Heavy element enriched atmospheres and where they are born

The heavy element content of giant exoplanets, inferred from structure models based on their radius and mass, often exceeds predictions based on classical core accretion. Pebble drift, coupled with volatile evaporation, has been proposed as a possible remedy to this with the level of heavy element enrichment a planet can accrete, as well as its atmospheric composition, being strongly dependent on where in the disc it is forming. We use a planet formation model which simulates the evolution of the protoplanetary disc, accounting for pebble growth, drift and evaporation, and the formation of planets from pebble and gas accretion. The growth and migration of planetary embryos is simulated in 10 different protoplanetary discs which have their chemical compositions matched to the host stars of the planets which we aim to reproduce, providing a more realistic model of their growth than previous studies. The heavy element content of giant exoplanets is used to infer their formation location and thus make a prediction of their atmospheric abundances. We focus here on giants more massive than Saturn, as we expect that their heavy element content is dominated by their envelope rather than their core. The heavy element content of 9 out of the 10 planets simulated is successfully matched to their observed values. Our simulations predict formation in the inner disc regions, where the majority of the volatiles have already evaporated and can thus be accreted onto the planet via the gas. As the majority of the planetary heavy element content originates from water vapour accretion, our simulations predict a high atmospheric O/H ratio in combination with a low atmospheric C/O ratio, in general agreement with observations. For certain planets, namely WASP-84b, these properties may be observable in the near future, offering a method of testing the constraints made on the planet’s formation.

💡 Research Summary

The paper tackles the long‑standing discrepancy between the heavy‑element masses (M Z) inferred for many giant exoplanets and the predictions of classical core‑accretion models. Observational studies (Thorngren et al. 2016; Bloot et al. 2023) have shown that a substantial fraction of transiting giants contain 50–100 M⊕ or more of heavy elements, far exceeding the amount that can be stored in a solid core alone. This implies that a large portion of the heavy elements must be mixed into the planetary envelope, a situation that classical models struggle to reproduce, especially for host stars with only modest metallicities.

The authors propose that pebble drift combined with volatile evaporation at snow lines can enrich the gas phase of the inner protoplanetary disc with heavy elements, which are then accreted by growing planets. To test this hypothesis they employ the Chemcomp planet‑formation code (Schneider & Bitsch 2021a,b), which self‑consistently evolves the gas surface density, temperature, dust growth, pebble drift, and the evaporation/condensation of key volatiles (H₂O, CO, CO₂). Crucially, the chemical composition of each simulated disc is matched to the measured stellar abundances of the host star (Teske et al. 2019), allowing a realistic, star‑specific initial dust‑to‑gas ratio and elemental inventory.

Ten planetary systems from the Thorngren sample with available host‑star abundances are modelled. The disc parameters are fixed to a mass of 0.128 M⊙, outer radius 137 AU, and a fragmentation velocity of 5 m s⁻¹. Three values of the Shakura‑Sunyaev viscosity parameter α (1×10⁻⁴, 5×10⁻⁴, 1×10⁻³) are explored. Planetary embryos start at 0.005 M⊕ after 0.1 Myr and grow by pebble accretion until they reach the pebble‑isolation mass (≈25 (H/r)⁰·⁵ M⊕). At that point pebble accretion stops, a gap opens, and gas accretion proceeds at a rate set by the disc’s viscous inflow. Migration is modelled with type‑I torques (including dynamical and heating torques) and, once a gap forms, type‑II viscous migration.

The simulations reveal that the timing and magnitude of heavy‑element accretion are highly sensitive to α. Low‑viscosity discs (α = 1×10⁻⁴) transport solids slowly; consequently, planets that start interior to the water ice line require unrealistically long disc lifetimes (up to ~17 Myr for a 2.5 M_J planet) to reach their observed masses, and the resulting M Z values fall below the observational constraints. High‑viscosity discs (α = 1×10⁻³) allow rapid gas inflow, so planets attain their final masses within the canonical 3 Myr disc lifetime, and the heavy‑element budget is dominated by the accretion of water vapour that has been released by drifting pebbles crossing the snow line.

For nine out of the ten planets, the model reproduces the observed heavy‑element mass within the quoted uncertainties. The inferred formation locations are all in the inner disc (well inside the water‑ice line), where most volatiles have already evaporated and are available as gas. Because the bulk of the heavy elements is supplied by water vapour, the resulting planetary atmospheres are predicted to have elevated O/H ratios (often several times stellar) and suppressed C/O ratios (well below the stellar value). This chemical signature matches recent atmospheric retrievals that find super‑stellar metallicities and low C/O in several hot Jupiters.

One particularly promising case is WASP‑84b. The model predicts a strong water‑vapour enrichment and a correspondingly high O/H, low C/O composition that should be detectable with JWST or ARIEL transmission spectroscopy in the near future, providing a direct test of the pebble‑drift enrichment scenario.

The study acknowledges several simplifications: (i) volatile evaporation is assumed to occur instantaneously within 10⁻³ AU of the snow line, (ii) a fixed refractory carbon fraction of 60 % (ISM‑like) is adopted and carbon‑burning fronts are ignored, and (iii) the disc lifetime and α are treated as free parameters that strongly influence the results. Nevertheless, by anchoring the disc chemistry to measured stellar abundances, the work improves upon earlier models that assumed solar‑scaled compositions and demonstrates that pebble‑drift‑driven volatile enrichment can naturally explain the heavy‑element excesses of massive giant planets.

In summary, the paper provides a coherent theoretical framework linking disc chemistry, pebble dynamics, and planetary migration to the observed heavy‑element content and atmospheric composition of giant exoplanets. It predicts that most heavy‑element‑rich giants form in the inner, volatile‑rich gas phase, leading to high O/H and low C/O atmospheres—a prediction that upcoming high‑precision spectroscopic observations can test.