Tracing exoplanets through time with TESS

In the thirty years since the discovery of the first exoplanet, over 5000 verified exoplanets have been discovered, unveiling a rich array of different exoplanetary architectures. However, there are still many unanswered questions regarding the formation and evolution pathways which have led to the observed population. To understand these processes it is imperative to trace exoplanets across time, both over galactic and human timescales. This thesis presents work in both of these areas, using data from the Transiting Exoplanet Survey Satellite (TESS). The bulk of this thesis focuses on the challenge of discovering new young exoplanets (age < 1 Gyr) and understanding the variability of their potential host stars. This begins with building an extended population of young stars around which to search for exoplanets, illustrating the kinematic power of the recently launched Gaia satellite and resulting in a target list of over three million young stars. A dedicated new young star detrending pipeline is then presented, which is in turn used to search for new young exoplanets in stellar associations within TESS sectors 1-5. Although no new exoplanets were found, the pipeline's effectiveness is demonstrated by recovering the previously known young exoplanets DS Tuc Ab and AU Mic b, alongside all other 2 min Targets of Interest (TOIs) from the 30 min cadence data alone. The completed young exoplanet search highlighted the challenging diversity of young stellar variability. To understand this variability, Kohonen Self-Organising Maps are used for the first time on a dedicated sample of young stars observed in the first year of TESS's primary mission, in order to sort light-curves by topology and look for distinct variability classes. This analysis forms the first step in the YOUNGSTER programme, aiming to use knowledge of young star variability to inform more targeted detrending in future young exoplanet searches. Finally, this thesis presents work on tracing exoplanets through time on human timescales, updating and improving the ephemerides for all previously known Kepler planets (22) and candidates (4) which were observed with sufficient signal to noise in TESS. It also explores any transit-timing variations seen for these objects, including intriguing results for HAT-P-7b, Kepler-411d, K00075.01 and K00076.01.