A Lava World With A Sulfur Sky Doesn’t Fit Any Planet Type Scientists Know

About 35 light-years from Earth, a rocky planet harbors a long-lived magma ocean deep in its interior, a churning reservoir of molten rock that researchers believe has persisted for billions of years. Its sky is laced with sulfur gases being forged by sunlight. And according to new research, it was born loaded with so much hydrogen and other lightweight gases that it makes early Earth look positively barren by comparison. L 98-59 d, a world roughly 1.6 times the size of our planet, breaks almost every rule scientists have developed for how small rocky planets are supposed to form and evolve.

For years, astronomers have sorted small rocky worlds beyond our solar system into two broad categories. Either a planet formed with a thick hydrogen-gas envelope and later lost most of it, earning the label “gas dwarf,” or it formed rich in water and goes by “water world.” L 98-59 d, detailed in new research published in the journal Nature Astronomy, fits neither box. Its chemistry points to a third kind of story: a world born saturated in volatile gases, kept perpetually molten by internal heat, and slowly transformed over billions of years into something no existing model predicted.

What brought the planet into focus was NASA’s James Webb Space Telescope (JWST), whose observations suggest an atmosphere rich in sulfur-bearing gases. Those findings sent researchers digging into how such a world could have ended up this way, and the answers proved far more layered than anyone anticipated.

How Scientists Modeled a Molten Planet’s Entire Life Story

Led by Harrison Nicholls of the University of Oxford and the University of Cambridge, the team used a computer modeling framework called PROTEUS to simulate how L 98-59 d might have evolved from birth to the present day. Rather than running a single scenario, they built a grid of 900 separate simulations, each starting with different conditions: how much hydrogen the planet began with, the ratio of sulfur to hydrogen, and the chemistry of the molten interior. Every simulation was then compared against the planet’s current bulk density and the atmospheric composition suggested by JWST.

L 98-59 d has an unusually low density for a rocky planet, far less than a pure rock-and-iron world of its mass should have. That signals it still holds a meaningful amount of lightweight gases. To reproduce that measurement, the researchers had to start the planet with more than 100 times the estimated hydrogen content of early Earth’s mantle. L 98-59 d orbits extremely close to its host star, far closer than Earth is to the Sun, which means it has spent billions of years bathed in intense radiation. High-energy starlight essentially boils the lightest gases off into space, a process called photoevaporation. As the outer atmosphere was stripped away and the hot interior gradually cooled and contracted, the planet shrank from something larger into the smaller world it is today, with both processes contributing to that transformation.

Early in its life, L 98-59 d would have been classified as a “sub-Neptune,” a category of planet slightly larger than Earth but smaller than Neptune. Its gradual shrinkage into what now looks like a super-Earth took several billion years, raising an uncomfortable question: how many worlds currently labeled sub-Neptunes are simply super-Earths in progress, still mid-transformation?

A Permanent Ocean of Molten Rock Beneath a Sulfur Sky

About 45% of L 98-59 d’s mantle is still molten today, and the researchers found this is not a temporary phase. Two forces are keeping it that way. Gravitational forces from its host star continuously squeeze and flex the planet’s interior, generating heat, while a thick atmosphere acts as a heat-trapping blanket that prevents the surface from cooling. A fully solidified mantle is strongly disfavored by the planet’s density measurements, even before factoring in the JWST data.

That subterranean sea of molten rock also drives the planet’s chemistry. Sulfur dissolves more readily into liquid rock than hydrogen does. As the planet slowly cooled over billions of years, sulfur was gradually released from the magma into the atmosphere, while hydrogen, being lighter, escaped into space. Over time, the atmosphere grew progressively more sulfur-rich.

Sulfur dioxide was the gas that really puzzled researchers. It cannot be explained by gases bubbling up directly from below, since it is too heavy to drift upward on its own. Instead, the team found that sunlight is actively manufacturing it in the upper atmosphere. Ultraviolet light splits water vapor molecules into reactive fragments, which then react with hydrogen sulfide rising from below to produce sulfur dioxide. Without that photochemical process, models show sulfur dioxide would be present only in negligible amounts. A similar sunlight-driven mechanism has been identified on a much larger hot Jupiter called WASP-39b, suggesting this kind of atmospheric chemistry may be more widespread among distant rocky planets than anyone expected.

What This Lava World Means for Planet Science

Neither standard model accounts for L 98-59 d’s history. A gas dwarf would have shed its hydrogen far too quickly. A water world requires an oxygen-rich interior chemistry that is flatly incompatible with the sulfur-heavy atmosphere JWST data suggest. Instead, this world followed a third path: born with an enormous reservoir of volatile gases, kept molten by tidal heating and atmospheric insulation, and sculpted over billions of years by processes working in a combination existing models have struggled to capture.

That has real consequences for how astronomers read the exoplanet census. A planet’s size category has long been treated as a fairly stable property. L 98-59 d suggests it can be a moving target, shaped as much by a world’s age and internal conditions as by what it was born with. Planets that look like super-Earths today may have looked like something else entirely a billion years ago.

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