Webb Detects Hydrogen Sulfide Gas on Three Super-Jupiters

Astronomers using the NASA/ESA/CSA James Webb Space Telescope have for the first time identified hydrogen sulfide gas in the atmospheres of three gas giant exoplanets orbiting HR 8799, a 30 million-year-old star in the constellation Pegasus. They discovered that the sulfur must have come from a solid material present in the planets’ native disks.

Artist’s rendering of the HR 8799 planetary system at an early stage in its evolution, showing the planet HR 8799c, a disk of gas and dust, and inner planets (Dunlap Institute for Astronomy & Astrophysics/Mediafarm)

HR 8799 resides approximately 129 light years away in the constellation Pegasus.

The star hosts a huge debris disk and four super-Jupiters: HR 8799b, c, d and e.

The smallest planet is five times more massive than Jupiter and the largest, ten times more.

The planets are far from their star – the closest one is 15 times further away than Earth is from the Sun.

Unlike most exoplanet discoveries, which are inferred from data analysis, these worlds are directly visible using ground-based telescopes.

“HR 8799 is somewhat unique because, so far, it is the only system imaged with four massive gas giants, but there are other known systems with one or two even larger companions whose formation remains unknown,” said Dr. Jean-Baptiste Ruffio, an astronomer at the University of California, San Diego.

Using Webb’s unprecedented sensitivity, Dr. Ruffio and his colleagues measured in detail the chemical composition of three planets orbiting HR 8799: HR 8799c, d and e.

Planets are about 10,000 times fainter than their star, and to extract the faint signal from the Webb data, researchers developed new data analysis techniques.

“The carbon and oxygen of 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 disk solids, or from gas,” said Dr. Jerry Xuan, a postdoctoral researcher at the University of California, Los Angeles and Caltech.

“But sulfur is unique because, at the distance between these planets and their star, it must be present in solids.”

“It is impossible that these planets accumulated sulfur in the form of gas.”

The discovery of hydrogen sulfide means that sulfur accumulated, or accumulated, as solid matter from solids already present in the disk around the star from which the planets were born.

These solids were swallowed up during the formation of the planet, and because the core and atmosphere of the young planet were extremely hot, the solids evaporated into the sulfur gas present today.

The ratio of sulfur to hydrogen, as well as that of carbon and oxygen to hydrogen, is much higher than that found in the star; the composition of the planets must therefore be very different from that of the star.

The same puzzling pattern of uniform enrichment of heavy elements is also found in Jupiter and Saturn.

“It is not easy to explain Jupiter’s uniform enrichment of carbon, oxygen, sulfur and nitrogen, but the fact that we are observing this in a different system suggests that there is something universal going on in the formation of planets, that it is completely natural for them to accumulate all heavy elements in almost equal proportions,” Dr Xuan said.

According to the authors, their discovery would facilitate the search for exoplanets similar to Earth.

“The technique applied here, which allows researchers to visually and spectrally separate the planet from the star, will be useful for studying exoplanets located at large distances from Earth in a clear and detailed manner,” said Dr Xuan.

“The method is still limited to the study of gas giants, but eventually, as telescopes become larger and instruments improve, scientists will be able to apply this type of technique to study Earth-like planets.”

“Finding an Earth analogue is the holy grail for exoplanet research, but we are probably decades away from achieving it.”

“But maybe in 20 to 30 years we will obtain the first spectrum of an Earth-like planet and look for biosignatures like oxygen and ozone in its atmosphere.”

The results were published on February 9, 2026 in the journal Natural astronomy.

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JB. Ruffio and others. Uniform enrichment of Jupiter-like metals in a system of several giant exoplanets. Nat Astronpublished online February 9, 2026; doi: 10.1038/s41550-026-02783-z