Spectroscopy of exoplanets at high resolution

The characterisation of a diverse set of exoplanet atmosphere observations, ranging from hot gas giants to small temperate rocky worlds, will be one of the legacies of exoplanet science in the coming decade. Our understanding and interpretation of such observations will hinge on our ability to link observations with atmospheric theoretical studies that critically rely on fundamental molecular and atomic opacities. Computing such opacities is a highly inaccessible process which requires several terabytes of available disk space, hours of CPU time per pressure-temperature combination, and requires users to carefully aggregate line lists data from various sources. Ultimately, this limits access to data and model intercomparison studies within the exoplanet community. To interpret next generation exoplanet observations successfully and robustly, the community needs access to validated and up-to-date opacities. This was the motivation for the MAESTRO (Molecules and Atoms in Exoplanet Science: Tools and Resources for Opacities) database, a new NASA-supported opacity service that can be accessed by the community via a web interface. Professor Lewis will present MAESTRO and discuss the outcome of a multi-year collaboration to develop community standards in computing opacities. This is timely because subtle choices in opacities lead to model inconsistencies in the analysis of both high-resolution (R>10,000) and low resolution (JWST-like) observations. The MAESTRO team reflects a diversity in expertise: the maintainers of ExoMol, HITRAN/HITEMP, and various exoplanet and stellar atmosphere models. With its recent release, MAESTRO will prove to be an invaluable community resource in the era of JWST and beyond. 

In recent years ground-based high-resolution spectroscopy has revolutionised how we detect and constrain constituent chemical species in the atmospheres of exoplanets. In the last five years alone a range of new spectrographs have come online, capable of higher sensitivity and spectral coverage. These facilities allow for very high spectral resolution observations (R>20,000) to detect many thousands of individual line features from spectrally active species in the atmosphere. A wide array of molecular, atomic and ionic species have been clearly observed in numerous transiting exoplanets, ranging from the optical to infrared, most notably for ultra-hot Jupiters with temperatures in excess of 2000K. From recent work on retrievals we have also been able to determine chemical abundances and explore a wide range of parameter space through robust statistical frameworks. These frameworks are essential for accurate characterisation, determining formation and migration of exoplanets as well as contextualising our own Solar System. In addition, with the spectral power and sensitivity of high-resolution spectroscopy we have been able to explore atmospheric dynamics in exoplanets for the first time. This due to atmospheric winds which result in Doppler shifts and broadening of signals of exoplanetary spectra. Dr Gandhi will discuss the latest developments in theoretical modelling of exoplanets at high resolution as well as the constraints and trends that we observe in the chemistry and dynamics from observations. They will also discuss the next generation of large ground-based facilities, which will enable us to explore the atmospheres of much cooler, rocky planets for the first time when they come online at the end of the decade.

Studies of exoplanets have accelerated in recent years thanks to improved observation and analysis techniques. Exoplanet atmospheres have been shown to be diverse, not just in temperature, but also composition, and the latest analyses from JWST spectra are already revealing tantalising discoveries. Characterising these atmospheres is dependent on the unique molecular signatures that are present in their spectra. Therefore it is vital that the spectroscopic parameters used for interpreting atmospheric models are accurate and appropriate for these new worlds. The HITRAN and HITEMP databases provide line-by-line parameters for 55 molecules, experimental cross-sections for over 300 molecules for which no reliable quantum mechanical models exist, as well as collisional induced absorption data. This presentation will highlight the state-of-the-art data available in HITRAN and HITEMP with a particular focus on recent line list improvements and the addition of line-broadening parameters for planetary atmospheres (such as those dominated by H₂/He, or CO₂). This presentation will also contemplate the limitations of current spectroscopic data for the interpretation of planetary spectra.

The use of high resolution spectroscopy using cross correlation techniques to study the spectra of molecules in the atmospheres of exoplanets places significant demands on the accuracy of the laboratory data used to analyse these spectra. The ExoMol project provides molecular line lists for exoplanetary and other atmospheres which are computed using first principles quantum mechanical methods informed by available experimental data. The original ExoMol line lists focused heavily on providing completeness at higher temperatures allowing opacities to be generated and the construction of appropriate spectral models for analysing transit spectra. The much higher resolution needed for cross correlation spectroscopy means that these line lists need to be refactored. To do this we use available laboratory measurements which are used to provide high accuracy energy levels which in turn are used generate key portions of the molecular spectrum with required accuracy. The talk will describe the MARVEL (measured active rotation vibration energy levels) procedure used for this process and the development of appropriate high accuracy models for challenging molecules such as VO, as well as how this method can be used to provide accurate spectra of isotopically substituted molecules.

Exoplanet atmospheres are thought to host a rich variety of molecular species, and so having appropriate line list data (molecular energy levels and transitions) and opacities (as a function of pressure and temperature) is important for proper atmospheric characterisation. The requirements for molecular data differ depending on whether they are to be used for low- or high-resolution studies. Dr Chubb will talk about methods which use laboratory data to bring theoretically computed line lists towards spectroscopic accuracy, in order that they can be used in high-resolution cross-correlation studies. They have led such projects as part of the ORBYTS (Original Research For Young Twinkle Students) initiative, where groups of 16–18 year olds are involved in conducting and publishing real research. The ExoMolOP database, on the other hand, is primarily for retrieval codes which make use of low-resolution spectra, requiring the line lists to be complete up to high-temperatures (typically 1000-2000K). Dr Chubb will talk a bit about these data and how they have been used to search for molecules in exoplanet atmospheres. They will also talk a little on how high-resolution molecular data can be used in the modelling of polarised reflection spectra to determine cloud properties in exoplanet atmospheres.

Ultra-hot Jupiters tend to orbit hot early type stars in short periods and are heated to extreme temperatures far over 2,000 K on their day-sides. All but the most strongly bound molecules are dissociated and many atoms may be significantly thermally ionised. The dominant sources of line opacity are due to metals and some molecules including metal oxides. Much of these absorb efficiently at short wavelengths, causing strong thermal inversions. These inversions affect the atmospheric structure, chemistry, as well as global circulation of gas and heat. Excitingly, these thermal inversions can be observed very effectively using high-resolution spectroscopy of the day-side, where a multitude of metals exhibit line emission. Together with transmission spectroscopy that senses the day-to-night terminator, we can use these observations to constrain the chemical and thermal structure of the atmosphere in three dimensions. The group has analysed observations of a collection of ultra-hot Jupiters, and find tantalising commonalities and differences between them.

Spectroscopic studies of exoplanetary atmospheres by the next-generation space missions, such as the James Webb Space Telescope (JWST) and Atmospheric Remote-sensing Exoplanet Large-survey (ARIEL), will rely on spectroscopic data available at mid-infrared and visible/infrared wavelengths for the bulk of already detected or expected molecules. Due to the specific conditions of hot atmospheres and chemical reactions which lead to the formation of spectroscopically active species ('exotic' molecules and molecular ions), laboratory studies are extremely scarce in both infrared/microwave and visible/ultraviolet regions. Therefore, there is a huge demand for robust theoretical approaches and estimates that could provide line-shape parameters for wide ranges of temperatures and pressures and large variety of perturbers.

For the UV region, where the lack of the line shape data is especially critical, theoretical modelling, motivated by the known effect of the strongly dominating adiabatic collisions, is introduced based on the Fourier-integral/phase-shift theory and ab initio interaction potentials. For the IR/MW region, where many classical, quantum-mechanical, and semi-classical theoretical approaches to collisional line-broadening and shifting are available but require pre-computed reliable potential-energy surfaces and CPU-costly calculations for each collisional pair, Professor Buldyreva suggests a simple theoretical expression requiring a minimal set of input parameters (kinetic molecular properties and the character of the leading term in the intermolecular interaction potential). Examples for active molecules from the ExoMol database are given.