Molecular Characterisation of Exoplanetary Atmospheres using High-Resolution Transmission Spectroscopy

Exoplanetary science has entered an exciting new age of studying lower-mass exoplanets, with atmospheric characterization emerging as the frontier of this revolution. Whilst JWST has dominated the spotlight in recent years, high-resolution spectrographs on large ground-based telescopes are powerful tools with significant scientific potential. However, ground-based observations are complicated and limited by the Earth’s own atmospheric absorption. We here set out to advance high-resolution transmission spectroscopy into the next era of atmospheric characterisation, that of low-mass planets. In this thesis, we demonstrate that it is feasible to characterise the atmospheres of small and temperate exoplanets using ground-based high-resolution transmission spectroscopy. We investigate the atmospheric compositions for a range of exoplanets using high-resolution transmission spectroscopy in the near-infrared. In doing so, we develop and refine sensitive and robust techniques for achieving molecular detections using ground-based facilities, probing the lower limits of targets available for ground-based atmospheric characterisation. My thesis consists of three chapters of original work, as follows. In the first chapter, we aim to address the lack of robust and consistent detrending method to correct for telluric and stellar features in high-resolution transmission spectra of exoplanets. In particular, we investigate the robustness of metrics used to optimise PCA-based detrending in the near-infrared. We confirm that optimising PCA detrending parameters to maximise the S/N of a cross-correlation signal in the presence of noise has the potential to bias the detection significance, leading to amplified or even spurious molecular detections. We find that an alternative optimisation framework, using the difference between a signal-injected cross-correlation function and the direct cross-correlation function, is more robust against over-optimisation of noise and spurious signals. Our findings pave the way towards a robust framework for homogeneous characterisation of exoplanetary atmospheres using high-resolution transmission spectroscopy in the near-infrared. In the second chapter, we demonstrate the feasibility of high-resolution transmission spectroscopy for chemical detections in atmospheres of temperate exoplanets with low radial velocity variation during transit. This approach has traditionally relied on the large Doppler shifts of the planetary spectral lines induced by the high velocities of close-in planets. However, using simulations of the temperate sub-Neptune TOI-732 c, we show that planetary signals in transit may indeed be recovered when the change in the planet’s radial velocity is very small, down to subpixel velocities. We additionally explore novel metrics for finding such signals, and investigate trends in their detectability. This work extends the scope of high-resolution transmission spectroscopy and creates a pathway toward the characterization of habitable sub-Neptune worlds with ground-based facilities. Having established the potential for ground-based high-resolution transmission spectroscopy to robustly characterise the atmospheres of small and temperate planets, in the third chapter we apply the methods and techniques developed thus far to study a temperate Super-Earth. We here demonstrate the feasibility of atmospheric characterisation for Super-Earths with R ≤ 1.6 R⊕ using ground-based facilities, by confirming the presence of H2S (3.9σ) in the atmosphere of the temperate Super-Earth planet L 98-59 d using high-resolution transmission spectra. This is the first ground-based detection of a molecular species in the atmosphere of a Super-Earth planet, and reveals the sensitivity of spectrographs on 8 m-class telescopes to the atmospheric characterisation of such planets. We additionally aim to place constraints on the chemical abundances of various molecular species, as well as the cloud/haze deck pressure of the atmosphere. This work ignites a new era in the atmospheric characterisation of sub-Neptune and Super-Earth exoplanets using high-resolution spectroscopy with ground-based facilities.

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

• Cheverall, Connor

Advisor dc:contributor.advisor

• Madhusudhan, Nikku

• Exoplanet Atmospheres
• Exoplanets
• Remote Sensing
• Spectroscopy

Licence

• https://www.repository.cam.ac.uk/bitstreams/19ccfaa6-16a8-4c71-8f03-8b2ae82a69ea/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.124084

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/393962