New Kind of Planet May Lurk in Milky Way: Extreme World of Magma and Sulfur
New observations of L 98-59d, a member of the five-planet system L 98-59, suggest that it is home to a vast global magma ocean that traps sulfur deep down, forming a previously unknown class of alien worlds.
Artist’s impression of the planetary system L 98-59. Image credit: Mark A. Garlick / markgarlick.com.
L 98-59 lies about 34.5 light years away in the southern constellation Volans.
Also known as TOI-175, TIC 307210830, this bright M dwarf is about a third of the Sun’s mass.
The planetary system hosts at least three transiting and two non-transiting exoplanets: L 98-59b, c, d, e and f.
L 98-59d orbits the parent star once every 7.5 days, is about 1.6 times the size of Earth, and emits about four times as much radiant energy as Earth.
In new research, astronomer Harrison Nicholls of the University of Oxford and his colleagues sought to reconstruct the history of this super-Earth from its birth to the present day, a period of almost five billion years.
By directly linking the telescope’s observations to detailed physical models of the planetary interior and atmosphere, they were able to determine what must be happening deep within the planet.
Their results reveal that the mantle of L 98-59d is likely made of molten silicate (similar to lava on Earth), with a global magma ocean extending thousands of kilometers below.
This vast molten reservoir allows the planet to store extremely large quantities of sulfur deep within its interior, over geological timescales.
The magma ocean also helps L 98-59d retain a thick hydrogen-rich atmosphere containing sulfur gases such as hydrogen sulfide.
Normally this would be lost to space over time, due to X-ray radiation produced by the host star.
Over billions of years, chemical exchanges between its molten interior and its atmosphere shaped what telescopes observe today on L 98-59d.
The researchers suggest that L 98-59d may be the first recognized member of a larger population of gas-rich sulfur exoplanets that harbor long-lived magma oceans. If this is the case, the diversity of worlds in our Galaxy could be even greater than previously imagined.
“This finding suggests that the categories astronomers currently use to describe small planets may be too simple,” Dr Nicholls said.
“Although this molten planet is unlikely to support life, it reflects the great diversity of worlds that exist beyond the solar system. We can then ask: what other types of planets are waiting to be discovered?”
The article was published today in the journal Natural astronomy.
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H.Nicholls and others. Volatile-rich evolution of the molten super-Earth L 98-59 d. Nat Astronpublished online March 16, 2026; doi: 10.1038/s41550-026-02815-8
