New Advances in Exoplanet Spectroscopy from Space and Ground

Thousands of exoplanets have been discovered over the last thirty years, providing key insights into the population of planets in the galaxy. Today, atmospheric characterisation of these distant worlds is at the forefront of exoplanet science, with new observational capabilities revolutionising remote sensing of exoplanets. The James Webb Space Telescope (JWST) is delivering unprecedented atmospheric observations with unmatched precision and wavelength coverage in the infrared, while large ground-based telescopes are facilitating high-resolution spectroscopy in the optical and near-infrared. This surge of high-quality data is enabling precise atmospheric characterisation across a diverse range of exoplanets, from hot Jupiters to temperate sub-Neptunes with no counterpart in the Solar System. Together, these advancements promise to transform our understanding of exoplanets and help us contextualise our place within the broader universe. Given these developments, this thesis lays the groundwork for reliable atmospheric characterisation of exoplanet atmospheres at high precision, with a particular focus on small, temperate worlds. We aim to establish best practices for both ground- and space-based instruments through detailed instrument characterisation and the development of robust data reduction and analysis techniques. First, we assess the on-sky performance of the CRIRES+ spectrograph (0.95-5.3 μm, R = 100,000) for exoplanet spectroscopy and conduct a detailed assessment of the data reduction procedure. Using K-band (2.0-2.4 μm) science-verification observations, we find that CRIRES+ meets its expected performance metrics in terms of both spectral resolution and signal-to-noise ratio. As a case study, we present the first detection of multiple molecular emission features and a thermal inversion in the atmosphere of the ultra-hot Jupiter MASCARA-1 b, identifying CO and H2O at high significance. These results highlight the scientific potential of CRIRES+ for characterising diverse exoplanetary atmospheres. Second, we explore key diagnostics of JWST NIRISS (0.6-2.8 μm, R = 700) for exoplanet spectroscopy. Through early observations of giant exoplanets WASP-39 b and WASP-96 b, we conduct a detailed evaluation of different aspects of the data reduction and analysis procedures. We address key challenges, such as contamination from background field stars, overlapping spectral orders, and correlated noise, for accurate spectral extraction. We also conduct a comparative assessment of different data reduction pipelines and highlight important lessons for exoplanet spectroscopy with JWST NIRISS, providing a foundation for future high-precision JWST studies of exoplanet atmospheres. Third, we perform atmospheric characterisation of the temperate sub-Neptune TOI-270 d using JWST NIRSpec and HST observations. We detect strong features of CH4 and CO2 in the transmission spectrum and find no evidence for NH3. The retrieved atmospheric composition supports the interpretation of TOI-270 d as a candidate Hycean planet, characterised by a shallow H2-rich atmosphere over a planet-wide ocean - rather than a deep H2-rich atmosphere. We also report potential inferences of CS2 and H2O in the atmosphere and discuss possible interpretations. Finally, we conduct an observational and theoretical analysis of the temperate sub-Neptune K2-18 b. Using recent JWST NIRISS and NIRSpec observations, we confidently detect both CH4 and CO2 in the planet's atmosphere, with no evidence of NH3, reinforcing K2-18 b’s stand as a promising Hycean candidate. We also infer potential signs of DMS, a predicted biomarker in Hycean atmospheres, motivating future studies. In addition, we develop an integrated interior-atmosphere modelling framework to investigate the plausibility of a gas-dwarf scenario for planets like K2-18 b and find that it is unable to explain the abundances from the present observation. Overall, these findings represent a pivotal shift toward studying smaller and cooler exoplanets, heralding a new era in atmospheric characterisation of potentially habitable worlds with JWST.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Holmberg, Måns

Advisor dc:contributor.advisor

• Madhusudhan, Nikku

• Exoplanet Atmospheres
• Exoplanets
• James Webb Space Telescope

Licence

• https://www.repository.cam.ac.uk/bitstreams/92c6c422-d910-4da8-8f8a-a84ab11e0dd2/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.119753

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/386613