Rewinding Exoplanetary Clocks: L 98-59 D Opens up Research Into a New Type of Molten Worlds

Key Takeaways

• L 98-59 d likely began with high hydrogen, low oxygen interior.
• Models show planet retains a long-lived magma ocean today.
• Sulfur-rich atmosphere formed partly by stellar photochemistry, not just outgassing.
• Findings suggest a new class of molten super‑Earths in the radius valley.
• Coupled interior‑atmosphere modeling narrows viable formation pathways for L 98-59 d.

Pulse Analysis

L 98‑59 d sits at the heart of the exoplanet radius valley, a region where planets abruptly transition from rocky super‑Earths to gaseous sub‑Neptunes. Its unusually low density and modest size have long puzzled astronomers, prompting questions about whether it formed as a stripped core or retained a volatile envelope. The new study leverages this tension, positioning L 98‑59 d as a natural laboratory for testing competing formation scenarios and highlighting the importance of molten‑planet phases that have been largely overlooked in population‑level analyses.

The authors employ a one‑dimensional, coupled interior‑atmosphere evolution framework that simultaneously tracks magma‑ocean cooling, volatile outgassing, and atmospheric escape under intense stellar irradiation. By iterating hundreds of initial‑condition permutations—varying hydrogen inventory, oxygen fugacity, and mantle melt fraction—they generate forward‑evolved outcomes that can be directly compared to observed bulk density and atmospheric mean molecular weight. Only scenarios with high primordial hydrogen and low oxygen fugacity reproduce the data, implying that L 98‑59 d retained a substantial volatile layer while never fully solidifying, a conclusion reinforced by the presence of sulfur‑bearing gases that require ongoing photochemical processing.

These findings broaden the taxonomy of small exoplanets, suggesting that a subset of radius‑valley bodies may persist as partially molten, sulfur‑rich worlds rather than transitioning cleanly to either rocky or gaseous states. This has practical implications for upcoming missions such as JWST and Ariel, which will probe atmospheric compositions of similar planets. Detecting signatures like H₂S or SO₂ could become a diagnostic for identifying molten interiors, guiding target selection and refining models of planetary evolution across the galaxy.