Astronomers have discovered a bizarre exoplanet with a giant underground ocean of magma that traps sulphur and may represent an entirely new class of worlds.

A team of scientists led by researchers at the University of Oxford has uncovered evidence of a previously unknown type of planet beyond our Solar System. This unusual world appears to trap enormous quantities of sulfur deep beneath its surface inside a permanent ocean of molten rock. The research describing this discovery was published today (March 16) in Nature Astronomy.

The planet (known as an exoplanet because it orbits a star outside our Solar System) is called L 98-59 d. It circles a small red star about 35 light-years away from Earth. Observations from the James Webb Space Telescope (JWST), along with data from ground-based observatories, revealed something unexpected. Despite being about 1.6 times the size of Earth, the planet has an unusually low density, and its atmosphere contains large amounts of hydrogen sulfide.

A Strange Exoplanet That Fits No Known Category

Until now, astronomers would have classified a planet like L 98-59 d in one of two ways. It could be considered a rocky “gas dwarf” with a hydrogen-rich atmosphere, or a water-rich world dominated by deep oceans and ice.

However, the new research suggests that L 98-59 d does not match either of these categories. Instead, the planet appears to belong to a completely different class of world dominated by heavy sulfur compounds.

A Planet With a Vast Magma Ocean

To better understand the planet, researchers from the University of Oxford, the University of Groningen, the University of Leeds, and ETH Zurich used sophisticated computer simulations to trace its evolution from shortly after its formation to the present day, covering nearly five billion years.

By combining telescope data with detailed models of planetary interiors and atmospheres, the scientists reconstructed what is likely happening far below the planet’s surface.

Their analysis indicates that the mantle of L 98-59 d is probably made of molten silicate rock, similar to lava on Earth. Beneath the surface lies a global magma ocean stretching thousands of kilometers deep. This massive reservoir of molten rock allows the planet to store vast amounts of sulfur within its interior over geological timescales.

The magma ocean also helps maintain a thick atmosphere rich in hydrogen and sulfur-containing gases such as hydrogen sulfide (H₂S). Normally, radiation from the host star would gradually strip these gases away into space through X-ray-driven atmospheric loss.

Chemical Exchanges Between Atmosphere and Interior

Over billions of years, interactions between the planet’s molten interior and its atmosphere have shaped the chemical signals scientists detect today. These processes help explain the unusual atmospheric composition observed on L 98-59 d.

The researchers propose that this world may represent the first identified example of a larger group of gas-rich, sulfur-dominated planets that maintain long-lasting magma oceans. If that idea is correct, it would suggest that planets throughout the galaxy may be far more diverse than scientists previously believed.

Lead author Dr. Harrison Nicholls (Department of Physics, University of Oxford) said: “This discovery suggests that the categories astronomers currently use to describe small planets may be too simple. While this molten planet is unlikely to support life, it reflects the wide diversity of the worlds that exist beyond the Solar System. We may then ask: what other types of planets are waiting to be uncovered?”

How Sulfur Shapes the Planet’s Atmosphere

Observations from JWST in 2024 revealed sulfur dioxide and other sulfur-containing gases high in the upper atmosphere of L 98-59 d. According to the team’s models, these gases can form when ultraviolet radiation from the host star, the red dwarf L 98-59, drives chemical reactions in the atmosphere.

Meanwhile, the magma ocean beneath the surface acts as a huge storage system for volatile chemicals. It can absorb and release these gases over billions of years after the planet formed. This interaction between deep interior storage and ultraviolet-driven atmospheric chemistry helps explain the unusual properties scientists observe.

Simulations also suggest that L 98-59 d formed with a large supply of volatile materials and may once have resembled a larger sub-Neptune-type planet. Over time, the world gradually cooled and lost part of its atmosphere, causing it to shrink.

Scientists note that magma oceans are believed to be the initial state of all rocky planets (including the Earth and Mars). Because of this, studying magma oceans on distant planets can offer clues about the earliest stages of our own planet’s history.

Reconstructing the History of Distant Worlds

Co-author Professor Raymond Pierrehumbert (Department of Physics, University of Oxford) said: “What’s exciting is that we can use computer models to uncover the hidden interior of a planet we will never visit. Although astronomers can only measure a planet’s size, mass, and atmospheric composition from afar, this research shows that it is possible to reconstruct the deep past of these alien worlds – and discover types of planets with no equivalent in our own Solar System.”

New observations from JWST are already providing an expanding stream of data, and future missions, including Ariel and PLATO, are expected to deliver even more detailed measurements.

The research team plans to apply their simulations to these upcoming observations using machine learning techniques. Their goal is to map the wide range of planetary types beyond our Solar System and connect those worlds to their early histories. Understanding how planets form and evolve could also help scientists determine which environments might be capable of supporting life.

Dr. Richard Chatterjee (University of Leeds/ University of Oxford) said: “Our computer models simulate various planetary processes, effectively enabling us to turn back the clock and understand how this unusual rocky exoplanet, L 98-59 d, evolved. Hydrogen sulphide gas, responsible for the smell of rotten eggs, appears to play a starring role there. But, as always, more observations are needed to understand this planet and others like it. Further investigation may yet show that rather pungent planets are surprisingly common.”