Since the discovery of the first exoplanets in the 1990s, our knowledge of planets has expanded far beyond the Solar System, and surveys like Kepler and Tess have revealed a huge diversity of other worlds. In response to this new information, a novel field of planetary astronomy has sprung up to deal with the major questions, including: How do planets form out of proto-planetary discs? What are the bulk and atmospheric compositions of planets, and what are their building blocks? In this thesis, I contribute to the literature around both of these questions, by studying accretion processes across the lifetime of planetary bodies. My thesis is organized chronologically, starting with the birth of planetary building blocks, and ending with the destruction of fully-formed planets. Besides the shared topic of planetary astronomy, a second unifying theme in this thesis is the use of simple analytical methods to pursue novel research ideas.

The first strand of my research (Chapter 2) deals with the formation of planetesimals - a plausible starting point for planet formation. I develop a new theory that relates the formation of these planetesimals to the spinning motion around their own axis. Specifically, I show that a general mechanism exists, whereby objects that gravitationally collapse next to an external potential naturally acquire spin angular momentum that is aligned with their orbital angular momentum (prograde). Planetesimals in the Solar System have a strong prograde bias, and prograde spin-up, therefore, provides new evidence for the popular hypothesis that they formed via gravitational collapse. The second strand of my research (Chapter 3) deals with the formation of the planets themselves, which likely grow via the accretion of large planetesimals, as well as smaller particles called pebbles. In this work, I study how the accretion of pebbles changes the opacity of planetary envelopes during their formation, which crucially determines how quickly accretion heat is lost, and how much hydrogen and helium the planet can bind. I show that relatively low opacities are predicted from this process, unless the pebble accretion rate crosses a certain threshold. The implication of this work is that the accretion of nebular gas during planet formation might be more efficient than previously thought, especially during periods of slow pebble accretion.

The final strand of my research (Chapters 4, 5, and 6) takes us to the end of a planet’s lifetime, when its host star has left the main sequence and has shed its outer layers to become a white dwarf star. Many of these white dwarfs show metal absorption lines in their spectra, indicative of pollution with accreted planetary material. From the analysis of such spectra, the composition of exoplanetary material can be recovered. In this work, I explore how planetary material could have accreted onto these stars, and try to link this process to observable features, such as the accretion rate and infrared excess. I also explore the possibility that different components of a pollutant could accrete onto these stars asynchronously, over different periods of time, which is a crucial process to understand for the pollutant composition to be correctly interpreted based on the measured stellar abundances.