C.3: Planetary spectra in time
One of the most intriguing aspects of the recent observational progress on extrasolar planets are atmospheric spectra. In several cases, they reveal clear spectral features of key species like H2O, CH4, or CO, i.e., features that can help to quantitatively probe extrasolar atmospheres. This provides a window into the composition of extrasolar planets, which in turn may give critical insights into a planet’s formation and evolution history, as it is the case in the Solar System.
A planet’s atmospheric composition depends on a multitude of factors: the composition of the host star, the structure and chemistry of the protoplanetary disc, the locations where the planet accreted, the composition of the accreted gas and solids, and the long-term compositional interior and atmospheric evolution of the planet. The latter includes effects of mixing and separation of species together with the exchange between core, interior, and atmosphere. The atmospheric composition evolves through effects of (non-)equilibrium chemistry, photochemistry, settling, circulation, atmospheric escape and finally also biologic activity. Consequently, each formation and evolution track will leave a – potentially complex – imprint in the observable atmospheric composition. The perspective of a new powerful window just opening makes it clear that a combination of planet formation models with atmospheric models is the logical next step to go.
A large number of studies have tried to connect atmospheric composition and spectra to a planet’s formation and evolution history and therefore to the chemistry in the protoplanetary disc in which the planet was formed. This can also teach us about the history of our own solar system. Many of these works have focused on close-in planets for which transit spectroscopy is possible. Analyses of the atmospheric composition of directly imaged planets have also inferred C and O abundances for planets on much larger orbits. On the instrumental side, we are now approaching a turning point in the field of exoplanetary atmosphere observations. For space missions there is the James Webb Space Telescope and later the European Space Agency (ESA) ARIEL satellite. Regarding ground-based instrumentation, there will be instruments from the European Southern Observatory (ESO) like ANDES and RISTRETTO, but also SPHERE+, METIS, and later PCS. All these instruments have the spectroscopic characterisation of exoplanets as a key science case. The expected data from these new instruments provides a strong motivation to study planetary atmospheres as messengers of planet formation. In this project, in which we connect our planet formation-evolution models with state-of-the-art atmospheric models, we want to create the bases for the vision that eventually, formation and evolution models should predict the observable spectral fingerprint for every planet from the moment it becomes observable in the protoplanetary disc to an age of many gigayears. Our vision, as summarised in the four work packages (WP) presented below, is to create a gallery of planetary spectra in time, for comparison with observational spectroscopic constraints. To keep this approach practicable, we will first address atmospheres that are still more connected to formation (H/He-rich but also H2O-rich, i.e., giant planets to (sub)-Neptunes). Observationally, these are the planets that will be spectroscopically characterised first. Over longer timescales, we will expand to secondary atmospheres like for example super Earths and terrestrial planets, paralleling the expected future observational progress.
Work Package 1: Atmospheric structures from detailed non-grey atmospheric models.
Currently our evolution model uses as outer boundary conditions an analytical semi-grey model. The most important parameter of this model, the ratio of the visible opacity to the thermal opacity was, however, calibrated only for a specific surface gravity and a solar-composition atmosphere. We found that this can result in significant differences compared to fully non-grey calculation. In turn this strongly affects the predicted mass-radius relation and the temporal evolution of the planets. To solve this problem we had to replace the semi-grey approach for hot Jupiters by the coupling to fully non-grey atmospheric models. We will generalise this more realistic approach to lower mass planets, planets at larger orbital distances and to different atmospheric compositions.
Work Package 2: Extension to accreting planets.
The atmospheric models in WP1 apply to non-accreting planets. Spectroscopic observations of actively accreting planets yield additional key constraints for formation models like the gas accretion rate, the accretion geometry, the structure of the accretion shock, or the planet mass. A fascinating first example for this are the planets around PDS 70, for which observations have revealed the photospheric spectrum in the near-infrared, the resolved Hα line in the visual from the accretion shock, and the circumplanetary disc in the K-band and submillimetre. Here, we will calculate atmospheric structures and spectra for all components, coupling them self-consistently with our formation model. The results will be first compared to the observations of PDS 70bc as the benchmark case. Then, these calculations will be generalised and included as an output of our population synthesis models. This anticipates the expected discovery of additional forming planets.
Work Package 3: Gallery of planetary spectra in time and comparison with observations.
By linking WP1 and WP2 with our formation-evolution model, we will follow the thermodynamic and compositional evolution of planets from their origins to the present day. At each moment during a planet’s formation and evolution we will be able to calculate its atmospheric spectrum. The predicted spectra will then be compared with observed spectra of individual planets obtained by all the aforementioned instruments, and associated results like retrieval analyses. We will also use these calculations to study how atmospheric compositions and spectra statistically and systematically depend on planetary properties and associated formation pathways. The calculated spectra will be included in the DACE database. Finally, in the long term and beyond the NCCR PlanetS timeframe, these models will be used to compute the posterior distribution of planetary formation tracks given spectrally resolved planetary observations. At that time, we will also extend the framework to lower mass planets for which (geophysical) mechanism like outgassing, geochemical cycles but also atmospheric escape are crucial. X-rays and EUV-driven atmospheric escape was already included in our evolution models for H/He-dominated atmospheres and is currently being extended to water-dominated atmospheres.
Work Package 4: Later changes of planetary atmospheres by irradiation.
Whereas the focus of WP1 – WP3 is on the link of formation and planetary atmospheres, WP4 addresses possible recent and ongoing changes of the atmospheres that might also affect the spectra and therefore bias the interpretation, like in particular effects caused by stellar and/or galactic cosmic rays. Ionisation is especially relevant for the troposphere and the stratosphere, which are both well shielded from the EUV. The ionisation finally affects chemical reaction rates. Considering the possible effects on atmospheric chemistry, the processes need to be understood and quantified before spectra can be interpreted either with respect to planet formation and atmospheric evolution (WP1 – WP3) and in particular before biosignatures can properly and reliably be interpreted. In this WP we will use our versatile particle production and transport model which includes magnetic fields of various configurations and atmospheres of various compositions.
