🪐 Surprising rocky worlds revealed around a small star

A team led by the Trottier Institute for Research on Exoplanets at the University of Montreal publishes the most precise study to date of the planetary system around the star LP 98-59. This study notably confirms the existence of a fifth planet located in the habitable zone, where conditions could allow for the presence of liquid water.


Located just 35 light-years from Earth, the small star L 98-59, a red dwarf, hosts three small exoplanets discovered in 2019 by NASA's TESS space telescope and a fourth planet, identified using the radial velocity method with the ESPRESSO spectrograph of the European Southern Observatory (ESO). These four planets orbit very close to their star, all within a distance five times smaller than that separating Mercury from the Sun.

Volcanic worlds, a "sub-Earth" and an "ocean planet"

By reanalyzing a vast set of data from ground-based and space telescopes, a team led by Charles Cadieux, a researcher at the University of Montreal and iREx, was able to determine the sizes and masses of these planets with unprecedented precision.

"These new results give us the most complete picture to date of the L 98-59 system," says Charles Cadieux. "It's a great example of the power of combining data from space telescopes and high-precision ground-based instruments. And the planets in this system are promising targets for observation with the James Webb Space Telescope."

All planets in the system have sizes and masses consistent with rocky worlds. The closest to the star, L 98-59 b, measures just 84% of Earth's diameter and has about half its mass, making it one of the few well-characterized "sub-Earths" to date.

The first two planets could be sites of intense volcanic activity caused by tidal heating, much like Jupiter's moon Io. As for the third, its surprisingly low density suggests it could be an "ocean planet," rich in water, different from all planets in our solar system.

Thanks to refined measurements, researchers also showed that the orbits of the inner planets are almost perfectly circular, an advantage for future atmospheric observations.

"With its diversity of rocky worlds that likely present a range of compositions, the L 98-59 system is a unique laboratory for exploring some of the big questions in our field: what are super-Earths and sub-Neptunes made of? Do planets form differently around small stars? And can rocky planets around red dwarfs retain their atmospheres over time?" adds René Doyon, professor at UdeM, director of iREx and co-author of the study.

A fifth planet in the habitable zone

Rather than requesting new observation time, the team leveraged a rich set of archival data from the TESS space telescope, the HARPS and ESPRESSO spectrographs in Chile, and the James Webb Space Telescope.

L 98-59 f receives an amount of energy comparable to what Earth receives from the Sun, placing it right in the temperate zone (sometimes called the habitable zone) where water could exist in liquid form.

"Finding a temperate planet in such a compact system makes this discovery particularly exciting," emphasizes Charles Cadieux. "This highlights the full diversity of exoplanetary systems and reinforces the interest in conducting studies on potentially habitable worlds around low-mass stars."

Revealing the invisible in existing data

Rather than requesting new observation time, the team leveraged a rich set of archival data from the TESS space telescope, the HARPS and ESPRESSO spectrographs in Chile, and the James Webb Space Telescope.

Scientists used an innovative radial velocity analysis method, developed by the iREx team in 2022, which significantly improves data accuracy. By also using a new technique to finely measure variations in the star's temperature, they were able to better isolate and remove noise due to stellar activity, thus revealing planetary signals with unprecedented clarity.

By integrating these refined measurements with the analysis of transits detected by the Webb telescope, the team doubled the precision of mass and radius estimates for the known planets.

"These techniques were designed to exploit all the hidden potential in archival data," specifies Étienne Artigau, researcher at UdeM and co-author of the study. "It's a great demonstration that improving analysis tools can allow us to push the boundaries of what we know, simply by revisiting already available data."

Next step: Webb

These discoveries place L 98-59 as one of the most interesting planetary systems near Earth for studying the diversity of rocky planets, and potentially for searching for signs of life.

Its proximity, the small size of its star, and the variety of planetary compositions and orbits make it an ideal system for probing planetary atmospheres with the James Webb Space Telescope - observations that have already been initiated by the iREx team.

"With these new results, L 98-59 joins the select group of compact planetary systems close to us whose mysteries we hope to unravel in the coming years," concludes Alexandrine L'Heureux, PhD student at UdeM and co-author of the study. "It's exciting to see this system join TRAPPIST-1 on the list of places that will allow us to better understand the nature and formation of small planets around red dwarfs."