Astronomers have detected a smelly gas best known for the odor of rotten eggs—hydrogen sulfide—in the atmospheres of four giant planets far beyond our solar system.
While the gas may be unpleasant on Earth, the discovery is exciting for scientists because it helps explain how massive gas planets are born.
The planets orbit a young star called HR 8799, about 133 light-years away in the constellation Pegasus.
Each of these worlds is enormous, ranging from five to ten times the mass of Jupiter, and they travel far from their star—at least 15 times the distance between Earth and the Sun. Because they are so distant and faint, studying them has been extremely challenging.
Using the powerful James Webb Space Telescope, a team from UCLA and the University of California San Diego analyzed the planets’ light to identify the chemicals in their atmospheres.
Every gas absorbs light in a unique way, leaving a kind of fingerprint in the spectrum.
By carefully separating the planets’ faint signals from the much brighter starlight, the researchers confirmed the presence of hydrogen sulfide for the first time on gas giants outside our solar system.
This finding matters because sulfur behaves differently from other common elements such as carbon and oxygen.
At the planets’ great distances from their star, sulfur would have existed as solid material—tiny grains or rocks—rather than gas when the planets formed. That means the planets likely grew by sweeping up solid matter from the disk of dust and debris around the young star.
As the growing planets heated up, those solids vaporized, creating the sulfur gas now seen in their atmospheres.
Scientists have long debated how to distinguish giant planets from brown dwarfs, objects that are too large to be planets but too small to become true stars.
Traditionally, anything more than about 13 times Jupiter’s mass was considered a brown dwarf because it could briefly fuse deuterium, a form of hydrogen. However, some smaller objects blur this boundary, making it unclear whether they formed like planets or stars.
The new results suggest that at least these four massive worlds formed in a planet-like way, by gathering solid material first.
The study also revealed that the planets contain unusually high amounts of heavy elements compared with their star, a pattern also seen in Jupiter and Saturn. This hints that giant planets across the galaxy may form through similar processes, accumulating many different elements in roughly equal proportions.
Beyond solving formation mysteries, the research demonstrates a powerful technique for studying distant worlds. By separating a planet’s light from its star and analyzing its chemical signature, astronomers can learn what these remote atmospheres are made of. For now, the method works best on large gas giants, but future telescopes could apply it to smaller, Earth-like planets.
Scientists hope that within the next few decades, they may capture the first detailed spectrum of a planet similar to Earth and search for signs of life, such as oxygen or ozone. For now, the unexpected discovery of a foul-smelling gas on distant giants is a promising step toward understanding how planets—including our own—come to exist.
