A New Class of Molten Worlds: Astronomers Discover Sulfur-Rich Exoplanet

Astronomers have identified a unique exoplanet, L 98-59 d, that challenges existing classifications of planetary bodies. Located approximately 35 light-years from Earth, this super-Earth possesses a surprisingly sulfur-rich atmosphere and a likely molten interior, suggesting a new category of molten worlds. The discovery, made using data from the James Webb Space Telescope (JWST) and ground-based observatories, was published in Nature Astronomy on March 16, 2026.

Challenging Planetary Classifications

Previously, modest exoplanets were generally categorized as either rocky “gas-dwarfs” with hydrogen-dominated atmospheres or water-rich worlds covered in deep oceans and ice. L 98-59 d defies both of these classifications. At approximately 1.6 times the size of Earth, its density is too low to be primarily composed of rock and iron, Earth.com reports.

A Magma Ocean and Sulfur’s Role

The key to understanding L 98-59 d lies in its interior. Researchers believe the planet harbors a permanent magma ocean that has persisted for billions of years. This molten layer stores vast amounts of sulfur, which is then exchanged with the planet’s atmosphere. The presence of sulfur-bearing gases, such as hydrogen sulfide, in the atmosphere is a direct indicator of this ongoing process, according to the University of Oxford.

Volatile Retention and Atmospheric Composition

The magma ocean plays a crucial role in retaining volatiles within the planet’s mantle over extended periods. This allows L 98-59 d to maintain a thick, hydrogen-rich atmosphere despite the potential for atmospheric erosion. The in-situ photochemical production of sulfur dioxide (SO2) in a hydrogen background further supports this scenario, indicating a chemically reducing mantle and substantial sulfur and hydrogen content, as detailed in the Nature Astronomy publication.

Implications for Exoplanet Diversity

The discovery of L 98-59 d suggests that our understanding of exoplanet formation and evolution may be incomplete. This planet represents a new evolutionary pathway where volatile-rich atmospheres are sustained by long-term magma ocean degassing, shaped by cooling, erosion, and photochemistry. The findings highlight the diversity of super-Earth and sub-Neptune exoplanets and suggest that many worlds previously thought to be understood may have more complex interiors than initially assumed, EarthSky.org notes.

Key Takeaways

• L 98-59 d is a super-Earth exoplanet located 35 light-years from Earth.
• It possesses a sulfur-rich atmosphere and a likely molten interior.
• The planet challenges existing classifications of exoplanets.
• A permanent magma ocean is believed to store and exchange sulfur with the atmosphere.
• This discovery highlights the diversity of exoplanets and the need for further research.