is comparable to the solar value. [ABSTRACT FROM AUTHOR]

Transmission spectroscopy of WASP-107b revealed 7–8 per cent absorption at the position of metastable He  i triplet at 10 830 Å in the velocity range of [−20; 10] km s−1. This absorption is stronger than that measured in other exoplanets. With a dedicated 3D self-consistent hydrodynamic multifluid model, we simulated the expanding upper atmosphere of WASP-107b and reproduced within the observations accuracy the measured He  i absorption profiles, constraining the stellar X-ray and UV radiation (XUV) flux to 6–10 erg cm−2 s−1 at 1 au, and the upper atmosphere helium abundance He/H to 0.075–0.15. The radiation pressure acting on the metastable He  i atoms was shown to be an important factor influencing the absorption profiles. Its effect is counterbalanced by the collisional depopulation of the He  i metastable state. Altogether, the observed He  i absorption in WASP-107b can be interpreted with the expected reasonable parameters of the stellar–planetary system and appropriate account of the electron and atom impact processes. [ABSTRACT FROM AUTHOR]

Warm Neptune GJ3470b has been recently observed in 23S-23P transition of metastable helium, yielding absorption of about 1 per cent in Doppler velocity range of [−40; 10] km s−1. Along with previous detection of absorption in Ly α with depth of 20–40 per cent in the blue and red wings of the line, it offers a complex target for simulation and testing of the current models. Obtained results suggest that absorption in both these lines comes from interaction of expanding upper planetary atmosphere with stellar plasma wind, allowing to constrain the stellar plasma parameters and the helium abundance in planet atmosphere. [ABSTRACT FROM AUTHOR]

We present a 3D fully self-consistent multifluid hydrodynamic aeronomy model to study the structure of a hydrogen-dominated expanding upper atmosphere around the hot Jupiter HD 209458b and the warm Neptune GJ 436b. In comparison to previous studies with 1D and 2D models, this work finds such 3D features as zonal flows in upper atmosphere reaching up to 1 km s−1, the tilting of the planetary outflow by Coriolis force by up to 45 degrees and its compression around equatorial plane by tidal forces. We also investigated in detail the influence of helium (He) on the structure of the thermosphere. It is found that by decrease of the barometric scale height, the He presence in the atmosphere strongly affects the H2dissociation front and the temperature maximum. [ABSTRACT FROM AUTHOR]