<scp>sponchpop</scp> : population synthesis to investigate volatile sulphur as a fingerprint of gas-giant formation histories

ABSTRACT Planet population synthesis is an integral tool for linking exoplanets to their formation environments. Most planet population synthesis studies have focused on the carbon-to-oxygen ratio in gas or solids, yet more insight into planet formation may be afforded by considering a wider suite of elements. Sulphur is one such key element. It has been assumed to be entirely refractory in population synthesis models, restricting it to being a tracer of accreted rocky solids. However, sulphur also has a volatile reservoir dominant at the onset of star and planet formation, which is then converted into refractories. We investigate sulphur’s wider potential as a formation history tracer by implementing a gas-grain chemical conversion, the first multi-phase treatment of S in a planet population synthesis model. We also present the planet formation module of sponchpop and its first predicted planet growth tracks and populations. We apply these to explore the diversity of the planetary sulphur budget. We show that planets can inherit a wide range of core and envelope sulphur content, depending on their formation environment and accretion history including late-stage infall, demonstrating sulphur’s new potential as a diagnostic tool for planet formation. Our models predict that some rocky planets are born sulphur-poor, which may have significant implications for their geochemistry and habitability. Enhanced sulphur abundances in gas-giant atmospheres, such as in our Solar system, may result not only from accretion of rocky planetesimals, but also from formation beyond the $\mathrm{H_2S}$ ice line.