A newly discovered exoplanet has a unique feature never observed before: it is completely molten. This discovery could reveal the existence of a new family of planets.
This exoplanet, named L 98-59 d, orbits a red dwarf star located 35 light-years away from us. Data from the James Webb Space Telescope allowed the measurement of its size and atmosphere. Researchers were surprised by this body's low density, as well as by the significant presence of hydrogen sulfide in its gaseous envelope.
Artist's impression of the exoplanet L 98-59 d.
Credit: Mark A. Garlick / markgarlick.com
To understand these characteristics, an international team used computer simulations. These indicate the planet is likely almost entirely made of liquid silicate. That is, a magma ocean extending thousands of miles deep (thousands of kilometers). This immense reservoir can store sulfur for billions of years.
This internal ocean is in constant interaction with the atmosphere. It releases and absorbs gases like hydrogen sulfide, helping to maintain a thick envelope. This relationship explains the particular chemical composition detected by the instruments. Ultraviolet radiation from the parent star also triggers reactions in the upper layers, forming other sulfur compounds.
The lead researcher of the study, quoted in
Nature Astronomy, states that this molten world most certainly cannot harbor life. Nevertheless, it bears witness to the great diversity of environments that exist beyond our stellar neighborhood.
The models indicate that L 98-59 d may have once resembled a larger sub-Neptune, before cooling and losing part of its atmosphere. Studying such magma oceans on distant worlds offers clues about the early phases of rocky planet history, including Earth's.
The role of magma oceans in planetary evolution
Magma oceans represent a common state at the birth of rocky planets. Shortly after their formation, these worlds are often entirely molten, allowing materials to mix and separate according to their density. This process, called differentiation, allows the creation of a distinct core, mantle, and crust.
On Earth, this primordial ocean lasted millions of years before solidifying. Gases trapped in the magma were then released, contributing to the formation of the primitive atmosphere. On other planets, conditions can allow this ocean to remain liquid for much longer. High pressure, significant internal heat, or proximity to the star can keep the mantle molten.
This internal liquid state has major consequences. It allows a continuous exchange of materials between the depths and the surface. So-called volatile elements, like water or sulfur, can be stored in the magma and then released into the atmosphere over time.
By comparing different worlds, scientists can reconstruct how factors like size or distance to the star affect the lifespan of the magma. This helps understand why some planets become Earth-like, and others, like L 98-59 d, follow such a different trajectory.