Finding an exoplanet in a star’s habitable zone always generates interest. Each of these planets has a chance, even if it’s an infinitesimal one, of hosting simple life. While the possibility of detecting life on these distant planets is remote, finding them still teaches us about exoplanet populations and solar system architectures.
When TESS found three planets orbiting the M-dwarf L 98-59 in 2019, then a fourth planet in 2021, the detections generated interest. Now that a fifth planet has been detected, a super-Earth in the habitable zone, the system is garnering renewed interest.
The discovery is reported in research that will appear in The Astronomical Journal titled “Detailed Architecture of the L 98-59 System and Confirmation of a Fifth Planet in the Habitable Zone.” The lead author is Charles Cadieux, a researcher at the University of Montreal and Trottier Institute for Research on Exoplanets (IREx). The paper is currently available on arxiv.org.
This figure shows transit data from TESS for the three innermost planets in the system, and radial velocity measurements for the two outermost planets. Image Credit: Cadieux et al. 2025.
L 98-59 is an M3V star, a red dwarf, about 34.5 light-years away. It has about 0.3 solar masses and measures about 0.31 solar radii. Its first three planets, L 98-59 b, c, and d, were found by TESS with the transit method. The other two planets, e and f, were found with the radial velocity (RV) and transit timing variations (TTV) methods.
“These new results paint the most complete picture we’ve ever had of the fascinating L 98-59 system,” said lead author Cadieux in a press release. “It’s a powerful demonstration of what we can achieve by combining data from space telescopes and high-precision instruments on Earth, and it gives us key targets for future atmospheric studies with the James Webb Space Telescope [JWST].”
While the potentially habitable planet is intriguing, the overall architecture of the system might be even more intriguing. The system is a tightly-packed grouping of terrestrial planets with some dramatic compositional differences, despite their close proximity to each other. The system is reminiscent of the TRAPPIST-1 system discovered in 2016/17, which contains seven terrestrial planets. Its discovery generated a wave of interest in the space science and exoplanet community.
“Multiplanetary systems offer a unique opportunity to study the outcomes of planetary formation and evolution within the same stellar environment,” the authors write in their paper. “One hypothesis is that planet formation around metal-rich M dwarfs may favor giant planets in ‘single’ configurations, while lower metallicity (and less massive disks) could lead to multiple rocky planets in stable, compact, and coplanar arrangements.”
Above view of the L 98-59 planetary system. The Habitable Zone is shown in green for runaway/maximum greenhouse (conservative) and pale green for early recent Venus/early Mars (optimistic).Image Credit: Cadieux et al. 2025.
The innermost planet, L 98-59 b, has an Earth-like density, but is only about 84% its mass and half its size. It’s a rare sub-Earth with well-understood parameters. It takes only about 2.25 Earth days to orbit its star, and atmospheric study suggests it might be a very volcanically active world due to tidal heating.
L 98-59 c is also likely volcanic due to tidal heating. It’s about 1.3 Earth radii and two Earth masses, and completes an orbit in about 3.7 Earth days. L 98-59 d has about 1.6 Earth radii, about 1.6 Earth masses, and an orbital period of about 7.4 days. It may be a water world, or hycean world. The fourth planet, L 98-59 e, has about 1.4 Earth radii, a minimum mass of about 2.8 Earth masses, and an orbital period of about 12.8 days.
The newly-detected planet, L 98-59 f, is in the optimistic habitable zone of the star. It has a minimum mass of about 2.80 Earth masses, about 1.4 Earth radii, and follows a 28 day orbit.
One of the interesting things about this system is that they follow near circular orbits. This means they’re amenable to atmospheric spectroscopic studies by the JWST or other telescopes. Observations also show that the three inner transiting planets have increasing water-mass fractions with orbital distance.
This figure shows Mass–radius constraints on the L 98-59 planets (b: green, c: purple, d: red, e: blue, f: teal) with other exoplanets around M dwarfs in the background (gray points). Exoplanet mass on the x-axis and radius on the y-axis. The orange region delimits a degeneracy (H2- or H2O-rich) in composition. “The planets around L 98-59 are seemingly showing diverse compositions,” the authors write. Image Credit: Cadieux et al. 2025.
“With its diversity of rocky worlds and range of planetary compositions, L 98-59 offers a unique laboratory to address some of the field’s most pressing questions: What are super-Earths and sub-Neptunes made of? Do planets form differently around small stars? Can rocky planets around red dwarfs retain atmospheres over time?” said René Doyon, co-author of the study, who is a professor at the University of Montreal and the Director of IREx.
“Finding a temperate planet in such a compact system makes this discovery particularly exciting,” said lead author Cadieux. “It highlights the remarkable diversity of exoplanetary systems and strengthens the case for studying potentially habitable worlds around low-mass stars.”
Exoplanet habitability around low mass M-dwarfs is a contentious idea. Since they’re so dim, their habitable zones are close to the stars. This creates a couple of potential obstacles to habitability as scientists understand it.
Because of their proximity to their stars, these planets may be tidally-locked. It’s difficult to say whether that’s a serious barrier to habitability, or if an exoplanet’s atmosphere would somehow spread the heat around. It’s possible that habitable zones on these planets are limited to a pole-to-pole terminator zone around the planet’s surface that is temperate.
M-dwarfs are also known for their powerful flaring, which can strip atmospheres away. Without an atmosphere, an exoplanet is unlikely to be habitable, though scientists can’t completely eliminate the possibility.
On the other hand, M-dwarfs are extremely long-lived stars that burn their fuel very slowly. That means they offer long-lived stability to any planets in their habitable zones.
This system will no doubt attract further attention from the exoplanet science community. In fact, further study with the JWST is already underway. “The atmospheric characterization of the L 98-59 planets is already underway with JWST, using both transmission and emission spectroscopy,” the authors write. ” Such comprehensive characterization is key to advancing our understanding of planet formation and evolution around low-mass stars.”
“With these new results, L 98-59 joins the select group of nearby, compact planetary systems that we hope to understand in greater detail over the coming years,” says Alexandrine L’Heureux, co-author of the study and Ph.D. student at the University of Montreal. “It’s exciting to see it stand alongside systems like TRAPPIST-1 in our quest to unlock the nature and formation of small planets orbiting red dwarf stars.”