The latest observations show that on an exoplanet about 35 light-years away from the Earth, "the ground is really a sea of magma." The research team believes that the planet named L 98-59 d is only about 1.6 times the size of the Earth, but has a mantle filled with silicate lava and an unusually sulfur-rich interior and atmosphere. It may represent a new type of "sulfur-rich magma ocean world" that has never been formally identified before.
The relevant results have been published in the journal Nature Astronomy, which was launched on March 16. Some of the data come from joint observations of the James Webb Space Telescope (JWST) and ground observatories. Harrison Nichols, the paper's lead author, who completed the research as a PhD student at the University of Oxford and is now a postdoctoral fellow at the University of Cambridge, told Refractor that the discovery shows "there is still so much we don't know about how planets form and evolve." He emphasized that the types of planetary environments in the Milky Way are far more diverse than what the current classification system presents, and this diversity must be fully taken into account when discussing planets in the habitable zone.
A key feature of L 98-59 d is that its mantle is similar to silicate lava from volcanic eruptions on Earth, but expanded into a "global magma sea" covering the planet's surface and storing large amounts of sulfur at depth. The researchers concluded that the planet was most likely born in a protoplanetary disk environment richer in sulfur than in the solar system. From the perspective of planet formation theory, this means that in the Milky Way, there may be many terrestrial planets with elemental compositions that are completely different from the Earth, and even sulfur-dominated planets, which will lead to the concept of new types of rocky planets such as "sulfur worlds."
What’s even more puzzling is that the planet seems to have managed to retain a hydrogen-rich, extremely high-pressure atmosphere for billions of years despite the constant “blow” of high-energy radiation from its star. Generally speaking, rocky planets like the Earth will gradually lose light volatile components such as hydrogen and sulfur during the evolution process, but L 98-59 d violates this "convention", forcing scientists to resort to high-precision numerical simulations to reconstruct its evolutionary history.
Models show that the planet was hotter and more "expanded" in its early days, and its appearance was closer to a "sub-Neptune". It then gradually cooled and contracted over the long years, but the overall density was still low, pointing to a thick and high-pressure atmospheric package. This hydrogen-rich, high-pressure atmosphere will make the outer layer of the planet highly opaque and produce an extreme greenhouse effect similar to Venus. Under the combined action of stellar radiation and tidal heating, the "primordial magma sea" on the planet's surface will be maintained in a liquid state for a long time. The research team pointed out that this mechanism of "locking" the magma sea by a thick atmosphere, moderate radiation and tides has not been fully considered by the existing planet classification framework.
Previously, the astronomical community has discovered magma ocean worlds near other stars, such as 55 Cancri e, but these planets are often close to the star, have extremely short orbital periods, and mainly rely on the strong radiation of the star to heat the surface. In contrast, the radiation received by L 98-59 d is relatively mild, but it relies on the triple mechanism of "atmosphere-radiation-tide" to jointly maintain the magma sea, presenting a new stable model. This has led researchers to wonder whether the current classification of "super Earth", which uses planet size as the only classification criterion, is no longer enough to describe these groups of planets with very different internal structures and compositions.
On a planetary chemistry level, L 98-59 d, although extremely hot and unsuitable for known life forms, could still provide scientists with important clues about the role of sulfur in planetary systems. Sulfur can participate in a variety of life-related geochemical cycles under appropriate conditions. The research team used the model to infer that the abundance of sulfur in the birth environment of this planet was significantly higher than that in the solar system, providing a theoretical reference for the future search for smaller and milder "sulfur-rich Earth-like planets".
In the next few years, scientists plan to continue to search for more magma ocean worlds similar to L 98-59 d through JWST, and look forward to the upcoming "Infrared Remote Sensing Survey of Exoplanet Atmospheres" (ARIEL) mission implemented by the European Space Agency to systematically sort out the differences in the composition and internal structure of different super-Earths from larger samples. Nichols said that by modeling the entire super-Earth population and comparing it with the current exoplanet census data, it is expected to identify multiple "subcategories" with different compositions and structures, and correlate them with different planet formation and evolution paths.
The research team is also using machine learning methods to build a more complex planetary evolution simulation framework to absorb more new data from space telescopes and large-scale sky survey missions in the future. In their view, L 98-59 d is just the beginning of many "outlier" planets, and these worlds that deviate from the traditional paradigm will in turn push scientists to rewrite the basic picture of planetary diversity, habitability, and potential habitats for life.