Led by researchers from the Trottier Institute for Research on Exoplanets (IREx) at Universite de Montreal, the study confirms that all five known planets in the L 98-59 system are terrestrial in nature. Among them is L 98-59 b, a rare sub-Earth measuring just 84% of Earth's size and half its mass. The third planet in the system likely contains a significant amount of water, suggesting it may be a "water world." The two innermost planets may experience intense volcanic activity driven by tidal heating, similar to Jupiter's moon Io.

"These new results paint the most complete picture we've ever had of the fascinating L 98-59 system," said lead researcher Charles Cadieux. "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]."

The fifth planet, L 98-59 f, was detected using radial velocity techniques with data from the HARPS and ESPRESSO spectrographs. Though it does not transit its star, it receives a similar amount of stellar energy as Earth, placing it squarely within the system's habitable zone, where liquid water could exist.

"Finding a temperate planet in such a compact system makes this discovery particularly exciting," said Cadieux. "It highlights the remarkable diversity of exoplanetary systems and strengthens the case for studying potentially habitable worlds around low-mass stars."

Rather than collect new observations, the team relied on existing datasets from NASA's TESS mission, ESO's HARPS and ESPRESSO instruments, and JWST. They applied novel analysis methods developed at IREx to isolate planetary signals with greater precision than ever before. This approach doubled the accuracy of known planet mass and radius estimates.

"We developed these techniques to unlock this kind of hidden potential in archival data," said co-author Etienne Artigau. "It also highlights how improving analysis tools allow us to improve upon previous discoveries with data that is just waiting to be revisited."

The unique characteristics of the L 98-59 system-its proximity, star size, and variety of planet types-make it a prime target for future studies with JWST.

"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," said Alexandrine L'Heureux, IREx Ph.D. student and study co-author. "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."

Research Report:Detailed Architecture of the L 98-59 System and Confirmation of a Fifth Planet in the Habitable Zone

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