An exoplanet 35 light-years away doesn't fit any known category - it may be a new type of world

How do you classify a planet that refuses to fit? L 98-59 d orbits a small red star about 35 light-years from Earth. It's roughly 1.6 times the size of our planet. And according to new research published in Nature Astronomy, it belongs to none of the categories astronomers typically use for small exoplanets.

It's not a rocky gas-dwarf wrapped in hydrogen. It's not a water world with deep oceans and ice. Instead, it appears to be something new entirely: a molten, sulfur-rich planet sustaining a permanent ocean of magma thousands of kilometers deep.

What JWST saw - and what it didn't explain

The James Webb Space Telescope observed L 98-59 d in 2024, along with ground-based observatories. Two things stood out. First, the planet's density is unusually low for its size - too low for a purely rocky composition but not low enough for a traditional gas envelope. Second, its atmosphere contains significant amounts of hydrogen sulfide, the gas responsible for the smell of rotten eggs.

Neither observation fit neatly into existing models. A standard rocky planet shouldn't have that much hydrogen sulfide. A hydrogen-rich gas dwarf shouldn't have such a high density. Astronomers needed a new explanation.

Reconstructing five billion years of planetary history

A team from the University of Oxford, the University of Groningen, the University of Leeds, and ETH Zurich built detailed computer simulations that reconstructed L 98-59 d's evolution from shortly after its formation to the present day - spanning nearly five billion years.

By linking telescope observations to physical models of planetary interiors and atmospheres, they could work backward from what we see today to determine what must be happening deep inside the planet.

Their results reveal that L 98-59 d's mantle is likely molten silicate - essentially a global ocean of lava extending thousands of kilometers below the surface. This magma ocean acts as a massive reservoir, storing extraordinarily large amounts of sulfur deep within the planet's interior over geological timescales.

The magma ocean also helps the planet retain a thick hydrogen-rich atmosphere containing sulfur-bearing gases like hydrogen sulfide and sulfur dioxide. Normally, the X-ray radiation from the host red dwarf star would strip such an atmosphere away over billions of years. But the continuous chemical exchange between the molten interior and the atmosphere replenishes what radiation removes, creating a dynamic equilibrium that has persisted for billions of years.

Sulfur dioxide from above, sulfur storage from below

JWST detected sulfur dioxide high in L 98-59 d's upper atmosphere. The team's models explain this through photochemistry: ultraviolet light from the host star triggers chemical reactions that produce sulfur dioxide from other sulfur compounds. Meanwhile, the magma ocean below buffers and regulates the volatile gases, storing and releasing them in a cycle that has operated since the planet formed.

This combination - deep volatile storage within the interior and ultraviolet-driven atmospheric chemistry - explains both the planet's low density and its sulfur-rich atmospheric signature.

A planet that shrank

The simulations suggest L 98-59 d likely formed with a much larger volatile envelope and may once have resembled a sub-Neptune - a common class of exoplanet larger than Earth but smaller than Neptune. Over billions of years, it gradually cooled, lost some of its atmosphere, and contracted to its current size.

This evolutionary pathway is important because magma oceans represent the universal initial state of all rocky planets, including Earth and Mars. Every rocky world begins molten. Understanding how magma oceans persist and evolve on other planets informs our understanding of how our own world went from a ball of magma to a habitable planet with oceans and atmosphere.

A new category - or just the first member we've found?

Lead author Harrison Nicholls at the University of Oxford suggested that L 98-59 d may be the first recognized member of a broader population of gas-rich, sulfurous planets sustaining long-lived magma oceans. If so, the diversity of worlds in our galaxy may be even greater than current classification schemes allow.

Co-author Raymond Pierrehumbert, also at Oxford, highlighted the method's power: using computer models to uncover the hidden interior of a planet that humans will never visit. Although astronomers can only measure size, mass, and atmospheric composition from afar, this research shows it's possible to reconstruct the deep history of alien worlds.

The team intends to apply their simulations to new data from JWST and upcoming missions like Ariel and PLATO, using machine learning to map the diversity of exoplanets and connect their current properties to their formation histories.

Whether L 98-59 d is genuinely unique or merely the first example of a common planetary type is a question that more observations will answer. But its existence is already a reminder that the categories we've built to describe planets - rocky, gaseous, watery - may be too simple for a galaxy that keeps surprising us.