Astronomers from the University of California Los Angeles (UCLA) and the University of California San Diego (UCSD) have detected hydrogen sulfide gas in the atmospheres of four distant gas giant exoplanets. This marks the first time that hydrogen sulfide has been identified in such planets outside our solar system, offering new insights into how gas giants form.
Gas giants like Jupiter and Saturn are composed mainly of hydrogen and helium surrounding a dense core. Traditionally, objects more massive than about 13 times Jupiter’s mass are classified as brown dwarfs because they can ignite deuterium fusion, but this boundary is not well-defined.
“The boundary between star formation and planet formation is quite fuzzy at these middle mass ranges,” said UCLA postdoctoral researcher Jerry Xuan, who is a first co-author of a paper announcing these discoveries published in Nature Astronomy. “The definition that says a brown dwarf is an object more massive than 13 Jupiter masses is fairly arbitrary. It’s not based on knowledge of how planets and stars form.”
Xuan and his colleagues focused on four large gas giant planets orbiting HR 8799, a star located about 133 light-years away in the constellation Pegasus. The smallest of these planets is five times as massive as Jupiter; the largest is ten times as massive. These planets orbit far from their star—the closest one sits 15 times farther from its star than Earth does from the Sun.
“For a long time, it was kind of unclear whether these objects are actually planets or brown dwarfs,” said Xuan.
Using spectral data from the James Webb Space Telescope (JWST), researchers were able to identify hydrogen sulfide in these planetary atmospheres. The detection relied on new data analysis techniques developed by Jean-Baptiste Ruffio, research scientist at UCSD, to extract faint signals from JWST observations. Xuan created detailed atmospheric models for comparison with the spectra.
“Carbon and oxygen in these planets have been studied from Earth-based observations in the past, but they’re not good signatures for solid matter because they can come from both ice or solids in the disk, or from gas,” said Xuan. “But sulfur is unique because at the distance these planets are from their star, it has to be in the solids. There’s no way these planets could have accreted sulfur as gas.”
The discovery suggests that sulfur present today originated from solid material accumulated during planet formation—material that evaporated into gas due to high temperatures inside young planets’ cores and atmospheres.
The ratio of sulfur to hydrogen—and also carbon and oxygen to hydrogen—is much higher than what is found in HR 8799 itself, indicating that planetary composition differs significantly from their host star’s makeup. A similar pattern has been observed with Jupiter and Saturn.
“It’s not easy to explain the uniform enrichment of carbon, oxygen, sulfur and nitrogen for Jupiter, but the fact that we’re seeing this in a different system is suggesting that there’s something universal going on in the formation of planets, that it’s quite natural to have them accrete all heavy elements in nearly equal proportions,” said Xuan.
Ruffio noted HR 8799’s uniqueness: “I think the question is, how big can a planet be? Can a planet be 15, 20, 30 times the mass of Jupiter and still have formed like a planet? Where is the transition between planet formation and brown dwarf formation?”
Xuan added that this research could aid future searches for Earth-like exoplanets by improving techniques used to separate planetary signals from those of their stars over great distances—a process currently limited to studying large gas giants but expected to advance with better telescopes.
“Finding an Earth analog is the holy grail for exoplanet search, but we’re probably decades away from achieving that,” said Xuan. “But maybe in 20-30 years, we’ll get the first spectrum of an Earth-like planet and search for biosignatures like oxygen and ozone in its atmosphere.”
This work was funded by NASA.
UCLA has built its reputation through notable achievements including Nobel laureates and MacArthur Fellows according to its official website. The university operates within a larger academic system while fostering diverse perspectives through its programs (source). Its campus supports extensive research activities (source).
