Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b

Abstract

Context. Ultra-hot Jupiters are the hottest exoplanets that have been discovered so far. They present a unique possibility to explore hot and cold chemistry on one object. The tidally locked ultra-hot Jupiter HAT-P-7b has a day-to-night temperature difference of ≃2500 K, confining cloud formation to the nightside and efficient ionisation to the dayside. Both have distinct observational signatures. Aims. We analyse plasma and magnetic processes in the atmosphere of the ultra-hot Jupiter HAT-P-7b to investigate the formation of an ionosphere and the possibility of magnetically coupling the atmospheric gas as the base for an extended exosphere. We show which ions and atoms may be used as spectral tracers, and if and where conditions for lightning may occur within the clouds of HAT-P-7b. Methods. We used 3D modelling results as input for a kinetic cloud formation code and evaluated characteristic plasma and magnetic coupling parameters. A local thermodynamical equilibrium radiative transfer was solved for the ionised gas phase. This study is confined to thermal ionisation only. Results. The ionisation throughout HAT-P-7b’s atmosphere varies drastically between day- and nightside. The dayside has high levels of thermal ionisation and long-range electromagnetic interactions dominate over kinetic electron–neutral interactions, suggesting a day–night difference in magnetic coupling. K+, Na+, Li+, Ca+, and Al+ are more abundant than their atomic counterparts on the dayside. The minimum magnetic flux density for electrons for magnetic coupling is B < 0.5 G for all regions of HAT-P-7b’s atmosphere. Conclusions. HAT-P-7b’s dayside has an asymmetric ionosphere that extends deep into the atmosphere, the nightside has no thermally driven ionosphere. A corresponding asymmetry is imprinted in the ion and neutral composition at the terminators. The ionosphere on HAT-P-7b may be directly traced by the Ca+ H&K lines if the local temperature is ≥5000 K. The whole atmosphere may couple to a global, large-scale magnetic field, and lightning may occur on the nightside.

Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b

“…For example, due to the large temperature contrasts expected between the dayside and nightside hemispheres, refractory species could condense on the nightside and settle to deeper layers of the atmosphere, despite dayside temperatures being high enough to maintain them in the gas phase. However, day-night cold trapping of this kind might be avoided if vertical mixing is vigorous within the atmosphere, allowing condensates to be suspended aloft long enough for lateral winds to return them to the dayside hemisphere 26,27 . Alternatively, condensates may gravitationally settle to deeper layers of the atmosphere and subsequently re-enter the gas phase as they are returned to lower pressures by updrafts 12 .…”

Aerosols in Exoplanet Atmospheres

“…Alternatively, parameterized cloud distributions are prescribed onto the 3D grid of the GCM based on observations (M. Roman & Rauscher, 2017) or as 1D columns in which cloud formation is evaluated based on whether condensate vapor is locally supersaturated without advection of the clouds (Harada et al., 2019; Parmentier et al., 2016, 2018, 2021; M. Roman & Rauscher, 2019; M. T. Roman et al., 2020; Tan & Showman, 2017, 2020). More complex 1D cloud models, like DRIFT (Helling et al., 2016, 2019a, 2019b, 2020; G. Lee et al., 2015) and the Ackerman and Marley (2001) model (Lines et al., 2019) have also been incorporated into GCMs in this fashion. Both radiatively active and post‐processed clouds (i.e.…”

Diurnal variations in the stratosphere of an ultrahot exoplanet

“…However, day-night cold trapping of this kind might be avoided if vertical mixing is vigorous within the atmosphere, allowing condensates to be suspended aloft long enough for lateral winds to return them to the dayside hemisphere. 30,31 Alternatively, condensates may gravitationally settle to deeper layers of the atmosphere and subsequently re-enter the gas phase as they are returned to lower pressures by updrafts. 13 Condensation curves for relevant refractory species [31][32][33] are shown in Fig.…”

“…30,31 Alternatively, condensates may gravitationally settle to deeper layers of the atmosphere and subsequently re-enter the gas phase as they are returned to lower pressures by updrafts. 13 Condensation curves for relevant refractory species [31][32][33] are shown in Fig. 3a, namely, corundum (Al2O3), perovskite (CaTiO3), VO, Fe, forsterite (Mg2SiO4), and enstatite (MgSiO3).…”

“…34 However, non-detections of Ti and TiO at the day-night terminator of WASP-121b complicate this picture, [14][15][16][17] as these gases should also form condensates such as perovskite and TiO2 on the nightside. [31][32][33] It would be surprising if Ti-bearing condensates are efficiently cold trapped while other refractory species avoid a similar fate. This is especially true for V, which is chemically similar to Ti but an order of magnitude less abundant in the solar neighbourhood.…”