In the deep cold darkness beyond our solar system, Oxford-led astronomers have discovered a world that defies easy classification: a small, rocky world underlain by molten rock thousands of kilometers below its surface. The world, named L 98-59d, orbits a small star 35 light-years away from Earth as part of a small, closely clustered system, but in terms of what’s happening beneath its surface, this world is in a class of its own. Using complex computer modeling combined with telescope observations, Oxford-led astronomers have determined through a study published in Nature Astronomy that this world is dominated by a long-lived magma ocean, filled with sulfur, which helped harbor a dense atmosphere composed primarily of hydrogen gas. The world itself is part of a new class of “magma ocean” worlds, which may be more common than astronomers long thought.
L 98-59: A rocky exoplanet covered in a global magma ocean
L 98-59d, a rocky exoplanet slightly larger than Earth, has conditions more akin to a permanent lava world than the familiar, cold, crusty land we know of. Inside, the mantle is composed primarily of molten silicate, as seen in Earth’s volcanoes, forming a global “magma ocean” that reaches almost to the core. Surface temperatures can reach 1,900 °C, causing the rock to be in a “mushy” state rather than solid or liquid.Unlike Earth, which cooled over billions of years to form a stable crust and tectonic plates, L 98-59d has maintained a molten state for a large portion of its lifetime. Its molten state means that the planet is able to retain large amounts of sulfur in its interior. From here, sulfur gradually comes to the surface in the form of hydrogen sulphide gas. The “sulphur-rich lava world” is a unique combination that we don’t see anywhere else in our solar system.
How Oxford scientists discovered the planet was molten
Scientists didn’t actually see inside the planet. Instead, they employed sophisticated computer models that combined observations from telescopes with models of the planets’ interiors and atmospheres. From a period not long after the planet began forming, about five billion years ago, they were able to explore what effect time has on its interior. By comparing these predictions to data from observatories such as the James Webb Space Telescope, they were able to determine that a magma ocean existed.Physics.Lead author Dr Harrison Nicholls, from Oxford’s Department of Physics, in the study of the volatile-rich evolution of the molten super-Earth L98-59d, said the discovery “suggests that the categories astronomers currently use to describe minor planets may be too simplistic.” Co-author Professor Raymond Pierrehumbert said the work shows how “computer models can uncover the hidden interior of a planet we will never see”, turning remote data into a picture of oceans of molten rock and strange chemistry.
Why does this change our view of exoplanets?
The structure of L 98-59d also likely prompts astronomers to rethink the way they classify “rocky” or “super-Earth” planets. A planet may resemble Earth in terms of mass and size, but it may be permanently molten beneath a dense atmosphere, making it less habitable than it seems. Sulphur-based gases, including the well-known smell of rotten eggs from hydrogen sulfide, may be indicative of other planets of molten type.As Dr. Richard Chatterjee, a planetary physicist at the Universities of Leeds and Oxford, explained, “We need to make more observations to determine how common these ‘sharp planets’ really are, but initial findings suggest they may be surprisingly common.” What does this mean for the search for habitable planets? The lesson here is to be cautious. A rocky planet that appears habitable may, in fact, be a burning molten sulfur world, not a water-based world suitable for life.
What this means for the future of space science
The research, published in Nature Astronomy, signals a shift in terminology from basic descriptors like “rocky” and “oceanic” to more detailed and physics-based nomenclature for exoplanets, including “magma ocean world” and “sulfur lava planet.” This is important because it allows scientists to better understand observations made by powerful telescopes and plan appropriately targeted follow-up observations. It also serves to remind us that our universe is home to planets that don’t exist in our world, including ones dominated by scorching lava and ones we can only dream about.For the public, the idea of ​​a world whose surface is actually a continent-scale ocean of glowing rocks, and a thick, acrid atmosphere, makes one realize how alien and yet familiar exoplanets can be. The discovery of L 98-59 d is not just a scientific achievement; It’s a step toward having a more honest conversation about what another Earth really means.
