Rocky planets are depleted in volatile elements relative to their host stars, and the standard way to predict a terrestrial exoplanet's bulk composition applies an empirical devolatilisation relation calibrated on a single pair, the Sun and the Earth. Wang et al. (2022, MNRAS) combined this relation with high-precision abundances of 13 Sun-like planet hosts to model the mineralogy and two-layer interior structure of hypothetical habitable-zone planets, and explicitly acknowledged the field's central gap: a comprehensive quantitative theory of devolatilisation that couples nebular condensation, disk evolution, accretionary dynamics, hydrodynamic escape, and impact processing across different stellar and orbital environments 'is still awaiting formalisation'. The gap remains open: individual legs are advancing — disk-composition-dependent condensation temperatures (Spaargaren et al. 2025), a Bayesian pebble-accretion devolatilisation model for Earth and Mars (Wang et al. 2026), condensation chemistry of reduced high-C/O systems (Zaveri et al. 2026) — but every application paper still uses the empirical Sun-to-Earth calibration, and a recent review of rocky-exoplanet interior modelling describes devolatilisation as a poorly known aspect of planet formation models. The problem as stated — establishing a validated, physics-based stellar-to-planetary compositional mapping, and thereby how far host-star abundances can constrain the interiors of terrestrial-type exoplanets — is open.

Background

A star and its planets condense from the same molecular cloud, so their refractory elements — those that condense at high temperature, such as Mg, Si, Fe, Ni, Al, and Ca — are expected to occur in nearly the same relative proportions. Volatile elements (e.g. H, C, N, S, Na), by contrast, are strongly fractionated between star and rocky planet: planets are depleted in them to degrees that grow with elemental volatility, conventionally expressed through the 50% condensation temperature TcT_c, the temperature at which half of an element has condensed out of a gas of solar composition in the equilibrium condensation sequence. This depletion is called devolatilisation.

Building on condensation-sequence simulations of extrasolar terrestrial planets (Bond et al. 2010), Wang, Lineweaver and Ireland (2019) quantified devolatilisation empirically by comparing the protosolar and bulk-Earth abundance tables: the bulk abundance ratio ff between Earth and proto-Sun follows log⁡f=αlog⁡Tc+β\log f=\alpha\log T_\mathrm{c}+\beta with α=3.676±0.142\alpha=3.676\pm0.142 and β=−11.556±0.436\beta=-11.556\pm0.436 across rock-forming and moderately volatile elements. Applying this Sun-to-Earth relation to another star's photospheric abundances yields a first-order estimate of the bulk composition of terrestrial planets around that star. Wang et al. (2019) coupled such devolatilised abundances with an extended core chemical network (Fe, Ni, S, Si, O) and thermodynamic mineralogy to constrain exoplanet interior structure, extending earlier Bayesian interior retrieval, in which mass, radius, and stellar abundance ratios Mg/Si/Fe are used as priors on mantle and core composition (Dorn et al. 2015).

The source (Wang et al. 2022, the second paper of the series) applied this pipeline to 13 Sun-like planet hosts with high-precision differential abundances, modelling hypothetical habitable-zone 'exo-Earths'. The interior model is a rigid two-layer structure — a silicate mantle plus a metallic core — with self-consistent density, pressure, and temperature profiles and Gibbs-free-energy minimisation (computing the equilibrium mineral assemblage by minimising thermodynamic free energy) for mantle mineralogy. Most predicted exo-Earths are broadly Earth-like in composition and structure (consistent with independent evidence from polluted white dwarfs, whose atmospheres retain the refractory debris of accreted rocky bodies); the exceptions, the Kepler-10 and Kepler-37 analogues, are predicted to be strongly oxidised with metallic cores much smaller than Earth's. Systems with stellar C/O > 0.8, where carbide (carbon-rich) rather than silicate planets may form, were excluded because the model is built on silicate chemistry.

The paper notes at the outset that its devolatilisation model is empirical, calibrated on one Solar-System pair: 'a sophisticated model of devolatilisation that involves disc evolution, accretion and hydrodynamic escape processes is still awaiting formalisation' (Section 2.1), and its Discussion calls for a comprehensive investigation of nebular condensation, disc evolution, hydrodynamic escape, accretionary dynamics, and impacts (Section 4.1). The Discussion then states what the method cannot yet do. First, the two-layer interior model omits crust, water, and a chemically stratified (inner solid / outer liquid) core. Second, the phase of super-Earth cores — planets of roughly 1–10 Earth masses — is genuinely uncertain, with conflicting modelling preferences for solid versus liquid cores, and 'this issue will remain open for further discussion'. Third, systematic differences between reference solar-abundance scales (at the ~0.03 dex level; a dex is a factor of ten in abundance, so 0.03 dex is roughly a 7% difference) propagate directly into inferred planetary compositions, tightening the required stellar-abundance precision from ~0.04 dex to ~0.025 dex. Fourth, the silicate-chemistry domain excludes non-silicate planetary compositions and likely extreme formation pathways (super-Mercuries, i.e. iron-enriched analogues of Mercury; planets around active M dwarfs). Fifth, observational degeneracies from planetary mass and radius uncertainties (typically 10–20% and 5–10% respectively) limit interior inference. The overarching scientific question that frames these gaps is how far host-star abundances, combined with planetary mass and radius, can in principle constrain the interior structure of terrestrial exoplanets once the stellar-to-planetary compositional mapping rests on physics rather than a Solar-System calibration.

Problem Statement

Develop and validate a quantitative, physics-based theory of the stellar-to-planetary devolatilisation that replaces the empirical Sun-to-Earth calibration (Wang, Lineweaver and Ireland 2019) as the standard mapping between host-star photospheric abundances and the bulk compositions of terrestrial exoplanets. The theory must couple, in one self-consistent framework, all of the relevant formation and loss processes whose comprehensive, coupled treatment Wang et al. (2022) identified as still warranted: nebular condensation (with its dependence on disk composition and pressure), protoplanetary disk evolution, accretionary dynamics (pebble accretion, the rapid aerodynamic-drag-assisted accretion of centimetre-sized solids onto planetary embryos, together with planetesimal-based collisional assembly), hydrodynamic escape (thermally driven, fluid-like bulk atmospheric loss), and impact processing, and must predict how the resulting volatile depletion varies across different stellar environments (e.g. FGK versus M dwarfs, high-C/O reduced systems) and orbital regimes. The single overarching objective is to establish, on this validated foundation, how far high-precision host-star abundances — combined with planetary mass and radius and their observational uncertainties — can constrain the interior structure of terrestrial-type exoplanets, including the treatment of interior layering (crust, mantle, water, stratified core) and the phase state of super-Earth cores that the empirical two-layer modelling of Wang et al. (2022) had to hold fixed. No coupling of these processes validated across stellar and orbital environments yet exists. That coupled mapping must be a single self-consistent framework in which all five processes are required components rather than a menu; it must reproduce the observed volatile depletion of the Solar-System terrestrial planets from first principles rather than from an empirical calibration on one Sun–Earth pair; and it must yield a quantitative statement of how far host-star abundances, together with planetary mass and radius and their uncertainties, constrain the interior structure of terrestrial-type exoplanets. The stellar-environment and orbital-regime examples above are illustrative rather than exhaustive.

References

  1. Wang, H. S., Quanz, S. P., Yong, D., Liu, F., Seidler, F., Acuña, L., Mojzsis, S. J. (2022). 'Detailed chemical compositions of planet-hosting stars: II. Exploration of the interiors of terrestrial-type exoplanets.' Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stac1119; arXiv:2204.09558. https://doi.org/10.1093/mnras/stac1119
  2. Wang, H. S., Lineweaver, C. H., Ireland, T. R. (2019). 'The volatility trend of protosolar and terrestrial elemental abundances.' Icarus 328, 287–305. DOI: 10.1016/j.icarus.2019.03.018. https://doi.org/10.1016/j.icarus.2019.03.018
  3. Wang, H. S., Liu, F., Ireland, T. R., Brasser, R., Yong, D., Lineweaver, C. H. (2019). 'Enhanced constraints on the interior composition and structure of terrestrial exoplanets.' Monthly Notices of the Royal Astronomical Society 482(2), 2222–2233. DOI: 10.1093/mnras/sty2749. https://doi.org/10.1093/mnras/sty2749
  4. Wang, H. S. (2022). 'Devolatilisation during planet formation: A hybrid model of chemistry and dynamics.' EGU General Assembly 2022, abstract EGU22-10146. DOI: 10.5194/egusphere-egu22-10146. https://doi.org/10.5194/egusphere-egu22-10146
  5. Spaargaren, R. J., Wang, H. S., Mojzsis, S. J., Ballmer, M. D., Tackley, P. J. (2023). 'Plausible Constraints on the Range of Bulk Terrestrial Exoplanet Compositions in the Solar Neighborhood.' The Astrophysical Journal 948, 53. DOI: 10.3847/1538-4357/acac7d. https://doi.org/10.3847/1538-4357/acac7d
  6. Spaargaren, R. J., Herbort, O., Wang, H. S., Mojzsis, S. J., Sossi, P. (2025). 'Proto-planetary disk composition-dependent element volatility in the context of rocky planet formation.' Astronomy & Astrophysics 703, A218. DOI: 10.1051/0004-6361/202556011; arXiv:2509.03724. https://doi.org/10.1051/0004-6361/202556011
  7. Wang, H. S., Johansen, A., Xu, Z., Steinmeyer, M.-L., Lambrechts, M., van Kooten, E., Yang, C.-C., Zhu, Z., Lauretta, D. S., Bizzarro, M. (2026). 'Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion.' arXiv:2608.24407. https://arxiv.org/abs/2608.24407
  8. Baumeister, P., Miozzi, F., Guimond, C. M., Steinmeyer, M.-L., Dorn, C., Karato, S.-I., Bolmont, É., Revol, A., Thamm, A., Noack, L. (2025). 'Fundamentals of Interior Modelling and Challenges in the Interpretation of Observed Rocky Exoplanets.' Space Science Reviews 221, 123. DOI: 10.1007/s11214-025-01248-5. https://doi.org/10.1007/s11214-025-01248-5
  9. Plotnykov, M., Valencia, D., Ross, A., Reggiani, H., Schlaufman, K. C. (2026). 'Evidence of 1:1 Slope between Rocky Super-Earths and Their Host Stars.' The Astrophysical Journal 997, 253. DOI: 10.3847/1538-4357/ae200b. https://doi.org/10.3847/1538-4357/ae200b
  10. Ross, A., Reggiani, H., Schlaufman, K. C., Plotnykov, M., Valencia, D. (2025). 'Terrestrial Exoplanet Internal Structure Constraints Enabled by Comprehensive Host Star Characterization Reveal That Terrestrial Planets in Mean Motion Resonances Are Water Rich.' The Astrophysical Journal 993, 235. DOI: 10.3847/1538-4357/ae03a5. https://doi.org/10.3847/1538-4357/ae03a5
  11. Plotnykov, M., Valencia, D. (2024). 'Observation uncertainty effects on the precision of interior planetary parameters.' Monthly Notices of the Royal Astronomical Society 530, 3488–3499. DOI: 10.1093/mnras/stae993. https://doi.org/10.1093/mnras/stae993
  12. Guimond, C. M., Wang, H., Seidler, F., Sossi, P., Mahajan, A., Shorttle, O. (2024). 'From Stars to Diverse Mantles, Melts, Crusts, and Atmospheres of Rocky Exoplanets.' Reviews in Mineralogy and Geochemistry 90, 259–300. DOI: 10.2138/rmg.2024.90.08. https://doi.org/10.2138/rmg.2024.90.08
  13. Krishna, H., Gupta, A., Bhowmik, S., Sahu, C. K., Mridha, S., Majumdar, L. (2026). 'Stellar Abundances as Probes of Rocky Exoplanet Interiors: The Mantle Composition and Mineralogy of GJ 486b.' arXiv:2608.18457. https://arxiv.org/abs/2608.18457
  14. Zaveri, U., Wang, H. S., Sossi, P. A. (2026). 'A chemical perspective on planet formation in reduced systems.' Astronomy & Astrophysics 709, A223. DOI: 10.1051/0004-6361/202558126. https://doi.org/10.1051/0004-6361/202558126
  15. Wang, H. S., Quanz, S. P., Mahadevan, S., Deal, M. (2024). 'Towards characterising rocky worlds: Trends in chemical make-ups of M dwarfs versus GK dwarfs.' Astronomy & Astrophysics 688, A225. DOI: 10.1051/0004-6361/202449467. https://doi.org/10.1051/0004-6361/202449467
  16. Dorn, C., Khan, A., Heng, K., Connolly, J. A. D., Alibert, Y., Benz, W., Tackley, P. (2015). 'Can we constrain the interior structure of rocky exoplanets from mass and radius measurements?' Astronomy & Astrophysics 577, A83. DOI: 10.1051/0004-6361/201424915. https://doi.org/10.1051/0004-6361/201424915
  17. Bond, J. C., O'Brien, D. P., Lauretta, D. S. (2010). 'The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations.' The Astrophysical Journal 715, 1050–1070. DOI: 10.1088/0004-637x/715/2/1050. https://doi.org/10.1088/0004-637x/715/2/1050
  18. Lichtenberg, T., Clement, M. S. (2022). 'Reduced Late Bombardment on Rocky Exoplanets around M Dwarfs.' The Astrophysical Journal Letters 938, L3. DOI: 10.3847/2041-8213/ac9521. https://doi.org/10.3847/2041-8213/ac9521
  19. Byrne, X., Guimond, C. M., Bonsor, A., Wang, H. S., Vaughan, S. R., Rogers, J. G. (2026). 'The Barnard's Star planetary system: stability, composition, and evolution of four sub-Earth exoplanets.' Monthly Notices of the Royal Astronomical Society 550. DOI: 10.1093/mnras/stag1207. https://doi.org/10.1093/mnras/stag1207