Astronomers Discover New Hydrogen Sulfide-Rich Planet Type

For decades, astronomers have sorted the thousands of worlds beyond our solar system into a tidy handful of categories: gas giants, ice giants, super-Earths, and rocky terrestrials. Now, a bizarre planet drenched in hydrogen sulfide — the compound responsible for the stench of rotten eggs — is forcing scientists to tear up that taxonomy. The discovery of L 98-59 d, a molten world with a permanent magma ocean and a sulfur-choked atmosphere, suggests that the Milky Way harbors an entirely new class of planet that existing models never predicted.

A World That Defies Classification

L 98-59 d orbits a small red dwarf star roughly 35 light-years from Earth in the constellation Volans [1]. First identified by NASA's Transiting Exoplanet Survey Satellite (TESS) in 2019 as part of a multi-planet system, the world initially appeared unremarkable — one of several terrestrial-sized planets making tight orbits around their host star [2]. But when the James Webb Space Telescope turned its infrared gaze on the system in 2024, the data that came back didn't make sense.

The planet measures approximately 1.6 times Earth's radius and 1.64 times its mass, placing it squarely in the size range astronomers call "super-Earths" [3]. Yet its density — roughly 2 grams per cubic centimeter — is startlingly low, less than half of Earth's 5.5 g/cm³ and substantially less dense than what standard rocky-planet models would predict [4]. Something was inflating this world from the inside.

"This discovery suggests that the categories astronomers currently use to describe small planets may be too simple," said Dr. Harrison Nicholls, the lead researcher from the University of Oxford's Department of Physics [1].

Rotten Eggs and Molten Rock

The answer, published on March 16, 2026, in Nature Astronomy, is as dramatic as it is pungent [5]. Using data from JWST and an array of ground-based observatories, a team from the University of Oxford, the University of Groningen, the University of Leeds, and ETH Zurich reconstructed the planet's five-billion-year evolutionary history through advanced computer simulations. What they found was a world unlike anything in our solar system or in the existing exoplanet catalog.

L 98-59 d's atmosphere is laden with hydrogen sulfide (H₂S), along with sulfur dioxide and water vapor [3]. At concentrations that would be overwhelming to any human nose — which can detect hydrogen sulfide at just one part per billion — the planet would, in the words of multiple researchers, "smell like rotten eggs" on a scale almost impossible to imagine [1].

But the atmosphere is just the surface expression of something far more extraordinary happening below. The planet's mantle is almost entirely molten — a global magma ocean extending thousands of kilometers beneath the surface [4]. This vast reservoir of liquid silicate rock acts as a planetary-scale sulfur sink, chemically exchanging volatile gases with the atmosphere over geological timescales.

"The mantle has a viscosity not like water and not like rock — it's more like molasses," Nicholls explained [4].

Why the Magma Ocean Persists

One of the study's most surprising findings is that L 98-59 d remains semi-molten despite being approximately five billion years old — roughly the same age as our own solar system. Two mechanisms keep the planet's interior from solidifying.

First, its thick hydrogen-rich atmosphere creates an intense greenhouse effect, trapping heat that would otherwise radiate into space. Second, tidal forces from neighboring planets in the L 98-59 system generate internal friction that continuously heats the mantle — a process analogous to what keeps Jupiter's moon Io volcanically active [4].

The magma ocean, in turn, serves a critical atmospheric function. By continuously releasing sulfur-bearing gases while also preventing them from escaping into space, it sustains the thick, sulfurous atmosphere that JWST detected. Without this molten reservoir, the planet's close orbit and its star's intense X-ray radiation would likely have stripped the atmosphere away billions of years ago [5].

Neither Gas Dwarf Nor Ocean World

The significance of L 98-59 d extends well beyond its striking chemistry. For years, astronomers studying small exoplanets have relied on two dominant formation models. In one, a rocky core accumulates a modest envelope of hydrogen and helium, producing what's known as a "gas dwarf." In the other, a planet rich in water ice migrates inward from its system's outer reaches, creating a "hycean" ocean world [1].

L 98-59 d fits neither template. Its low density rules out a purely rocky composition, but the sulfur-dominated atmospheric chemistry is inconsistent with a water-rich interior. The researchers argue it likely began life as a sub-Neptune — a class of planet common in the galaxy but absent from our own solar system — before losing much of its original atmosphere over billions of years. What remains is something transitional: a world slowly evolving from a sub-Neptune toward a super-Earth, with its magma ocean acting as a chemical buffer that slows this transformation [5].

"The way we've been doing it in the last few years is suggesting that planets are either very hydrogen-rich gas dwarfs or water-rich worlds," Nicholls said. This planet, he argued, demonstrates that reality is considerably more complex [4].

A Growing Catalog of Sulfurous Signatures

The L 98-59 d discovery arrives amid a broader revolution in exoplanet atmospheric chemistry, driven almost entirely by JWST's unprecedented sensitivity to infrared light.

In July 2024, a team led by researchers at Johns Hopkins University published the first-ever detection of hydrogen sulfide in an exoplanet's atmosphere — the hot Jupiter HD 189733 b, a gas giant 64 light-years away with temperatures exceeding 1,700°F and rain made of molten glass [6]. That Nature paper established hydrogen sulfide as a detectable marker in exoplanet spectroscopy for the first time.

Then, in early 2026, astronomers at UCLA and UC San Diego announced they had detected hydrogen sulfide in the atmospheres of four massive gas giants orbiting the star HR 8799, located 133 light-years away in the constellation Pegasus [7]. These planets — ranging from five to ten times Jupiter's mass — represent the first directly imaged exoplanets where H₂S has been identified.

"Different molecules show up as bands of different wavelengths of light in this data, allowing scientists to precisely identify which elements are present," explained Jerry Xuan, a UCLA postdoctoral researcher and co-first author of that study [7].

The HR 8799 detection was particularly significant because it demonstrated that the sulfur in these worlds was accreted as solid matter from the protoplanetary disk during formation — proving conclusively that they are genuine planets rather than brown dwarfs, which would have formed differently [7].

Source: GDELT Project

Data as of Mar 17, 2026CSV

Rewriting Planetary Taxonomy

With more than 6,250 confirmed exoplanets now cataloged by NASA — up from just 1,000 a decade ago — the pressure on existing classification systems has been building for years [8]. The standard taxonomy groups worlds into four broad types: terrestrial, super-Earth, Neptune-like, and gas giant. But as JWST peels back the atmospheres of these distant worlds one by one, the boundaries between categories are blurring.

L 98-59 d is not an isolated anomaly. The Oxford team suggests it may be the first recognized member of a broader population of sulfurous, magma-ocean planets that have been hiding in plain sight throughout the galaxy [5]. If correct, this would represent the first genuinely new category of exoplanet identified in years — a class defined not by size or orbital distance, but by interior chemistry and geological state.

"While this molten planet is unlikely to support life, it reflects the wide diversity of the worlds which exist beyond the Solar System," Nicholls noted [1].

Source: NASA Exoplanet Archive

Data as of Mar 17, 2026CSV

What Comes Next

The discovery opens several avenues for future investigation. ESA's upcoming Ariel mission, designed specifically for atmospheric characterization of exoplanets, and the PLATO mission, focused on finding habitable-zone planets, will both be capable of surveying large numbers of small exoplanets for similar sulfurous signatures [4].

Meanwhile, continued JWST observations of the L 98-59 system could reveal whether other planets in the system share similar characteristics. The system is known to contain at least five planets, including L 98-59 f, a non-transiting super-Earth on a 23-day orbit inside the habitable zone [2].

Professor Raymond Pierrehumbert, a co-author on the study from Oxford's Department of Physics, emphasized the broader methodological breakthrough: "What's exciting is that we can use computer models to uncover the hidden interior of a planet we will never visit" [1].

The approach — linking telescope observations directly to detailed physical models of planetary interiors — could be applied to dozens of other enigmatic exoplanets whose densities and compositions don't fit neatly into existing categories. In a galaxy that keeps surprising us with its diversity, the rotten-egg planet may be just the beginning of a much larger olfactory — and scientific — awakening.

The Bigger Picture

The discovery of sulfurous magma worlds arrives at a moment when exoplanet science is undergoing its most rapid transformation since the field's birth three decades ago. JWST has moved the discipline from simply counting and measuring planets to actually tasting their atmospheres — detecting specific molecules that reveal how worlds formed, how they evolved, and what drives their present-day chemistry.

Hydrogen sulfide, once considered a minor curiosity in planetary science, has emerged as a powerful tracer of planetary formation and evolution. Its detection on hot Jupiters, super-Jupiters, and now a super-Earth-sized magma world demonstrates that sulfur chemistry is far more prevalent and diverse across planetary systems than anyone expected just a few years ago.

For the thousands of sub-Neptunes and super-Earths scattered across JWST's target lists — the most common type of planet in the galaxy, yet entirely absent from our own solar system — the lesson of L 98-59 d is clear: the universe's planetary palette is richer, stranger, and far more sulfurous than we ever imagined.