🛢️ Some planets behave like giant diesel engines

Astrophysicists from the University of Chicago suggest that certain mini-Neptune-sized planets could hide, deep in their atmospheres, actual soot clouds. These particles would arise from chemical reactions very similar to those occurring in our combustion engines.

These mini-Neptunes are planets of intermediate size between Earth and Neptune, very common in our galaxy but still poorly understood. Their exact nature is debated: are they miniature versions of Jupiter, rich in hydrogen and helium? Worlds similar to Uranus and Neptune, teeming with volatiles like water? Or perhaps ocean planets covered by a hydrogen atmosphere? Scientists do not know, because their atmosphere appears opaque, hiding their true composition.


It was while analyzing data from the James Webb Space Telescope that Jeehyun Yang, a former specialist in exhaust gases, recognized a signature that was familiar to him: the flat spectra of mini-Neptunes showed a curve typical of soot from diesel engines. These soots consist of polycyclic aromatic hydrocarbons (PAHs), carbon-based molecules.

These PAHs form when carbon, hydrogen, and oxygen react at high temperature and under high pressure, conditions met in the depths of mini-Neptunes... and diesel engines. Researchers estimate that these reactions create huge soot clouds that rise into the atmosphere through convection, giving that opaque appearance observed by telescopes.

The carbon/oxygen ratio in this soot would change depending on the distance at which the planet formed in its protoplanetary disk. By measuring this ratio, one could determine the origin of mini-Neptunes, which formed more or less far from their star before migrating inward. This would finally allow categorizing the different possible types of these worlds.


This discovery, published in The Astrophysical Journal Letters, shows the importance of an interdisciplinary approach. As Yang notes, this is the first time that petrochemical techniques have been applied to the study of exoplanets. It could also explain why our Solar System contains no mini-Neptunes, while they are so frequent elsewhere.