L 98-59 b
A small, volcanic exoplanet with a possible sulfur dioxide atmosphere.
L 98-59 b is an exoplanet that falls between Earth and Mars in size, with a mass just half that of Venus. It circles the red dwarf star L 98-59, which lies 34.6 light-years away in the constellation Volans. The system contains at least four other planets—c, d, e, and f—plus a possible fifth, designated ".06". The planet's discovery was made public on June 27, 2019, through both *The Astronomical Journal* and a NASA press release. At the time, it was the smallest planet found by TESS, a record later broken by LHS 1678 b, and it held the title of the lowest-mass planet measured via radial velocity until Proxima Centauri d took that spot in 2022.
L 98-59 b completes an orbit every 2.25 days, staying so close to its star that it receives 22 times more energy than Earth gets from the Sun. None of the system's four confirmed planets lie within the habitable zone. TESS measured the planet's temperature at 330 °C. In 2022, transmission spectroscopy suggested it either has no atmosphere or one that is opaque due to high-altitude hazes. However, observations with the James Webb Space Telescope's NIRSpec instrument, published in 2025, favor the presence of a sulfur dioxide atmosphere, likely produced by volcanism. This indicates that L 98-59 b experiences at least eight times the volcanic and tidal heating of Jupiter's moon Io.
Quick Facts
- Discovered
- March 2019
- Discovery Method
- Transit
- Eccentricity
- 0.103 · 0.117 · 0.045
- Star
- L 98-59
Facts from the source article.
Lore & Background
L 98-59 b orbits its star every 2.25 days, staying so close that it receives 22 times more energy than Earth receives from the Sun. The planet's temperature, as detected by TESS, is 330 °C. There are at least four confirmed planets in the system (L 98-59 c, d, e, f) and possibly a fifth unconfirmed planet designated ".06"; none of these planets lie in the habitable zone of the host star. In 2022, transmission spectroscopy indicated that the planet has either no atmosphere or an opaque atmosphere with high-altitude hazes. However, transmission spectroscopy observations with the James Webb Space Telescope NIRSpec published in 2025 favor the presence of a sulfur dioxide atmosphere, likely driven by volcanism. This implies that L 98-59 b experiences at least eight times as much volcanism and tidal heating as Io.
Reader's Guide
L 98-59 b is notable as one of the smallest exoplanets discovered by TESS and, for a time, the lowest-mass planet whose mass was measured using radial velocities. Its significance deepened with the 2025 James Webb Space Telescope observations, which suggested a sulfur dioxide atmosphere—a signature that points to active volcanism. The planet's extreme tidal heating, at least eight times that of Jupiter's moon Io, makes it a key target for understanding volcanic processes on exoplanets. The system's multiple planets, though not in the habitable zone, offer a laboratory for studying planetary formation and atmospheric evolution around red dwarfs. The uncertainty about whether the planet has no atmosphere or an opaque haze, later resolved in favor of a sulfur dioxide atmosphere, highlights the iterative nature of exoplanet characterization.
Did You Know?
- L 98-59 b has a mass only half that of Venus.
- It receives 22 times more energy from its star than Earth receives from the Sun.
- The planet's temperature is 330 °C.
More in Transiting Exoplanets, Part 5 1-24
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