NextFin News - Astronomers have identified a new class of exoplanet that challenges the fundamental taxonomy of worlds beyond our solar system, discovering a "sulfur world" 35 light-years away that has maintained a global magma ocean for nearly five billion years. The planet, designated L 98-59 d, was analyzed using data from the James Webb Space Telescope (JWST) and ground-based observatories, revealing a composition that defies the traditional binary classification of small planets as either rocky "super-Earths" or volatile-rich "water worlds."
The discovery, led by researchers at the University of Oxford and published this week in Nature Astronomy, centers on a world roughly 1.6 times the size of Earth but with only 40% of its density. While such low density typically suggests a thick envelope of hydrogen and helium or a deep global ocean of water, L 98-59 d presents a third, more pungent reality. Its atmosphere is saturated with sulfur-rich gases, including hydrogen sulfide—the compound responsible for the smell of rotten eggs—and sulfur dioxide. This chemical signature is not a remnant of the planet's birth but a product of a massive, enduring reservoir of molten rock that extends thousands of kilometers beneath the surface.
On Earth, the initial magma ocean phase lasted only a few million years before the crust solidified. L 98-59 d has remained in this molten state for billions of years, a feat made possible by its extreme sulfur content. Sulfur acts as a powerful flux, significantly lowering the melting point of silicate rocks and allowing the interior to remain liquid even as the planet ages. This deep magma ocean serves as a planetary-scale storage tank, continuously outgassing sulfur into the atmosphere and replenishing volatiles that would otherwise be stripped away by the intense radiation of its host red dwarf star.
The implications for planetary science are structural. For decades, the "radius gap"—a observed scarcity of planets between 1.5 and 2 Earth radii—has been explained by the loss of hydrogen atmospheres from rocky cores. However, L 98-59 d suggests that many planets in this size range may not be losing their atmospheres at all, but rather sustaining them through internal chemical cycles. This "magma-atmosphere coupling" creates a stable equilibrium that can preserve thick, toxic atmospheres over geological timescales, effectively creating a new category of "molten sulfur worlds."
This discovery forces a recalibration of how we interpret low-density exoplanets. If sulfur-rich interiors are common, many worlds previously flagged as potential "water worlds" or "ocean planets" may in fact be hellish landscapes of molten rock and sulfuric acid. The research team utilized advanced computer simulations to trace the planet's five-billion-year evolution, concluding that L 98-59 d likely began as a larger, Neptune-like planet before shrinking to its current state. The persistence of its molten interior suggests that the thermal history of a planet is dictated as much by its chemical impurities as by its distance from a star.
While L 98-59 d is decidedly inhospitable to life as we know it, it provides a rare window into the "primordial" state of rocky planets. Earth and Mars are believed to have passed through similar magma ocean phases in their infancy. By observing a world where this phase has been "frozen" in time—or rather, kept liquid—scientists can study the chemical exchanges that eventually lead to the formation of crusts, atmospheres, and potentially habitable environments. The James Webb Space Telescope continues to peel back the layers of these distant atmospheres, but the discovery of L 98-59 d serves as a reminder that the most significant secrets of a planet often lie buried deep beneath its surface.
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