Stars are born with gas-rich protoplanetary discs that typically survive for a few Myr before dispersing, after which they are left with the planetary bodies that formed within the protoplanetary system: planets, and belts of planetesimals (km-sized rocky and icy bodies). Planetesimals within belts readily collide and produce detectable levels of dust and gas, known as debris discs, around 20% of main sequence stars. Despite the ubiquity of planets and planetesimal belt detections, these appear quite unlike the planetary system around our Sun.

In my thesis, I explore how this planetary system diversity arises, with high-resolution observational studies of planetesimal belts around young stars immediately after the dispersal of the protoplanetary disc (class III stars), and around stars on the main sequence. I introduce the background to my research in chapter 1. I present my published results analysing an ALMA survey of class III stars in chapter 2. In chapter 3, I present a detailed investigation of CO gas observations around the class III star, NO Lup. In chapter 4, I present ALMA and HST images of the debris disc around the main sequence star q1 Eri. Finally, in chapter 5, I summarise my key findings. By observing planetesimal belts at the class III epoch, I interpret these as having formed rapidly within the 2Myr age of Lupus and discuss this in the context of recent theoretical work on planet and planetesimal formation pathways. By analysing the gas of NO Lup, I show this to be outflowing, the first such example at the class III epoch, and discuss this in the context of recent theoretical work on disc dissipation mechanisms, and planetary system formation. By exploring late-stage planet-belt interactions, I set new constraints on the planetary architectures around q1 Eri after 1.4Gyr of evolution. I conclude in my final chapter by summarising these results in the context of recent work by others, and discuss future work to take these analyses further, either with existing instrumentation, or with upcoming/proposed telescopes/observatories.