Many planets in the universe might be shrouded in thick soot-laden hazes, according to a recent study by scientists at the University of Chicago. Their research suggests that extreme temperatures and intense pressures inside these planets could create “soot factories,” similar to combustion engines on Earth, which produce smog and haze that obscure the planets’ surfaces and atmospheres.
When astronauts take pictures of Earth from space, they see beautiful blue oceans, white clouds, and lush green land. But this clarity might be rare elsewhere. Many distant planets could be covered in dense, smoky clouds, making them look more like giant smog machines than the stunning worlds depicted in science fiction.
Published in The Astrophysical Journal Letters, this study could help explain a mystery that has puzzled astronomers for years. Scientists have identified thousands of exoplanets—planets orbiting stars beyond our solar system—and among them, a common type known as “mini-Neptunes.” These are planets larger than Earth but smaller than Neptune, with thick atmospheres. Interestingly, there’s no direct equivalent of mini-Neptunes in our solar system.
As astronomers have studied these planets more closely—especially with NASA’s James Webb Space Telescope—they’ve noticed something strange. Instead of clear atmospheric signatures, many of these planets appear oddly featureless, cloaked in opaque atmospheres that hide their true nature.
The surprising explanation may come from an unlikely source: internal combustion engines. Jeehyun Yang, a postdoctoral researcher at the University of Chicago and a trained chemical engineer, observed a pattern in the data that reminded him of engine behavior. The relationship between temperature and atmospheric opacity on these planets resembled the patterns seen in gasoline, diesel, and jet engines.
On Earth, when fuel combusts at high temperatures, it creates tiny carbon particles called soot—complex molecules known as polycyclic aromatic hydrocarbons. These black particles dominate emissions from engines and industrial processes. Initially, the temperatures on these exoplanets didn’t seem hot enough to form soot. But Yang and team realized that conditions deep inside a planet’s thick atmosphere—where pressure is high—can generate the necessary heat for soot formation.
The scientists found a close match between the conditions predicted for soot production and what the telescopes observed. They suggest that soot forms deep within the atmosphere and slowly rises, creating a dense haze that blocks light and makes the atmosphere appear opaque from afar.
Most of these worlds are inhospitable to life. Many orbit very close to their stars, experiencing scorching heat, some with oceans of molten rock, or subjected to pressures so intense that materials behave in bizarre ways. Despite this, understanding their atmospheric composition offers important clues about planetary formation and evolution.
The presence of soot in an exoplanet’s atmosphere might reveal details about the original balance of carbon and oxygen during the planet’s formation. These chemical clues could help scientists unravel the history of distant planetary systems and refine their search for worlds that might support life.
This discovery also highlights the value of interdisciplinary research—learning from engine technology on Earth can shed light on how alien worlds behave light-years away.