2026ApJ...997L..29L

Some low-density exoplanets are thought to be water-rich worlds that formed beyond the snow line of their protoplanetary disk, 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.