Heavy Atmospheres and Hidden Birthplaces: Tracing Where Giant Planets Form
Over the past two decades, astronomers have discovered that many giant exoplanets contain surprisingly large amounts of “heavy elements,” meaning elements heavier than hydrogen and helium. In this paper, Barry O’Donovan and Bertram Bitsch investigate why these planets are so enriched and where in their natal protoplanetary discs they are most likely to have formed. Observations show that some giant planets contain tens to hundreds of Earth masses of heavy elements, far more than classical planet formation models predict. This discrepancy suggests that key physical and chemical processes during planet formation are missing from simpler models, motivating the detailed simulations presented in this study.
Beyond Classical Core Accretion
The authors begin by reviewing the standard picture of giant planet formation through core accretion, where solid material builds a core that later accretes gas. While pebble accretion can speed up this process, it still struggles to explain planets whose gaseous envelopes themselves are strongly enriched in heavy elements. A promising solution involves pebble drift and evaporation: as small solid particles drift inward through the disc, they evaporate at specific temperature-dependent “evaporation fronts” (such as those for water and carbon-bearing species). This process enriches the surrounding gas with heavy elements, which can then be accreted directly into a growing planet’s atmosphere, naturally increasing its heavy element content.
Modeling Planet Growth with Realistic Chemistry
To test this idea, O’Donovan and Bitsch use the Chemcomp planet formation model, which simulates the evolution of a protoplanetary disc, pebble growth and drift, gas and pebble accretion onto planets, and planetary migration. A key improvement over earlier studies is that each simulated disc is given a chemical composition matched to the observed elemental abundances of the planet’s host star. Since stars and their discs form from the same material, this approach provides a more realistic starting point. The authors focus on planets more massive than Saturn, because in these objects the heavy elements are expected to reside mainly in the gaseous envelope rather than just in a solid core.
Reproducing Observed Heavy Element Masses
The simulations explore how planets grow and migrate in discs with different viscosities, a parameter that controls how fast gas flows inward. The results show that low-viscosity discs are most successful at reproducing the observed heavy element masses within realistic disc lifetimes. By placing planetary embryos at different starting distances from the star, the authors identify which initial locations lead to planets that end up with the observed masses and heavy element contents. Using this method, they successfully match the observed heavy element masses of 9 out of the 10 giant planets they simulate, with one notable exception likely requiring additional processes such as atmospheric stripping or giant impacts.
Where Planets Form and What Their Atmospheres Reveal
A central result of the paper is that most of the successfully matched planets are predicted to form in the inner regions of the disc, often near or interior to the water ice line. In these hot regions, pebbles have already lost their volatile components, enriching the gas in heavy elements like water vapor. As a result, growing planets accrete large amounts of heavy elements directly with the gas. Because water dominates this enrichment, the simulations predict atmospheres with high oxygen-to-hydrogen (O/H) ratios and relatively low carbon-to-oxygen (C/O) ratios. These predicted atmospheric properties broadly agree with current observations of giant exoplanet atmospheres.
Robustness of the Results
The authors then examine how sensitive their conclusions are to assumptions in the model, such as the size of pebbles or the initial mass and formation time of planetary embryos. While these factors can influence growth details, the main outcome remains robust: if giant planets form in the inner disc and accrete gas enriched by evaporating pebbles, they naturally acquire large heavy element contents. Importantly, this conclusion does not depend strongly on exactly how the planetary core formed, as long as it became massive enough to accrete gas efficiently.
Implications and Future Tests
In their discussion and conclusions, O’Donovan and Bitsch emphasize that a planet’s heavy element content can act as a fossil record of its birthplace. By combining interior structure measurements with atmospheric observations, especially of the C/O ratio, astronomers can begin to reconstruct where giant planets formed and how they migrated. The paper highlights WASP-84b as a particularly promising target for future atmospheric characterization, which could directly test the predictions made here. Overall, the study shows that pebble drift and evaporation offer a powerful and testable explanation for the heavy element–rich atmospheres of giant exoplanets.
Source: O’Donovan
