Universe Today
Earth was once a magma ocean world, just as all rocky worlds were early in their development, according to theory. As the very young Earth cooled, only the outer core remained molten, wrapped around a solid inner core and covered by the solid mantle and crust. This arrangement, along with coriolis forces, is what sustains our protective magnetosphere.
Sulphur plays an important role in this. It's a siderophile element, meaning it loves iron. As iron sank to the core of magma ocean Earth, it dragged an enormous amount of sulphur with it. Scientists think that up to 2% of the core is sulphur, which is a huge amount. Since all of this sulphur lowers the melting point of the molten outer core, sulphur plays an important role in habitability.
This relationship between sulphur and magma may be behind an entirely new class of exoplanet.
New research in Nature Astronomy shows that the exoplanet L 98-59 d, discovered by TESS in 2019, is representative of a new class of exoplanets. These planets remain as magma oceans far longer than planets like Earth, and its because of their sulphur content.
The research is titled "Volatile-rich evolution of molten super-Earth L 98-59 d," and the lead author is Harrison Nicholls. Nicholls is from the Department of Physics at Oxford University.
L 98-59 d orbits an M-dwarf star about 35 light-years away. When astronomers used the JWST and other telescopes to study the exoplanet, they found that it has an extremely low density. It has 1.64 Earth masses and 1.627 Earth radii, giving it an approximate density of 2.2 g cm−3. That's only about 40% of Earth's density, indicating that there's something very different about this exoplanet.
Exoplanets with low densities and small sizes like this one are often slotted into one of two categories. They can have rocky interiors with Hydrogen/Helium atmospheres, sometimes called gas dwarfs. Or they can have bulk densities dominated by different phases of water, sometimes called water worlds.
But L 98-59 d doesn't fit comfortably into either definition. The researchers refer to it as a low-density super-Earth, and they used observations and models to figure out how it formed and evolved.
"Here we constrain the possible range of evolutionary histories linking the birth conditions of low-density super-Earth L 98-59 d to recent observations using a coupled atmosphere–interior evolutionary model," the researchers write.
The researchers modelled 5 billion years of the planet's history to understand how it reached its current state. Their work allowed them to look inside the planet as it changed over time.
They found that the planet has a mantle made of molten silicate, which is similar to lava here on Earth. But the real surprise was found under the mantle. Their work showed that L 98-59 d has a vast magma ocean that extends for thousands of kilometers below the mantle. The key to the planet is that this magma ocean contains lots of sulphur, which shapes its atmosphere.
The planet has a thick atmosphere that's rich in hydrogen. It also contains sulphur-bearing atmospheric gases like hydrogen sulphide (H2S). These gases are prone to being stripped away by stars in exoplanet atmospheres, especially around red dwarfs. But at almost 5 billion years old, the H2S remains. Why is that?
Earth's magma ocean lasted for millions of years before it cooled. As it cooled, the magma crystallized and gases were emitted to the surface by outgassing, and sulphur-bearing gases were part of the process.
The same thing is happening on L 98-59 d, but since its magma ocean is so vast, the atmosphere is constantly replenished with sulphur-bearing molecules. This ongoing replenishment explains the JWST's observations from previous research. The space telescope found an atmosphere rich in sulphur dioxide (SO2) and the research shows how it's produced from H2S by UV-induced photolysis from the star.
*This figure from previous research shows JWST NIRSPec data for L 98-59 d. The brown line from Seligman et al. 2024 indicates what a 98% SO2 atmosphere would look like, and the green line shows a self-consistent photochemical model assuming an SO2-dominated atmosphere. Image Credit: Bello-Arufe et al. 2025. ApJL*
"What’s exciting is that we can use computer models to uncover the hidden interior of a planet we will never visit," said co-author Raymond Pierrehumbert, a Professor in the Department of Physics at the University of Oxford. "Although astronomers can only measure a planet’s size, mass and atmospheric composition from afar, this research shows that it is possible to reconstruct the deep past of these alien worlds - and discover types of planets with no equivalent in our own Solar System."
The simulations show that the exoplanet must have formed with a large contingent of volatiles. It was likely larger in the past, more like a sub-Neptune. It shrank over billions of years due to atmospheric loss and cooling, and we see it now as a result of those mechanisms. "Our analysis reveals an evolutionary pathway in which planets host volatile-rich atmospheres sustained by long-term magma-ocean degassing, shaped by secular cooling, atmospheric erosion and photochemistry," the researchers explain.
It's not surprising that this research is uncovering a newly-recognized class of exoplanets. We're really only at the beginning of understanding the exoplanet population, and our understanding of exoplanet types is bound to become more granular and detailed. With more data on the way from missions like PLATO and ARIEL, our exoplanet categories will become more fine-grained. PLATO will study terrestrial planets in the habitable zones of Sun-like stars, while ARIEL will study the atmospheres of thousands of exoplanets in the Milky Way.
"This discovery suggests that the categories astronomers currently use to describe small planets may be too simple," said lead author Harrison. "While this molten planet is unlikely to support life, it reflects the wide diversity of the worlds which exist beyond the Solar System. We may then ask: what other types of planet are waiting to be uncovered?"
