L98-59d: A New Class of Molten Worlds Discovered by JWST

Astronomers have identified L98-59d, an exoplanet orbiting a red dwarf star 35 light-years from Earth, as a potentially molten world with a sulphur-rich atmosphere. This groundbreaking discovery, made with observations from the James Webb Space Telescope (JWST) and advanced computer simulations, suggests a new category of planet.

This new class is characterized by deep magma oceans and the ability to retain heavy sulphur molecules over billions of years.

Researchers propose that such molten planets may be common and indicate a greater diversity of worlds in the galaxy than previously recognized.

A Fiery Realm: Discovery and Characteristics

L98-59d is approximately 1.6 times the size of Earth and possesses an unusually low density. Initial observations had led astronomers to consider the possibility of a deep ocean of liquid water; however, subsequent analysis revealed characteristics inconsistent with typical rocky or water worlds.

The exoplanet's atmosphere has been observed to contain notable quantities of hydrogen sulphide (H2S), a compound associated with a rotten egg odor, and sulphur dioxide. Surface temperatures are estimated to reach 1,900°C (3,500°F).

According to Dr. Harrison Nicholls, an astrophysicist at the University of Oxford, the planet is likely in a "mushy, molten state," comparable to molasses, with its core potentially also molten.

Conditions on L98-59d are considered unfavorable for life.

Deep Magma Oceans and Sulphur Storage

Researchers from institutions including the University of Oxford, University of Groningen, University of Leeds, and ETH Zurich utilized computer simulations to reconstruct the planet's history over nearly five billion years.

Their findings indicate that L98-59d likely has a mantle composed of molten silicate, similar to Earth's lava, and a global magma ocean extending thousands of kilometers beneath its surface.

This vast molten reservoir is believed to store significant amounts of sulphur over geological timescales. The magma ocean also plays a role in enabling L98-59d to retain a thick hydrogen-rich atmosphere containing sulphur-bearing gases. This retention mechanism is crucial, as such an atmosphere would typically be lost to space due to X-ray radiation from its host star. Chemical exchanges between the molten interior and the atmosphere over billions of years are understood to have shaped the planet's current observable properties.

A Fiery History: Formation and Evolution

Simulations suggest that L98-59d likely formed with a substantial amount of volatile material, possibly resembling a larger sub-Neptune planet. Over billions of years, it is thought to have gradually shrunk as it cooled and lost some of its atmosphere. The combination of deep volatile storage within the magma ocean and ultraviolet (UV) light-driven atmospheric chemistry is proposed to explain the planet's unique characteristics.

Magma oceans represent the universal initial states of all rocky planets, including Earth and Mars, implying that insights into magma ocean physics can inform understanding of our own world's primordial history.

Redefining Planetary Categories

The identification of L98-59d as a sulphur-rich magma ocean world suggests that current astronomical categories for small planets may be oversimplified.

It does not fit existing classifications such as rocky 'gas-dwarfs' with hydrogen atmospheres or water-rich "hycean" ocean worlds, indicating a potential new class of exoplanets rich in heavy sulphur molecules.

The discovery implies that molten planets might be common and highlights the wide diversity of worlds beyond our solar system. Dr. Nicholls noted that some planets located within the traditionally defined habitable zone might, in fact, be molten. The research suggests a need for astronomers to exercise greater caution when classifying exoplanets as potentially habitable.

Professor Raymond Pierrehumbert from the University of Oxford emphasized the capability of computer models to reveal the hidden interiors of distant planets and reconstruct their deep past, uncovering planet types with no equivalents in our solar system.

Publication Details

The findings were published in the journal Nature Astronomy on March 16. Dr. Jo Barstow, a planetary scientist at the Open University who contributed to James Webb observations of L98-59d, indicated that the research offers a plausible explanation for the planet's properties.