sponchpop II: Population Synthesis to Investigate Volatile Sulfur as a Fingerprint of Gas Giant Formation Histories

TL;DR

This work advances planet population synthesis by incorporating a first multi-phase sulfur chemistry that treats volatile and refractory sulfur reservoirs in tandem, linking disk chemistry to planetary core and envelope compositions. By simulating 45,000 planets across three disk models and varying late-stage planetesimal ablation (p_ratio), it demonstrates that envelope sulfur content can record formation location and accretion history, especially for gas giants forming beyond the H2S iceline. The results show that envelope sulfur enrichment is highly sensitive to late-stage solid accretion and migration, producing a spectrum from near-solar to multi-solar abundances, while core sulfur exhibits strong spatial dependence and potential depletion in inner disk regions. Overall, sulfur chemistry emerges as a complementary diagnostic to C/O for reconstructing giant-planet formation pathways, with implications for interpreting JWST/Ariel atmospheric data and informing future chemical extensions of population synthesis models.

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 (C/O) in gas or solids, yet more insight into planet formation may be afforded by considering a wider suite of elements. Sulfur 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, sulfur also has a volatile reservoir dominant at the onset of star and planet formation. We investigate sulfur's wider potential as a formation history tracer by implementing the first multi-phase treatment of S in a planet population synthesis model. We present the planet formation module of \textsc{sponchpop} and its first predicted planet growth tracks and populations. We explore the diversity of planet compositions in terms of their sulfur budget, including both refractory and volatile components, and apply a novel gas-grain conversion of sulfur to study how formation trajectories of giant planets relate to final core and envelope compositions. We show that planets inherit a wide range of core and envelope sulfur content related to accretion history while considering late-stage planetesimal infall, providing a new diagnostic tool for planet formation. The diverse sulfur content of planet cores suggests some rocky planets may be born sulfur-poor, with implications for their geochemistry and habitability. Enhanced sulfur abundances in gas-giant atmospheres can be attributed to formation beyond the H2S iceline, such as the giants in our Solar System.

Figures (13)

• Figure 1: Fractional representation of volatile and refractory reservoirs of sulfur throughout the disk evolution and planet formation processes. Values are normalised to solar. kama2019
• Figure 2: Flowchart showing the process undertaken by the fiducial case of the planet formation code.
• Figure 3: Surface density distribution and temperature profiles of each considered disk model. Line colour denotes disk model, and opacity denotes time in disk evolution.
• Figure 4: Planet formation tracks for the fiducial parameter set detailed in Table \ref{['table:1']}, using the viscous-irradiated disk model. The solid, dashed and dotted parts of each track represent the solid accretion, envelope contraction and runaway gas accretion, respectively. The coloured triangles are markers for every $0.5\,\text{Myr}$ from $t_0$ of planet birth time, ending after $3\,\text{Myr}$. The green lines denote the minimum and maximum pebble isolation masses for this specific disk model.
• Figure 5: Surface density distributions for the FeS, Fe and $\mathrm{H_2S}$ species in each considered disk model following $3\,\text{Myr}$ of disk evolution. The dashed lines show the surface densities at $t_0$ of the disk (only applicable for Fe and $\mathrm{H_2S}$, before any FeS is formed), and the solid lines denote the densities at $0.5\,\text{Myr}$. The more transparent the line, the earlier in the disk lifetime the distribution. These distributions are for disks sans planet formation, using the fiducial disk parameters outlined in Table \ref{['table:1']}