Proto-planetary disk composition-dependent element volatility in the context of rocky planet formation

TL;DR

This work demonstrates that elemental volatility during disk condensation strongly depends on the bulk disk composition, not just stellar abundances. By running GGchem condensation sequences across 1000 star-derived disk chemistries and deriving $T_{ ext{C}}^{i}$ parametrisations, the authors translate stellar abundances into projected rocky-planet bulk compositions under Earth-like devolatilisation. They find distinct planet populations governed by the disk C/O ratio, including Earth-like planets in low-$C/O$ disks, graphite-rich planets at high-$C/O$, and intermediate-$C/O$ planets with Mg/Si and Ca/Al enrichment, yielding a core-mass fraction dichotomy. The framework highlights that accounting for disk composition–dependent condensation temperatures expands the predicted compositional diversity of rocky exoplanets and can be extended to non–Earth-like devolatilisation trends and broader dynamical formation models, providing a practical baseline for interpreting future exoplanet observations.

Abstract

The compositions of the Solar System terrestrial bodies are fractionated from that of the Sun, where elemental depletions in the bulk rocky bodies correlate with element volatility, expressed in its 50% condensation temperature. However, because element volatility depends on disk gas composition, it is not mandated that elemental fractionation trends derived from the solar-terrestrial scenario apply to other planetary systems. Here, we expand upon previous efforts to quantify element volatility during disk condensation, and how this affects rocky planet compositional diversity. We simulate condensation sequences for a sample of 1000 initial disk compositions based on observed stellar abundances. We present parametrisations of how element 50% condensation temperatures depend on disk composition, and apply element fractionation trends with appropriate element volatilty to stellar abundances to simulate compositions of rocky exoplanets with the same volatile depletion pattern as the Earth, providing a robust and conservative lower limit to the compositional diversity of rocky exoplanets. Here we show that Earth-like planets emerge from low-C-to-O disks and graphite-bearing planets from medium-to-high-C-to-O disks. Furthermore, we identify an intermediate-C-to-O class of planets characterized by Mg and Si depletion, leading to relatively high abundances of Fe, Ca, and Al. We show that devolatilisation patterns could be adapted potentially with disk composition-dependent condensation temperatures to make predictions of rocky planet bulk compositions within individual systems. The outcomes of our analysis suggest that accounting for disk composition-dependent condensation temperatures means that we can expect an even broader range of possible rocky planet compositions than has previously been considered.

Figures (23)

• Figure 1: Histogram of stellar metallicity (top) in the GALAH catalogue with solar metallicity for comparison Asplund2009, and 50% condensation temperature of various elements as a function of host stellar metallicity (bottom). Each column of condensation temperatures is calculated from a condensation sequence with solar abundances scaled to the corresponding metallicity. Condensation temperatures are shown relative to the simulation with unscaled solar abundances (Table \ref{['tab:Tc_validation']}).
• Figure 2: Stellar abundances within the GALAH catalogue (orange; $N=91,202$) and within the sample for which we run condensation sequence models (blue; $N=1,000$). Given our adjusted selection procedure, the selected sample places more emphasis on the tails of the distribution presented by the GALAH catalogue. Solar abundances are shown as a black dashed line Asplund2009.
• Figure 3: Condensation temperatures ($T_{\mathrm{C}}^{}$) of Fe and Ni, plotted against the logarithmic abundances of Fe and Ni (normalised to H=12; $A_{\mathrm{Fe}}$, $A_{\mathrm{Ni}}$) in those disks. Condensation temperatures are derived from condensation sequence calculations at $P=10^{-4}$ bar.
• Figure 4: Condensation temperatures ($T_{\mathrm{C}}^{}$) of the rock-forming elements Mg, Si, Ca, and Al, plotted against the molar C/O ratio in disks, based on stellar $\epsilon_{\text{C}}/\epsilon_{\text{O}}$ values (top). Solar C/O=0.55 Asplund2009 is indicated as a dashed vertical line. All elements show a transition from high $T_{\mathrm{C}}^{}$ at low C/O to lower $T_{\mathrm{C}}^{}$ at higher C/O. For Mg, colouring condensation temperature by the disk Mg/O ($=\epsilon_{\text{Mg}}/\epsilon_{\text{O}}$; bottom) reveals that the C/O value at which this transition occurs depends on Mg/O. Condensation temperatures are derived from condensation sequence calculations at $P=10^{-4}$ bar.
• Figure 5: Condensation temperatures ($T_{\mathrm{C}}^{}$) of Mg and Si, after subtracting a fit to the C/O-dependence of $T_{\mathrm{C}}^{\text{Mg}}$ and $T_{\mathrm{C}}^{\text{Si}}$ (Fig. \ref{['fig:Tc_MgSiCaAl']}), plotted against the disk Mg/Si ratios. Here, a trend emerges between $T_{\mathrm{C}}^{}$ and the molar Mg/Si ratio of the disk (bottom) for Si, but not for Mg. Solar Mg/Si=1.23 Asplund2009 is indicated as a vertical dashed line. Condensation temperatures are derived from condensation sequence calculations at $P=10^{-4}$ bar.