'Mini-Neptune' exoplanets may have smoggy atmospheres similar to diesel exhaust

'Mini-Neptune' exoplanets may have smoggy atmospheres similar to diesel exhaust Lisa Lock Scientific Editor Robert Egan Associate Editor The astronauts circling Earth on the Artemis mission sent back beautiful clear photos of the continents, clouds, and oceans. But we might be the exception. Many planets in the universe may be hazed in clouds of soot, according to a new study by University of Chicago scientists. Their analysis explains a curious trend seen by astronomers training telescopes on distant planets beyond our own solar system. Many of these worlds had atmospheres that returned strangely featureless readings. According to the study, this may be because high temperatures and crushing pressures on these planets combine to make "soot factories," much like combustion engines here on Earth, which shroud the planets in smog. "It's like you have a natural diesel engine in the deep atmosphere of a planet," said UChicago postdoctoral scholar Jeehyun Yang, first author on the paper published May 18 in The Astrophysical Journal Letters. A clue from another field As scientists catalog the planets in distant star systems, they have found more than a third of them are a distinct type that has no equivalent near us. These planets, which scientists have labeled as "mini-Neptunes," are a little larger than Earth but smaller than Neptune, and are shrouded in thick atmospheres. But that's the end of what we definitively know about them. Since we cannot see faraway planets directly and must use clever workarounds to learn about them, more mysteries keep popping up as we build on our knowledge. One such mystery: As the powerful new James Webb Space Telescope has added more and more information about these mini-Neptune planets, a strange but persistent curve in the data appeared. Planets in a particular temperature range seemed to have atmospheres that were very opaque. No one knew for sure what might be in these atmospheres to cause these readings. But that curve rang a bell in Yang's mind. Yang had done his Ph.D. working on combustion engines as a chemical engineer. He had seen thousands of these curves. Jet engines, diesel engines, gasoline engines—they all produced a similar curve as you varied the temperatures at which they were burning. "When you burn these fuels you get black smoke, and if you look at the particles with advanced microscopes you see these beautiful honeycomb-like structures," Yang said. These honeycombs are a class of molecules known as polycyclic aromatic hydrocarbons, and they result when hydrogen, carbon, and oxygen interact at high temperatures. Combined, they make up the soot in truck exhaust or a car's engine oil filter. These temperatures didn't exactly match those of the planets. But Yang and his collaborators knew the atmospheres on these planets are thick and heavy. They realized that deeper down, the pressure and temperatures would be higher—and those temperatures would match the conditions to produce soot. "The peak exactly matches," Yang said. "All of the current observations for planets match with our framework." The haziness may well be clouds of soot produced from deeper within the planets that floats to the surface. 'A great case study' To be clear, these soot-filled atmospheres would make an already distinctly uninhabitable planet even less attractive to humans. These mini-Neptunes orbit so close to their stars that they are swelteringly hot, and the surfaces are probably covered in seas of magma or under such high pressures that they harden like a diamond. But the finding does intrigue scientists, because the ratio of carbon and oxygen on these planets can likely offer insight into how and where the planets originally formed in their solar systems. The amounts of soot could serve as a proxy to measure these ratios more precisely—and thereby learn more about planetary formation, including more clues to narrow the search for habitable planets. "As far as I know, this is the first time anyone has applied chemical engineering to the field of exoplanet study," Yang said. "I think it's a great case study that shows why having people from all different backgrounds can help us untangle these mysteries." Publication details Jeehyun Yang et al, Sub-Neptunes as Soot Factories: Deep Atmosphere Hydrocarbon Formation and Quenching as the Origin of Sub-Neptune Aerosol Trends, The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/ae6914 Journal information: Astrophysical Journal Letters Provided by University of Chicago