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.
Illustration of the soot line and water ice line structure of protoplanetary disks, and the three chemically distinct planet archetypes. The water ice line is also known as the water snow line or snow line. Compositions of the model rocky planet, soot planet, and soot-water worlds are shown as pie charts, with red, yellow, and blue segments representing the mass fractions of rock, soot, and water, respectively (Table A1). For the soot-water worlds, the pie chart and numbers refer to the end-member dry case and the numbers for the end-member wet case are: 36% rock, 14% soot, and 50% water. The soot line is defined by its sublimation temperature of 500 K under disk conditions (J. Li et al. 2021). The water ice line is placed at 150 K (M. Minissale et al. 2022). The locations of the soot lines and ice lines vary with time and across stellar mass range. For this study, it is the relative locations that matter. Figure credit Ari Gea and Sayo Studio. — astro-ph.EP
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.
Mass-Radius relations for model Earth-like rocky planets (black curves), soot planets (gray bands), and soot-water worlds (blue bands). Panel A is for multi-layer planets and Panel B is for single-layer planets. The shaded bands for the soot planets and soot-water worlds encompass end-member cases where the soot component varies from highly incompressible (diamond) to highly compressible (water ice) (Fig. A3). Shown for comparison are curves for model planets consisting of 50% Earth-like material and 50% water ice (blue dotted curves) along with Earth-like rock planets with 1% by mass H2 envelope (purple band), as well as exoplanets with up to 3 Earth radii and up to 10 Earth masses, and for which masses and radii are known to <20% and <10%, respectively (blue crosses, NASA Exoplanet Archive: https://exoplanetarchive.ipac.caltech.edu.)
Jie Li, Edwin A. Bergin, Marc M. Hirschmann, Geoffrey A. Blake, Fred J. Ciesla, Eliza M.-R. Kempton
Comments: 20 pages, 8 figures, submitted to ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.16781 [astro-ph.EP] (or arXiv:2508.16781v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.16781
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Submission history
From: Edwin A. Bergin
[v1] Fri, 22 Aug 2025 20:25:26 UTC (21,164 KB)
https://arxiv.org/abs/2508.16781
Astrobiology,
