Revealing the atmospheres of highly irradiated exoplanets: from ultra-hot Jupiters to rocky worlds

Astrophysics and Space Science (2023) 368:24 https://doi.org/10.1007/s10509-023-04183-5 RESEARCH Revealing the atmospheres of highly irradiated exoplanets: from ultra-hot Jupiters to rocky worlds Megan Mansfield1 Received: 29 November 2022 / Accepted: 18 March 2023 © The Author(s) 2023 Abstract Spectroscopy of transiting exoplanets has revealed a wealth of information about their atmospheric compositions and thermal structures. In particular, studies of highly irradiated exoplanets at temperatures much higher than those found in our solar system have provided detailed information on planetary chemistry and physics because of the high level of precision which can be obtained from such observations. Here we use a variety of techniques to study the atmospheres of highly irradiated transiting exoplanets and address three large, open questions in exoplanet atmosphere spectroscopy. First, we use secondary eclipse and phase curve observations to investigate the thermal structures and heat redistribution of ultra-hot Jupiters, the hottest known exoplanets. We demonstrate how these planets form an unique class of objects influenced by high-temperature chemical effects such as molecular dissociation and H− opacity. Second, we use observations of helium in the upper atmosphere of the exo-Neptune HAT-P-11b to probe atmospheric escape processes. Third, we develop tools to interpret JWST observations of highly irradiated exoplanets, including a data analysis pipeline to perform eclipse mapping of hot Jupiters and a method to infer albedos of and detect atmospheres on hot, terrestrial planets. Finally, we discuss remaining open questions in the field of highly irradiated exoplanets and opportunities to advance our understanding of these unique bodies in the coming years. Keywords Planets and satellites: atmospheres · Planets and satellites: gaseous planets · Planets and satellites: terrestrial planets 1 Introduction The main goals of exoplanet atmosphere spectroscopy are to determine exoplanets’ compositions and thermal structures in order to further our understanding of planetary formation, physics, and chemistry. The study of extrasolar planets offers an opportunity to investigate planetary origins and climate in a broader context and across a much wider population of planet types than studies of our solar system. In particular, spectroscopic observations of transiting planets can reveal information on their atmospheric compositions and thermal structures. In this review, we present observations of highly irradiated exoplanets aimed at addressing three large, open questions in exoplanet atmosphere spectroscopy. First, M. Mansfield: NHFP Sagan Fellow.  M. Mansfield 1 Steward Observatory, University of Arizona, Tucson, 85715, AZ, USA what are the primary processes impacting the thermal structures of ultra-hot Jupiters, gas giant planets with equilibrium temperatures above 2000 K, and how do those processes affect their observed emission spectra and phase curves? Second, how does atmospheric escape sculpt the population of hot exoplanets? And third, how can we use the new capabilities of JWST to further advance our understanding of highly irradiated exoplanets? In Sect. 2 we present a series of observations which reveal the thermal structures and heat transport of ultra-hot Jupiters. We present Hubble Space Telescope (HST) emission observations of the ultra-hot Jupiter HAT-P-7b, which along with other studies led to the realization that ultrahot Jupiter spectra are impacted by molecular dissociation (Mansfield et al. 2018; Arcangeli et al. 2018; Kreidberg et al. 2018; Parmentier et al. 2018). We next present a Spitzer Space Telescope phase curve of KELT-9b, the hottest known exoplanet, which shows enhanced energy transport due to dissociation (Mansfield et al. 2020). We then expand to a broad study of high-temperature chemistry through a popu- 24 Page 2 of 12 M. Mansfield Fig. 1 (a) Secondary eclipse spectrum of HAT-P-7b compared to a suite of theoretical models. Black points with 1σ error bars represent observations with HST/WFC3 (Mansfield et al. 2018) and Spitzer (Wong et al. 2016). The inset zooms in on the WFC3 data. The dark blue line represents the best-fitting 1D self-consistent model (Arcangeli et al. 2018; Mansfield et al. 2021), and the surrounding red lines show 500 spectra randomly drawn from the posterior. Blue points outlined in black show the best-fitting 1D model binned to the data resolution. The black line shows a best-fit blackbody. The orange line shows the expected emission spectrum for a model with a monotonically decreasing T-P profile, calculated using the methods of Fortney et al. (2008). (b) Corresponding T-P profiles for each model, with the red shaded area showing 1σ error bars on the best-fit model. The data are consistent with a blackbody and strongly reject the monotonically decreasing model. Figure from Mansfield et al. (2018) lation study of all HST hot Jupiter emission spectra (Mansfield et al. 2021, 2022). In Sect. 3, we present a discovery of helium escape in the HST transmission spectrum of the exo-Neptune HAT-P-11b (Mansfield et al. 2018). In Sect. 4, we present modeling tools to interpret future JWST observations of highly irradiated exoplanets. First, we present a data analysis pipeline that can be used to interpret JWST eclipse mapping observations of hot Jupiters, which will produce 3D maps of these planets’ daysides (Mansfield et al. 2020). We then present a model of inferred albedos for hot, terrestrial planets which can be used to determine whether such planets have atmospheres, a prerequisite for habitability (Mansfield et al. 2019). Finally, we conclude in Sect. 5. to investigate these predictions, we observed a secondary eclipse of the hot Jupiter HAT-P-7b, which has a dayside temperature of ≈ 2600 K, with the HST Wide Field Camera 3 (WFC3) instrument between 1.1 − 1.7 µm (Mansfield et al. 2018). The secondary eclipse spectrum is shown in Fig. 1 compared to several models. We found that the spectrum is blackbody-like and clearly rejects a monotonically decreasing T-P profile. We compared the data to both 3D general circulation models (GCMs) and 1D self-consistent thermochemical equilibrium models (Arcangeli et al. 2018; Mansfield et al. 2021). Both models preferred an inverted T-P profile, but in both cases the spectrum did not show the previously predicted water emission features because of water dissociation. In both models, the upper atmosphere was heated to high enough temperatures that water began to dissociate. Water dissociation becomes important to shaping ultrahot Jupiter thermal emission spectra at temperatures above ≈ 2200 K (Parmentier et al. 2018; Mansfield et al. 2021), which from Fig. 1 is below the dayside temperature of HATP-7b. The dissociation limited the range of pressures which could be probed in the WFC3 bandpass, which is primarily sensitive to water opacity. Therefore, the observations were rest (...truncated)