L 98-59 b is smaller and lighter than Earth, and it may have an atmosphere made by volcanoes. It is the innermost of five planets around L 98-59, a bright red dwarf about 35 light-years away, and passes in front of its star every 2.25 days. NASA's TESS satellite found it in 2019 along with its siblings L 98-59 c and d, in a study led by Veselin Kostov. Watching those transits, the small dips in starlight as a planet crosses its star, gives a planet's size directly. L 98-59 b is about 0.84 times Earth's width.
In 2021 a team led by Olivier Demangeon used the ESPRESSO spectrograph to measure its mass from the slight wobble it gives its star. At about half the mass of Venus, it was then the lowest-mass planet ever measured by that method. A 2025 study led by Charles Cadieux refined its mass to about 0.46 Earth masses and predicted that its slightly oval orbit flexes it enough to heat its interior, with heat flows comparable to those of Jupiter's volcanic moon Io.
That heating is why astronomers looked for a volcanic atmosphere. Hubble data from 2022 ruled out a clear atmosphere made mostly of hydrogen. In 2025 a team led by Aaron Bello-Arufe observed four transits with the James Webb Space Telescope. A bare rock still fits the data adequately, but an atmosphere of sulfur dioxide, a gas that volcanoes release, is preferred at a statistical confidence of 3.6 sigma, which is suggestive rather than conclusive. If it is real, the team estimates L 98-59 b would need at least eight times as much volcanism per unit mass as Io, and might hide a magma ocean beneath its surface.