How Giant Planets Collect Their Metals: A New Look at the Mass-Metallicity Relation

In this paper, Chachan and collaborators revisit how giant planets like Jupiter accumulate the heavy elements that shape their composition. Heavy elements in astronomy refer to everything heavier than hydrogen and helium. By studying these elements, scientists can piece together the history of how planets form and grow. Using updated models of planetary interiors and a much larger sample of exoplanets than before, the authors provide new insights into how metals are distributed across planets of different masses.

Introduction: Building Giant Planets

The formation of gas giants is a race against time. These planets must build up their massive envelopes before the surrounding gas in their star’s disk disappears after just a few million years. The main theory for this process is “core accretion”: small rocky or icy particles collide and stick together, gradually forming a solid core. Once the core is large enough, it begins to pull in gas from the disk. But how many heavy elements make it into the final planet depends on many complicated factors, including the sizes of planetesimals and the dynamics of the gas. Because these details are hard to predict, astronomers instead look at observations of giant planets’ masses and radii to infer their bulk metallicities.

Expanding the Planet Sample

Previous studies had measured metallicities for only a few dozen warm giant planets. Chachan and colleagues more than triple that number, studying 147 planets with well-measured masses and radii and temperatures below 1000 K (to avoid uncertainties from heat-driven radius inflation). This larger sample makes it possible to look for patterns in how metal content varies with planet mass. The team relies on updated planetary models, which account for the latest physics of hydrogen-helium mixtures and the effects of metal-rich atmospheres.

Methods: Modeling Planetary Interiors

The researchers modeled each planet’s interior structure and cooling history to estimate how many heavy elements it contains. They tested different assumptions, such as whether metals are concentrated in a core or mixed into the envelope, but found that these details make only small differences. Their models also update the equation of state for hydrogen and helium and include atmospheric boundary conditions that vary with metallicity, leading to more realistic predictions of how planets evolve over billions of years.

Results: The Mass-Metallicity Relation

The expanded dataset shows that planets less massive than about half the mass of Jupiter (∼150 Earth masses) tend to be dominated by metals, with metallicity values much higher than the Sun’s. As planet mass increases, the overall metallicity declines, but not all the way down. Instead, it flattens out at a super-solar value: roughly 7 times the Sun’s metallicity. This indicates that even massive giants continue to accrete material rich in heavy elements during their growth. The team also finds that a simple linear relation between metal mass and planet mass fits the data better than the power law used in earlier studies.

Special Populations and Exceptions

While most planets follow this broad pattern, there are some notable exceptions. A few low-mass planets show unexpectedly low metallicities, raising questions about why they didn’t gather more gas to become larger. Some high-mass planets, on the other hand, are surprisingly metal-rich, containing hundreds of Earth masses worth of heavy elements. The study also identifies planets whose apparent ages don’t match expectations, possibly because of measurement uncertainties or effects from stellar evolution.

Implications for Planet Formation

The results challenge the classical view that runaway gas accretion happens as soon as planets reach about 10 Earth masses. Instead, Chachan et al. find that planets typically reach the runaway threshold only between 30 and 60 Earth masses. This suggests that gas accretion is slower and more complex than previously thought, possibly delayed by ongoing solid accretion or high dust opacities. The finding that even the most massive planets remain metal-rich points to continued enrichment during later stages of growth, either through planetesimal capture or metal-rich gas. Altogether, these results strengthen the case for core accretion as the dominant formation pathway and provide new constraints on how giant planets evolve.

Source: Chachan