In a groundbreaking discovery that challenges our understanding of planetary evolution, astronomers have identified a remarkable world that defies conventional classification. The exoplanet L 98-59 d, located approximately 35 light-years from Earth, represents what scientists believe to be an entirely new category of planetary bodies—one that has maintained a vast subterranean magma ocean for nearly the entire history of our solar system. This finding, published in Nature Astronomy, reveals how sulphur chemistry can fundamentally alter the evolutionary trajectory of rocky worlds, keeping them in a perpetual state of molten fury for billions of years.
The discovery emerged from detailed observations by the James Webb Space Telescope combined with sophisticated computer modeling that allowed researchers to peer beneath the planet's surface and reconstruct its 5-billion-year history. What they found challenges the simple categories astronomers have traditionally used to classify small exoplanets, revealing a cosmos far more diverse and complex than previously imagined.
The Primordial Crucible: Understanding Magma Ocean Worlds
To appreciate the significance of this discovery, we must first understand the violent birth of rocky planets. According to planetary formation theory, all terrestrial worlds—including Earth—began their existence as magma ocean planets, with surfaces covered in seas of molten rock heated to thousands of degrees by the energy of colliding planetesimals and radioactive decay. Our own planet experienced this hellish phase approximately 4.5 billion years ago, when the proto-Earth was struck by a Mars-sized body called Theia, creating both our Moon and temporarily returning Earth to a completely molten state.
Over millions of years, Earth's magma ocean gradually cooled and solidified from the outside in, eventually forming the layered structure we know today: a solid inner core of crystallized iron and nickel, surrounded by a liquid outer core, wrapped in a solid rocky mantle, and topped with a thin crust. This specific arrangement, combined with Earth's rotation and the Coriolis effect, generates the convection currents in the outer core that power our planet's protective magnetic field—a crucial shield against harmful solar radiation that has helped preserve life for billions of years.
The element sulphur plays a surprisingly critical role in this process. As a siderophile element—meaning it has a strong chemical affinity for iron—sulphur was dragged down into Earth's core along with sinking iron during our planet's differentiation phase. Scientists estimate that sulphur now comprises approximately 2% of Earth's core mass, which may seem small but represents an enormous absolute quantity. This sulphur acts as a natural antifreeze, lowering the melting point of the iron-nickel alloy and helping maintain the outer core's liquid state, which is essential for generating Earth's life-protecting magnetosphere.
An Unexpected Discovery: The Enigma of L 98-59 d
The exoplanet L 98-59 d was first detected in 2019 by NASA's Transiting Exoplanet Survey Satellite (TESS), orbiting a red dwarf star in the constellation Volans. Initial observations revealed a world slightly larger than Earth, with 1.64 Earth masses and 1.627 Earth radii. However, when researchers led by Harrison Nicholls from Oxford University's Department of Physics calculated the planet's density, they encountered a profound puzzle.
With a density of approximately 2.2 grams per cubic centimeter—only about 40% of Earth's density—L 98-59 d clearly couldn't be a simple rocky planet like our own. Typically, astronomers categorize low-density small planets into two groups: gas dwarfs with rocky cores surrounded by thick hydrogen-helium atmospheres, or water worlds where various phases of water dominate the bulk composition. Yet detailed spectroscopic analysis revealed that L 98-59 d didn't fit comfortably into either category.
"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," explained co-author Raymond Pierrehumbert, Professor in the Department of Physics at the University of Oxford.
Computational Time Travel: Modeling 5 Billion Years of Planetary Evolution
To solve the mystery of L 98-59 d's unusual properties, the research team developed a sophisticated coupled atmosphere-interior evolutionary model that could simulate the planet's entire history from formation to its present state. This computational approach allowed them to essentially travel back in time, testing different initial conditions and evolutionary pathways to determine which scenarios matched current observations.
The modeling revealed a startling picture: beneath L 98-59 d's thick hydrogen-rich atmosphere and a relatively thin mantle of molten silicate (similar to terrestrial lava), lies an enormous magma ocean extending thousands of kilometers deep—far more extensive than anything that existed on early Earth. This subterranean ocean of molten rock has persisted for approximately 5 billion years, maintained in its liquid state by an extraordinarily high concentration of dissolved sulphur.
The planet's atmosphere provided crucial clues to this hidden interior. JWST observations detected significant quantities of sulphur dioxide (SO₂) and hydrogen sulphide (H₂S)—molecules that typically get stripped away rapidly from exoplanet atmospheres, especially around active red dwarf stars. The persistence of these sulphur-bearing gases at the planet's advanced age of nearly 5 billion years demanded an explanation: there must be an ongoing process continuously replenishing the atmosphere with sulphur compounds.
The Sulphur Cycle: A Self-Sustaining System
The answer lies in a dynamic cycle of volcanic outgassing unlike anything occurring on Earth today. As L 98-59 d's deep magma ocean slowly cools and partially crystallizes, dissolved gases—particularly sulphur-bearing compounds—are released and bubble up through the overlying layers to the surface. This process, called degassing, continuously supplies the atmosphere with fresh hydrogen sulphide.
Once in the atmosphere, these H₂S molecules are exposed to intense ultraviolet radiation from the host star, triggering photochemical reactions that convert hydrogen sulphide into sulphur dioxide—exactly matching the spectroscopic signatures observed by JWST. This creates a self-sustaining cycle: the vast magma ocean acts as an enormous reservoir of sulphur that continuously feeds the atmosphere, replacing molecules lost to space and maintaining the planet's distinctive chemical signature.
A New Planetary Class: Implications and Characteristics
The research team's analysis suggests that L 98-59 d represents an evolutionary pathway previously unrecognized in exoplanet science. The planet likely formed as a larger sub-Neptune world with a substantial envelope of hydrogen and other volatile compounds. Over billions of years, it has gradually shrunk through a combination of atmospheric erosion (particularly intense around red dwarf stars) and cooling, arriving at its current state as what the researchers term a volatile-rich molten super-Earth.
Key characteristics of this new planetary class include:
- Extended magma ocean longevity: Unlike Earth's magma ocean which solidified within millions of years, these worlds maintain deep molten layers for billions of years due to high sulphur content lowering melting points
- Continuous atmospheric replenishment: Ongoing volcanic outgassing from the deep magma ocean sustains thick, chemically active atmospheres rich in sulphur-bearing molecules
- Low bulk density: The combination of a thick atmosphere and sulphur-enriched interior results in surprisingly low overall density compared to typical rocky planets
- Photochemical activity: Intense stellar radiation drives complex atmospheric chemistry, particularly the conversion of H₂S to SO₂
- Evolutionary shrinkage: These planets gradually decrease in size over billions of years as they lose atmospheric mass and cool
Broader Implications for Planetary Science and Habitability
This discovery has profound implications for our understanding of planetary diversity and the potential for life beyond Earth. While L 98-59 d itself is almost certainly uninhabitable—with surface temperatures likely exceeding 1,000 degrees Celsius and a toxic atmosphere—the existence of this planetary class reveals that our current taxonomic schemes for exoplanets are overly simplistic.
The research also highlights the critical role of sulphur geochemistry in shaping planetary evolution and habitability. On Earth, the sequestration of sulphur in the core was essential for developing the solid mantle and crust that support plate tectonics—a process many scientists believe is crucial for maintaining long-term habitability. Worlds with different sulphur abundances or distribution patterns could follow radically different evolutionary paths, potentially remaining in magma ocean states indefinitely or developing entirely novel geological regimes.
Furthermore, the discovery underscores the importance of considering a planet's entire evolutionary history when assessing habitability. A world that appears inhospitable today might have passed through habitable phases earlier in its history, or conversely, a currently habitable-looking planet might be destined for dramatic changes.
Future Observations and the Quest for Planetary Diversity
The identification of L 98-59 d as a new planetary type is likely just the beginning. Upcoming missions promise to reveal even greater exoplanet diversity and refine our understanding of planetary categories. The European Space Agency's PLATO mission (PLAnetary Transits and Oscillations of stars), scheduled for launch in 2026, will focus on finding and characterizing terrestrial planets in the habitable zones of Sun-like stars, potentially discovering more unusual worlds that challenge our classifications.
Similarly, ESA's ARIEL mission (Atmospheric Remote-sensing Infrared Exoplanet Large-survey), planned for 2029, will conduct detailed atmospheric studies of approximately 1,000 exoplanets, creating an unprecedented census of exoplanet atmospheric chemistry. This data will help determine how common sulphur-rich magma ocean worlds like L 98-59 d might be and whether they represent a significant fraction of small exoplanets or a rare curiosity.
"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 which exist beyond the Solar System. We may then ask: what other types of planet are waiting to be uncovered?" noted lead author Harrison Nicholls.
Conclusion: Expanding Our Cosmic Perspective
The discovery of L 98-59 d as a representative of a new class of long-lived magma ocean worlds fundamentally expands our understanding of planetary possibilities. It demonstrates that the universe contains worlds following evolutionary pathways with no equivalent in our solar system—planets that remain in states our own world passed through billions of years ago, sustained in perpetual geological youth by the chemistry of sulphur.
As our observational capabilities continue to improve and our theoretical models become more sophisticated, we can expect many more surprises. The cosmos is revealing itself to be far stranger and more diverse than early exoplanet science suggested, with each new discovery adding complexity to our understanding of how planets form, evolve, and potentially host life.
This research, combining cutting-edge observations from JWST with advanced computational modeling, exemplifies the power of modern planetary science to reveal the hidden interiors and deep histories of worlds we can never visit. As we continue to discover and characterize thousands more exoplanets in the coming decades, our taxonomic schemes will undoubtedly become increasingly nuanced, reflecting the true richness of planetary diversity across the galaxy.
