Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021
EPSC Abstracts
Vol. 15, EPSC2021-853, 2021, updated on 22 Jul 2021
European Planetary Science Congress 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Forming planets around stars with non-solar composition

David Jorge1, Inga Kamp1, Rens Waters2,3, Peter Woitke4, and Rob Spaargaren5
David Jorge et al.
  • 1Kapteyn Astronomical Institute, University of Groningen, The Netherlands
  • 2Radboud University Nijmegen, The Netherlands
  • 3SRON, The Netherlands
  • 4SUPA, University of St Andrews, UK
  • 5ETH Zurich, Switzerland


We aim to understand the impact of different refractory element ratios such as Mg, Si, and Fe on the composition of planets. We use the thermodynamic equilibrium code GGchem to simulate the condensation of solids in a minimum mass solar nebula around main sequence G-type stars within 150pc. We extract the stellar elemental composition from the Hypatia database. We find that a lower Mg/Si ratio shifts the condensation sequence from forsterite and SiO to enstatite and quartz; a lower Fe/S ratio leads to the formation of FeS and FeS2 and little or no Fe-bearing silicates. Ratios of refractory elements translate directly from the gas phase to the condensed phase for T<1000K. However, ratios with respect to volatile elements (e.g. oxygen and sulphur) in the condensates – the building blocks of planets – differ from the original stellar composition. Our results can have important implications for planet interiors, which depend strongly on the degree of oxidization and the sulphur abundance.

1. Introduction

Planets form from solid material residing in the disk around a young star. Both the star and the disk are composed oft he same material (inherited from the collapsing cloud). In the inner disk (inside a few au), material is likely sublimating and solidifying repeatedly due to episodic accretion events and stellar luminosity peaks. Small variations from solar abundances (within 0.5dex), especially also for refractory elements, are ubiqutous in the solar neighborhood. The question is whether such small element abundance fluctuations have a noticeable impact on the composition of a planet condensing from the planet forming disk.

2. Methods

We selected from the Hypatia Database [4] single solar-type main-sequence stars within 150 pc. We select stars that have as complete as possible chemical abundance data and draw six sample stars, representative of the typical spreads in the elements ratios (see Fig. 1).

Figure 1: Ratios of the refractory elements Mg, Si and Fe, as well as S, for main-sequence G-type stars within 150pc. Black dots represent the location of the six sample stars. A red dot indicates a star for which we inferred missing abundances from the median of the known abundances. 

We use the GGchem code developed by [2] and re-written and updated by [6]. The thermo-chemical data are taken from the NIST-JANAF and the geophysical SUPCRTBL databases. The code has been successfully benchmarked against the public TEA code [1]. Given a specific temperature and pressure, the code calculates the various molecules and condensates forming by minimizing the Gibbs free energy. We choose 24 elements (H, He, Li, C, N, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Zr, W, F, P), leading to 552 molecules and 241 condensates. We use the pT-profile of the Minimum Mass Solar Nebula from [3] and assume that different parts of the disk do not mix and that planets are forming in situ. 

We sample the results of the condensation sequences by defining locations for the formation of hypothetical planets. Besides the current locations of Mercury, Venus, Earth, Mars, and the Asteroid belt in the Solar system, we also use 1000, 1250 and 1500K, mimicking that solar system planets likely formed at higher temperatures (dynamical evolution).

Figure 2: Condensation sequence for Mg-bearing species for the solar abundances and two extreme cases of Mg/Si ratio.