Soot Planets instead of Water Worlds

Some low-density exoplanets are thought to be water-rich worlds that formed beyond the snow line of their protoplanetary disc, possibly accreting coequal portions of rock and water. However, the compositions of bodies within the Solar System and the stability of volatile-rich solids in accretionary disks suggest that a planet rich in water should also acquire as much as 40% refractory organic carbon (``soot’’). This would reduce the water mass fraction well below 50%, making the composition of these planets similar to those of Solar System comets. Here we show that soot-rich planets, with or without water, can account for the low average densities of exoplanets that were previously attributed to a binary combination of rock and water. Formed in locations beyond the soot and/or snow lines in disks, these planets are likely common in our galaxy and already observed by JWST. The surfaces and interiors of soot-rich planets will be influenced by the chemical and physical properties of carbonaceous phases, and the atmospheres of such planets may contain plentiful methane and other hydrocarbons, with implications for photochemical haze generation and habitability.

💡 Research Summary

The paper “Soot Planets instead of Water Worlds” challenges the prevailing interpretation that some low-density exoplanets (sub-Neptunes) are primarily composed of rock and water, often termed “water worlds.” The authors propose an alternative hypothesis: these planets may be rich in refractory organic carbon, colloquially dubbed “soot,” potentially with little to no water.

The argument stems from cosmochemical evidence. Observations of Solar System comets and primitive meteorites indicate that volatile-rich regions beyond the snow line (where water ice condenses) also contain a substantial fraction—up to 40% by mass—of solid, carbon-rich organic material. This material, analogous to meteoritic insoluble organic matter (IOM), is refractory and can survive to much higher temperatures (~500 K) than water ice (~150 K), defining a “soot line” within the protoplanetary disk that is closer to the star than the snow line. Consequently, planetary building blocks accreted in different zones lead to three archetypes: rocky planets (inside the soot line), soot planets (between the soot and snow lines, containing rock and soot), and soot-water worlds (beyond the snow line, containing rock, soot, and water).

A key technical contribution is the estimation of the physical properties of this hypothetical “soot” component. By analyzing the correlation between the 1-bar density (ρ₀) and mean atomic number (Z) for a wide range of planetary materials, the authors derived a linear relationship (ρ₀ ≈ 0.317Z). Using a soot composition based on cometary IOM (C:H:O = 100:78:17 atomic ratio, Z=4.17), they estimated its ρ₀ to be about 1.32 g/cm³. To account for uncertainties in its compressibility at high pressures, they bounded its behavior between that of incompressible diamond and highly compressible water ice.

Using these properties, the team calculated mass-radius (M-R) relationships for soot-rich planetary models with different internal structures (fully differentiated vs. fully mixed). The critical finding is that the predicted M-R curves for soot planets and soot-water worlds are nearly indistinguishable from those previously calculated for simple 50% rock - 50% water planets. This M-R degeneracy means that many observed low-density exoplanets, previously interpreted as water worlds, could equally well be explained as soot-rich bodies.

The implications are significant. Soot-rich planets would have distinct interiors, surfaces, and atmospheres compared to water worlds. Their atmospheres are predicted to be rich in methane and other hydrocarbons, influencing climate, photochemical haze production, and ultimately assessments of habitability. The study concludes that soot-rich planets, formed beyond the soot line, are likely a common class of planets in our galaxy and may already be among the targets observed by JWST. It calls for a reevaluation of planetary composition models to incorporate the significant role of refractory carbon, moving beyond the simpler binary rock-water paradigm.