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.

Popular Abstract

Ever since the discovery of the first exoplanet, mankind has dreamed of discovering another planet similar to Earth. These could be the key for the future of our civilisation, or the first contact with extraterrestrial life. More than 5000 planets have been discovered, and this number is expected to soar with future missions. All different kinds of exoplanet have now been discovered: Extremely large, gassy planets like Jupiter, ice giants like Neptune, but also smaller planets, more similar to Earth. These planets can be covered with dense gas, liquid oceans, or even lava. In order to find habitable planets, we need to investigate their surface conditions. These are tightly bound to the planetary atmospheres. From the solar system, we know that atmospheres change over time. It is hypothesised that the now barren red planet Mars used to have an atmosphere. But, presumably due to the loss of its magnetic field, it was unable to retain most of it. On the other hand, despite not having an intrinsic magnetic field, Venus managed to keep a thick layer of atmosphere. How exactly magnetic fields affect the evolution of atmospheres is not fully understood. It is therefore important to investigate how atmospheres lose their mass, and the role of magnetic fields.
It is incredibly difficult to directly measure light from exoplanets, even more so from their atmosphere. Instead, scientists measure the change of the stellar light as a planet orbits across the stellar disc. Of course, this is only possible for planets that transit, i.e. cross the line of sight between observer and star. The imprint of a planetary atmosphere is then detectable in the spectrum of the stellar light. This is commonly known as transmission spectroscopy. Larger planets are more easily observed with this method. And yet, modern telescopes reach unprecedented levels of resolution, which opens the field for characterisation of smaller, terrestrial exoplanets.
In this work, we investigate the planet π Men c. It is roughly double the size of Earth, but orbits around its host star 20 times closer than Earth around the Sun. This is an extreme environment: Life like on Earth would not exist here. Observations of this planet suggest that a large structure surrounds the planet, known as an exosphere. This is the uppermost layer of the atmosphere, and populated by low-density charged particles, also called plasma. This plasma is created due strong stellar irradiation. Although ionised particles are affected by magnetic fields, previous modelling work on this planet does not include them. We use a three-dimensional plasma code that includes various physical effects, including magnetic fields. This allows us to create artificial observations of the exosphere of π Men c. We can then tweak various physical parameters, like the speed of the stellar winds, or the strength of the planetary magnetic field. This causes changes in the spectrum we calculate from our models. Doing this, we try to find the best fit of parameters to reproduce observations.
We find that magnetic fields affect the observed light from atmospheres. This could be a step towards confirming the existence of extraterrestrial magnetic fields. Furthermore, we might even be able put con- straints on the strength of these fields. By simulating different particles, we can make predictions about future observations. All of this will be important for other terrestrial exoplanets. Our simulation and modelling pipeline provides a new approach to investigate Earth-like planets.
We most likely will not be able to travel to other planets within our lifetime. Even more so, the planets we currently investigate are not habitable to humans. However, we help lay the groundwork for future scientists in the search of a second Earth. Without doubt, these planets exist. We just have to find them.

• author: Spitzner, Lucian
• supervisor: Jens Hoeijmakers
• organization: Astrophysics; Department of Physics
• course: ASTM32 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: astronomy, astrophyics, exoplanets, transmission spectroscopy, magnetic fields, atmospheres, ions
• report number: 2023-EXA211
• language: English
• id: 9133262
• date added to LUP: 2023-08-15 08:57:37
• date last changed: 2023-08-30 14:53:39