A Planet Made of Magma: How a Sulfurous World 35 Light-Years Away Is Rewriting the Exoplanet Rulebook

For decades, astronomers have sorted small exoplanets into a tidy pair of bins: rocky worlds with thin atmospheres, and water-rich worlds blanketed in deep oceans and ice. A study published today in Nature Astronomy argues that the universe has no interest in such neatness. An international research team, led by the University of Oxford, has revealed that the exoplanet L 98-59 d — a super-Earth orbiting a red dwarf star just 35 light-years away — belongs to neither category. Instead, it appears to be the founding member of an entirely new class: a molten, sulfur-saturated world sustained by a perpetual ocean of magma thousands of kilometers deep [1][2].

"This discovery suggests that the categories astronomers currently use to describe small planets may be too simple," said Dr. Harrison Nicholls, the study's lead author from Oxford's Department of Physics [3].

A World That Shouldn't Fit

L 98-59 d is roughly 1.6 times Earth's radius and 1.64 times its mass, yielding a bulk density of approximately 2.2 grams per cubic centimeter — barely 40% of Earth's 5.5 g/cm³ [4][5]. That anomalously low density has long puzzled astronomers. When L 98-59 d was first characterized, the leading explanation was water: up to 30% of the planet's mass might consist of H₂O, making it a candidate ocean world [5].

The new Oxford-led study upends that interpretation. Using data from the James Webb Space Telescope (JWST) and ground-based observatories, the team detected hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) in the planet's upper atmosphere — signatures that fit poorly with a water world but align strikingly well with a planet whose interior is dominated by molten rock [1][2].

Source: NASA Exoplanet Archive / Nature Astronomy

Data as of Mar 16, 2026CSV

An Ocean of Magma, Not Water

The researchers constructed detailed evolutionary models tracing L 98-59 d's history from formation to its present state, approximately five billion years later. Their conclusion: the planet's mantle is composed of molten silicate — essentially lava — forming a global magma ocean that extends thousands of kilometers beneath the surface. This vast molten reservoir comprises an estimated 70–90% of the planet's interior radius, reaching depths between roughly 4,465 and 5,740 kilometers [2][6].

Within this magma ocean, small crystals of solid rock may be trapped in the turbulent fluid, creating what the researchers describe as "a molten, mushy state" [6]. The planet's metallic iron core, by contrast, is relatively small — a departure from the large iron cores found in Earth and other rocky planets in our solar system.

"We can use computer models to uncover the hidden interior of a planet we will never visit," said Professor Raymond Pierrehumbert, a co-author from Oxford's Department of Physics [3].

Perhaps most remarkably, the magma ocean functions as a massive chemical reservoir. It stores enormous quantities of sulfur deep in the planet's interior over geological timescales, continuously exchanging volatile gases with the atmosphere above. Ultraviolet radiation from the host star, the red dwarf L 98-59, then drives photochemical reactions that produce the sulfur-bearing gases JWST detected in the upper atmosphere [1][2].

Why This Matters: A Third Category of Planet

Until this study, astronomers classifying a planet like L 98-59 d — a small, low-density world with a hydrogen-rich atmosphere — would have placed it into one of two existing categories. It could be a rocky "gas-dwarf," a dense planet wrapped in a primordial envelope of hydrogen and helium. Or it could be a water-rich world, its low density explained by vast oceans and ice layers [1][3].

L 98-59 d fits neither. Its atmosphere is not a simple hydrogen-helium remnant; it is rich in sulfur compounds that require an active geological source. And its internal structure is not dominated by water ice but by a permanent magma ocean that shapes the planet's chemistry from the inside out.

The researchers propose that L 98-59 d is the first recognized member of a broader population of "gas-rich sulphurous planets sustaining long-lived magma oceans" [1]. If they are right, the current exoplanet taxonomy — already strained by discoveries like "mini-Neptunes" and "super-puffs" — will need another major revision.

The L 98-59 System: A Laboratory Next Door

L 98-59 d does not exist in isolation. It is the third planet in a five-world system orbiting L 98-59, a small red dwarf star (spectral class M3V) in the southern constellation Volans [5][7]. The star has about 32% of the Sun's mass and a surface temperature of roughly 3,469 Kelvin — cool and dim by stellar standards, but an ideal target for JWST's infrared instruments.

The system's five confirmed planets span a remarkable range of compositions:

• L 98-59 b, the innermost, is just 84% the size of Earth and half its mass — one of the smallest exoplanets ever measured [5].
• L 98-59 c and e are likely hot, rocky worlds.
• L 98-59 d, the subject of the new study, orbits at 0.049 AU with a period of 7.45 days and an equilibrium temperature of roughly 416 Kelvin [4][5].
• L 98-59 f, confirmed in 2025, orbits in the star's habitable zone with a minimum mass 2.8 times Earth's, making it a prime target for future atmospheric characterization [7].

The diversity within a single system — from a sub-Earth to a magma world to a potentially habitable super-Earth — makes L 98-59 one of the most scientifically valuable planetary systems within reach of current telescopes.

JWST's Golden Age for Rocky Worlds

The L 98-59 d finding arrives amid a surge of JWST discoveries that are collectively transforming our understanding of small, rocky exoplanets.

In 2024, JWST detected what may be the first confirmed atmosphere on a rocky exoplanet: 55 Cancri e, a lava world that showed signs of carbon dioxide or carbon monoxide in its envelope [8]. Later that year and into 2025, a Carnegie-led team used JWST to observe TOI-561 b, an ultra-hot super-Earth that should reach roughly 2,700°C on its dayside but measured at only about 1,700°C — strong evidence for a thick, insulating atmosphere above a magma ocean [9].

These findings challenge a long-held assumption: that small planets orbiting extremely close to their stars cannot hold onto atmospheres. The intense radiation and stellar winds from host stars were thought to strip away any gaseous envelope over time. Instead, JWST is revealing that magma oceans can actively replenish atmospheres by outgassing volatiles from their molten interiors — a process that L 98-59 d appears to demonstrate on a grand scale [1][9].

"The JWST observations from 2024 pointed to the presence of sulphur dioxide, among other sulphur gases, high in L 98-59 d's upper atmosphere," the Oxford team noted. Their evolutionary models then showed how these gases can be sustained over billions of years through continuous chemical exchange between the magma ocean and the atmosphere [1].

Rethinking Planet Formation

The implications extend beyond taxonomy. If magma-ocean worlds like L 98-59 d are common — and the researchers suggest they may constitute "a broader population" — then current models of planet formation and evolution will need updating [1].

Standard models assume that small planets close to their stars lose their volatile elements relatively quickly, baked away by stellar radiation until only a bare rocky core remains. L 98-59 d suggests an alternative pathway: planets that formed with abundant volatile material can retain those volatiles for billions of years if they are locked in a deep magma ocean that slowly releases them into the atmosphere.

Dr. Richard Chatterjee from the University of Leeds, a co-author, noted that their models are "effectively enabling us to turn back the clock and understand how this unusual rocky exoplanet, L 98-59 d, evolved" [6]. The team's simulations show the planet gradually cooling and contracting over its five-billion-year history, with the magma ocean maintaining a melt fraction of approximately 45% even today — meaning nearly half the mantle remains liquid [4].

This persistence is itself remarkable. Earth's own magma ocean, which likely existed in the planet's earliest days, solidified within tens of millions of years. L 98-59 d's magma ocean has endured a thousand times longer, sustained by the planet's composition, orbital dynamics, and the ongoing tidal forces from its host star.

What Comes Next

The discovery opens several immediate research paths. Other planets in the L 98-59 system — particularly the inner worlds b and c — may also harbor magma oceans that could be probed with JWST. And the system's outermost confirmed planet, L 98-59 f, sitting in the habitable zone, becomes an even more compelling target: understanding the magma-rich chemistry of its neighbor could help calibrate atmospheric models for worlds where liquid water, rather than liquid rock, might persist.

Beyond this system, the study suggests astronomers should look for similar sulfur signatures in other low-density super-Earths that have resisted easy classification. With over 6,100 confirmed exoplanets cataloged as of early 2026 and JWST continuing to push the boundaries of atmospheric characterization, the universe's inventory of planet types is likely far from complete [10].

As Nicholls and his colleagues put it, the question is no longer just whether a planet is rocky or watery. It may also be molten — a perpetual ocean not of water, but of fire.