Hydrogen sulfide is what makes rotten eggs stink. Its detection in the HR 8799 system offers clues to how distant gas giants formed. A UCLA and University of California San Diego team analysed three of the system’s four planets: HR 8799 c, d and e. Hydrogen sulfide was detected in c and, at lower significance, d. It is not the first time the gas has been found on an exoplanet: a 2024 study of HD 189733 b had already identified it. The researchers say the technique they used to identify the gas could eventually help the search for life on other planets too.
Gas giants, such as Jupiter and Saturn in our own solar system, are large planets made mostly of helium and/or hydrogen around a dense core, but they have a bit of an identity crisis. Planets form in the swirling disk of dust and gas around a newborn star. If the planet is about 13 times more massive than Jupiter, deuterium fusion occurs, a process that ignites a star-like object known as a brown dwarf, which straddles the line between planets and stars.
But astronomers have also identified brown dwarfs smaller than 13 Jupiter masses.
“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 is not based on knowledge of how planets and stars form.”
Xuan and colleagues at UCSD studied the three inner planets of HR 8799, a four-planet system approximately 133 light-years away in the constellation Pegasus. The outer planet, b, was not part of their Nature Astronomy paper. Across the four-planet system, mass estimates range from roughly five to ten times Jupiter’s mass. The planets are far from their star: the closest one is 15 times farther away than Earth is from the Sun.
“For a long time, it was kind of unclear whether these objects are actually planets or brown dwarfs,” said Xuan.
The UCLA and UCSD team used spectral data gathered by the James Webb Space Telescope (JWST) to study the three planets’ atmospheres and identify hydrogen sulfide in c and d.
Different molecules show up as bands of different wavelengths of light in this data, allowing scientists to precisely identify which elements are present.
The planets are about 10,000 times fainter than their star, and to extract the weak signal from JWST’s data, first co-author Jean-Baptiste Ruffio, a research scientist at UCSD, developed new data analysis techniques. Xuan, a 51 Pegasi b Fellow at UCLA, created detailed atmospheric models that could be compared with the JWST spectra to see if sulfur was present.
The sulfur measurements support the interpretation that these planets accumulated solid material from the disk around their star as they formed. In that picture, the solids were gobbled up as the planets formed, and because the young planets’ cores and atmospheres were extremely hot, they evaporated into the sulfur gas present today.
“Carbon and oxygen in these planets have been studied from Earth-based observations in the past, but they are 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 is no way these planets could have accreted sulfur as gas.”
The ratio of sulfur to hydrogen, as well as that of carbon and oxygen to hydrogen, is much higher than what is found in the star. The composition of the planets therefore has to be quite different from that of the star. The same puzzling pattern of uniform enrichment in heavy elements is also found in Jupiter and Saturn.
“It is not easy to explain the uniform enrichment of carbon, oxygen, sulfur and nitrogen for Jupiter, but the fact that we are seeing this in a different system is suggesting that there is something universal going on in the formation of planets, that it is quite natural to have them accrete all heavy elements in nearly equal proportions,” said Xuan.
Ruffio said HR 8799 is somewhat unique because, thus far, it is the only imaged system with four massive gas giants, but there are other known systems with one or two even larger companions and whose formation remains unknown.
“I think the question is, how big can a planet be?” he said. “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 said that the research will help the search for Earth-like exoplanets. The technique applied here, which lets researchers visually and spectrally separate the planet from the star, will be useful for studying exoplanets at great distances from Earth in clear detail. The method is still limited to studying gas giants, but eventually, as telescopes get bigger and as instruments improve, scientists will be able to apply this kind of technique to study Earth-like planets, Xuan said.
“Finding an Earth analog is the holy grail for exoplanet search, but we are probably decades away from achieving that,” said Xuan. “But maybe in 20-30 years, we will get the first spectrum of an Earth-like planet and search for biosignatures like oxygen and ozone in its atmosphere.”
