L 98-59 d: Scientists Discover Unusual Exoplanet With Molten Lava Surface And 'Stinky' Atmosphere

Hyderabad: Scientists have identified a new type of planet beyond our solar system, which happens to store large amounts of sulphur deep within a permanent ocean of magma. Located about 35 light-years from Earth and first discovered in 2019, "L 98-59 d" is a super-Earth exoplanet which has a mass of 2.31 Earths. It orbits a small red M-type star at a distance of 0.0506 AU and takes 7.5 days to complete one orbit.

Recent observations from the James Webb Space Telescope (JWST) and ground-based observatories suggested that the exoplanet has an especially low density and contains significant amounts of hydrogen sulfide in its atmosphere, characterised by a pungent odour that resembles rotten eggs. The findings of the study, led by the University of Oxford, have been published in Nature Astronomy.

An artist’s impression of L 98-59 d in orbit around its host star, the red dwarf L 98-59. (Credit: Mark A. Garlick / markgarlick.com)

Until recently, astronomers would have placed a planet like this into one of two categories: either a rocky “gas dwarf” with a hydrogen-rich atmosphere or a water world covered in deep oceans and ice. However, the new findings reveal that L 98-59 d doesn't fit either description. Instead, it seems to represent an entirely different class of planet—one dominated by heavy sulfur compounds.

The James Webb Space Telescope’s advanced instruments allowed the team of researchers from Oxford, Groningen, Leeds, and ETH Zurich to analyse the planet’s atmospheric composition in unprecedented detail. They used advanced computer simulations to trace the planet’s evolution over nearly five billion years. By combining telescope observations with detailed physical models of planetary interiors and atmospheres, they reconstructed its internal history and current state.

A planet with an ocean of magma

The researchers found that L 98-59 d likely has a mantle made of molten rock, similar to lava on Earth, forming a huge ocean of magma that stretches thousands of kilometres below the surface. This molten layer allows the planet to trap large amounts of sulfur deep inside over billions of years. It also helps the planet keep a thick, hydrogen-rich atmosphere filled with sulfur gases like hydrogen sulfide (H₂S). Normally, such gases would escape into space over time due to the strong X-ray radiation from the planet’s star.

The conditions on L 98-59 d today are said to be the result of billions of years of chemical reactions between its molten interior and atmosphere. Researchers believe it could be the first known example of a new type of planet—one that is rich in gases and sulfur, with a long-lasting ocean of magma. If that’s true, it means there may be far more of these unusual worlds in our galaxy than scientists once thought.

Artistic conception of the James Webb Space Telescope. (Credit: NASA GSFC/CIL/Adriana Manrique Gutierre.)

"This discovery suggests that the categories astronomers currently use to describe small planets may be too simple... We may then ask: what other types of planet are waiting to be uncovered?" said lead author Harrison Nicholls from the Department of Physics, University of Oxford.

How sulphur shapes the planet

JWST observations from 2024 pointed towards the presence of sulfur dioxide and other sulfur gases in the upper atmosphere of L 98-59 d. Researchers showed these gases form through ultraviolet-driven chemical reactions from its star, while the planet’s deep magma ocean stores and releases them over billions of years. Simulations suggest L 98-59 d began with abundant volatile material, resembling a sub-Neptune, but gradually shrank as it cooled and lost atmosphere.

Since magma oceans are the starting point for all rocky planets, studying them offers insights into Earth’s early history. With new data from JWST and upcoming missions like Ariel and PLATO, scientists plan to use machine learning to explore planetary diversity, evolution, and potential habitability.