Mass Measurement and Characterisation of Transiting Exoplanets

The discovery of more than 5,000 exoplanets over the last three decades has led to the realisation that our galaxy contains billions and billions of planets – possibly even more planets than stars. Small planets, including rocky worlds like our own Earth, seem to be particularly abundant. While some of these distant worlds remind us of our own Solar System, more exotic findings that have no Solar System analogues, like hot Jupiters, super-Earths, and sub-Neptunes, have  challenged our understanding of the formation and evolution of planetary systems.

Until recently, our understanding of the exoplanet population was shaped by two distinct methods: Transit photometry, yielding the radius of the planet, and Doppler spectroscopy, measuring its mass. Combining the two gives an estimate
of the bulk density, which provides insight into the composition and nature of the discovered planets. Such measurements are only possible for a small and cherished fraction of planets, which must transit their host stars and be bright enough to allow detailed characterisation. These systems are therefore subjected to comprehensive observation programmes and careful analysis to allow determination of planetary parameters with the highest possible accuracy.

In this thesis, I first present my published work on the photon-weighted barycentric correction, eliminating a significant and previously overlooked source of systematic errors. This will be critical to address when future surveys attempt to detect the radial velocity signature of a true Earth analogue. Contributing to the ongoing efforts to characterise small exoplanets, I also present the confirmation of a newly discovered transiting sub-Neptune, EPIC 229004835 b, and use K2 and HARPS-N data to accurately determine its mass, radius, and bulk density. Finally, I describe my contributions to a large TESS follow-up programme at the Nordic Optical Telescope, and the efforts made to improve the radial velocity precision of the FIES spectrograph. The lessons learned from improvements of current precision radial velocity instruments and mass measurements of small planets are paving the way for next generation decade-long dedicated radial velocity surveys with the goal of discovering Earth-analogue planets in the habitable zones of Solar-like stars in our closest stellar neighbourhood.