Building Worlds: How Protoplanetary Disk Chemistry Shapes Rocky Planets
In their recent paper, Spaargaren and collaborators explore how the chemistry of the disks around young stars, the protoplanetary disks where planets form, can change the types of rocky planets that eventually emerge.
Why Composition Matters
The bulk composition of a planet determines what its interior is like, how its surface evolves, and even whether it might support life. For Earth, Mars, and other Solar System bodies, we see a pattern: compared to the Sun, they are depleted in elements that condense at relatively low temperatures. This depletion is called a devolatilization trend. Many earlier studies assumed that this Earth-Sun trend could be applied to planets everywhere. But Spaargaren and colleagues point out that this ignores how an element’s volatility depends on the gas composition of the disk where planets form. For example, if oxygen is tied up in carbon monoxide gas, it’s less available to form silicates, changing how elements condense.
Modeling Condensation in Disks
To study this, the team ran computer simulations using the code GGchem, which models chemical equilibrium as a gas cools. They simulated condensation sequences, what solids appear as the gas temperature falls, for 1,000 disk compositions based on observed stars from the GALAH catalogue. They paid special attention to elements important for rocks and metals, like magnesium (Mg), silicon (Si), iron (Fe), and aluminum (Al). These results let them create “parametrisations” that describe how condensation temperatures change depending on disk chemistry.
Shifting Behaviors of Elements
The study found that the critical ratio of carbon to oxygen (C/O) strongly controls which minerals form. At low C/O, planets form in a way like Earth, with silicate-rich mantles and iron cores. As the C/O rises, oxygen becomes scarce, and the condensation behavior of elements changes. For instance, Mg and Si can become more volatile, while Fe remains stable, leading to planets with relatively larger metallic cores. At still higher C/O, exotic minerals like graphite (pure carbon) or even silicon carbide can appear, creating planet types very different from anything in our Solar System.
Predicting Planetary Diversity
By applying these results to stellar abundances, the team identified distinct categories of rocky planets. Earth-like planets emerge in low-C/O disks (C/O ≤ 0.75). Graphite-bearing planets may form in higher C/O disks (C/O > 0.75). They also found an intermediate class (C/O between 0.84 and 1.04) with Mg and Si depletion, which leads to higher fractions of Fe, Ca, and Al. These planets could end up with unusually large iron cores or exotic mineral compositions.
Bigger Implications
This work suggests that the range of rocky planet compositions is broader than previously thought. Rather than assuming all rocky planets mirror Earth’s chemistry, Spaargaren and colleagues show that protoplanetary disk chemistry can push planets toward very different outcomes. Some may end up rich in carbon, some unusually metal-heavy, and others depleted in key rock-forming elements. These predictions provide a framework for interpreting the growing number of exoplanet discoveries.
Source: Spaargaren
