An animation of the L 98-59 system from NASA


In an article titled Volatile-rich evolution of molten super-Earth L 98-59 d, published yesterday in Nature Astronomy, a team of scientists described their discovery of a magma oceans and a sulphuring atmosphere on the ‘super-Earth’-sized exoplanet L 98-59 d. The study’s lead author is Harrison Nicholls, a Ph.D. student in physics at the University of Oxford.

The exoplanet L 98-59 d was discovered in 2019 by NASA’s Transiting Exoplanet Survey Satellite. Later observations of the planet were made with the Hubble Space Telescope and the James Webb Space Telescope. L 98-59 d is what astronomers call a “super-Earth” - that is, a planet larger than Earth but smaller than Neptune. These kind of planets appear to be very common in other solar systems, even there are no super-Earths orbiting the Sun in our local solar system.

Since super-Earths are bigger than Earth (the largest rocky planet in our solar system) and smaller than Neptune (the smallest gas planet in our system), it can be difficult for astronomers to infer what these planets are like, at least on the basis of analogy to our solar system. Oftentimes, astronomers believe these super-Earth’s to be “water worlds” covered with oceans or sometimes “gas dwarfs”, smaller versions of the gas giants like Jupiter and Saturn.

However, L 98-59 d appears to be something different. Nicholls believes that the data available on the planet is “inconsistent with both the gas-dwarf and water-world scenarios.” Instead, Nicholls and his co-authors suggest that the observations of L 98-59 d’s atmosphere are consistent with “photochemical production of SO2 [sulfur dioxide] in an H2 [hydrogen] background.” Nicholls also believes that the exoplanet’s interior “comprises a permanent magma ocean, allowing long-term retention of volatiles within its mantle over billions of years.”

The data used in the study is available online via Zenodo.