Researchers have utilized new coupled atmosphere-interior evolutionary models to better understand the formation and history of the exoplanet L 98-59 d. Located 35 light-years away, this super-Earth orbits a red dwarf star and has presented a challenge to conventional classification models. While many small exoplanets are categorized as either rocky gas dwarfs or water-rich worlds, recent analysis indicates that L 98-59 d does not fit strictly into these established frameworks. Instead, the data points toward a unique history defined by a persistent magma ocean that has existed for billions of years.
The findings, published in Nature Astronomy, suggest that the planet possesses a chemically reducing mantle with a significant concentration of sulfur and hydrogen. This mantle composition allows for the long-term retention of volatiles, which are continuously released through degassing processes. This cycle of volatile release is a primary driver behind the atmospheric chemistry observed by the James Webb Space Telescope, specifically the presence of sulfur dioxide produced through photochemistry within a hydrogen-rich background.
L 98-59 d is noted for its exceptionally low bulk density, which sits at approximately 2.2 grams per cubic centimeter. This density is inconsistent with a composition consisting purely of rock and iron, further supporting the theory that the planet hosts substantial amounts of atmosphere-forming volatiles. By employing the PROTEUS modeling framework, scientists simulated the planet's evolution from a molten state, accounting for factors such as mantle crystallization, tidal dissipation, and energy-limited atmospheric escape driven by stellar X-ray and extreme ultraviolet radiation.
The models indicate that the planet's physical state has changed significantly over its estimated 4.94 billion-year lifespan. During its early development, the planet likely maintained a much larger radius, potentially placing it firmly within the sub-Neptune regime before radiative energy losses drove atmospheric contraction and cooling. This evolutionary pathway highlights how internal geological processes, such as magma-ocean degassing, contribute to the observed diversity of exoplanets found in the radius valley.
Ultimately, the study suggests that L 98-59 d serves as a critical case for understanding the physics of planetary formation. Because it lacks a direct analogue in our own Solar System, the planet provides a rare opportunity to observe how secular cooling and atmospheric erosion shape the final composition of super-Earths. The research concludes that the interaction between the interior magma ocean and the atmosphere is a fundamental mechanism that maintains the planet's distinct volatile-rich characteristics over geological time scales.
