Simulations of the Tenuous Upper Atmospheres of Exoplanets

Abstract: Over the last decade, the interest in research on extraterrestrial planets has expanded dramatically. With the number of confirmed exoplanets having increased tenfold over the last ten years, we now know that many different types of exoplanets exist. Modern telescopes, both ground- and space-based, like the Very Large Telescope (VLT) or the James Webb Space Telescope (JWST) will drive forward the research on exoplanets discovered by missions like Kepler or TESS. Despite a present day bias towards detection of large Jupiter-like planets, a plethora of smaller, Earth-like planets are now being discovered. Research on those planets is especially interesting in the context of habitability and the search for potential extraterrestrial life. However, for most planets current technology is not precise enough to resolve light directly. Instead, the two main methods indirectly measure the effect of planets through variations in the stellar spectrum. For transiting planets, i.e. planets that orbit into the line of sight between star and observer, their shadow causes a momentary reduction in stellar flux. The reduction in flux is proportional to the ratio of planetary to stellar area. Surface conditions on planets are greatly affected by their atmospheres. A great diversity of atmospheres is known to exist, with different constituents, temperatures, chemistry and morphologies. In transmission spectroscopy, the stellar light filtered through the thin atmospheric annulus surrounding the planet is split into its spectrum to identify signatures from atoms and molecules. These can give insight about the properties of the atmospheres. However, it is not only the dense parts of atmospheres that contribute to this signal. The outer atmospheric layer, called the exosphere, is thinly populated by ionised particles, also called plasma. Many solar system bodies feature an exosphere, including Mars, Venus and Earth. The existence of exospheres has been confirmed for some exoplanets. Exospheres of planets may be very large due to strong incident stellar wind flux. Imprints of exospheric ions may be visible in transmission spectroscopy. In this work, three-dimensional models of the extraterrestrial planet π Men c have been created using the hybrid-kinetic code AMITIS. π Men c is a roughly 2 R⊕ super-Earth in a very close orbit around a Sun-like star. Previous research by García Muñoz et al. (2021) using the Hubble Space Telescope (HST) detects absorption by C II ions in the ultraviolet, with a peak absorption depth of 6 %. According to their models, these particles surround the planet in a large, 15 planetary radii exosphere. Particles are sourced from lower parts of the atmosphere, where they are photoionised and escape into the exosphere. There, interactions with the stellar wind cause them to accelerate, which is visible in the observed transmission spectrum. García Muñoz et al. vary parameters like particle densities and ionisation timescales to match their model to observations. However, the influence of magnetic fields is not included. Hence, our approach extends their research. AMITIS includes physical processes like magnetic fields, electron pressure or stellar wind pressure to compute the time-dependant evolution of a system. Using the assumption that π Men c is similar to Venus in its atmospheric composition, we create different models of the planet. Outputs of this code include densities and velocities of particles in the exosphere. These results are then used in radiative transfer. Here, we calculate the extinction of light through the exosphere. As a result, we obtain synthetic transmission spectra. Similar to García Muñoz et al., we then vary parameters in the plasma models to fit our results to their observations. While staying below the threshold of ion density proposed by García Muñoz et al., we are able to reach transit depths on the same order of magnitude as in the observations: With peak densities of C II around 105.5 cm−3, a maximum transit depth of 2 % is reached for a planet with no intrinsic magnetic field. We find that magnetic fields affect the shape and position of the ion absorption lines. For a non-magnetised planet, the peak of the absorption line is shifted by about 130 km s−1, while a planet with a dipole similar to Earth has peak line depth shifted by 100 km s−1. Most likely, shielding by the magnetosphere decreases entrainment and acceleration of planetary ions in the stellar winds. As a result, the position of the absorption line peak relative to its intrinsic centre may hold information about the magnetic field of the exoplanet. The properties of the stellar wind also significantly affect the observed transmission spectrum. We find that a change in angle of the stellar magnetic field also changes the absorption depth by about 25 %. For close orbit planets, the orientation of the stellar magnetic field can therefore not be ignored. Variations in stellar wind intensity are expected to change the line profile as well. Over multiple observations, changes in the absorption line could be used to reveal variations of stellar winds for other stars.