Astronomers Discover TOI-5573 b: A Miniature Saturn Orbiting a Red Dwarf Star

Astronomers have recently confirmed the existence of exoplanet TOI-5573 b, a miniature Saturn-like planet orbiting a red dwarf star located approximately 606 light-years from Earth. This discovery, made on June 26, 2025, adds a significant example to the limited catalog of giant exoplanets found around M-dwarf stars, contributing to ongoing research regarding planetary formation theories.

TOI-5573 b, which measures 0.87 times the radius of Jupiter and has a mass of just 0.35 times that of Jupiter, exhibits a bulk density characteristic of Saturn-like planets. It completes an orbit around its host star, TOI-5573, every 8.79 days at a distance of approximately 0.07 astronomical units. This close proximity results in an equilibrium temperature of about 528 Kelvin, which is relatively cool for a gas giant located so near its star.

The confirmation of TOI-5573 b was achieved through a combination of NASA's Transiting Exoplanet Survey Satellite (TESS) photometry and radial-velocity observations using the Habitable-zone Planet Finder and NEID spectrographs. Rachel B. Fernandes, an astrophysicist at Pennsylvania State University, emphasized the importance of this finding, noting that it adds to the sparse catalog of Giant Exoplanets around M-dwarf Stars (GEMS), which are critical for refining planet formation models. According to Fernandes, “Each new example helps refine models that tie disk mass, metallicity, and migration history into one picture.”

The host star, TOI-5573, is notably smaller and less massive than the Sun, showing no strong flares or significant rotation, which makes it an ideal target for further studies aimed at detecting atmospheric features on TOI-5573 b during transits. The star's faint luminosity, with an apparent magnitude near 15.6, positions it within the relatively quiet constellation of Lynx, devoid of bright neighboring stars.

The formation of giant planets around M dwarfs like TOI-5573 poses challenges due to their smaller, less massive protoplanetary disks. Most existing models suggest that building a solid core capable of attracting gas must occur rapidly before the disk dissipates. This is particularly challenging for stars with mass below 0.5 solar masses, such as TOI-5573, which resides just above this threshold.

The discovery of TOI-5573 b raises intriguing questions about the dynamics of planetary formation. Core-accretion models suggest that in metal-rich disks around stars of this type, a growing core can reach sufficient mass to initiate gas capture. As pointed out by Fernandes and her colleagues, “The odds are stacked against this happening in a low-mass disk, where the solid material is spread out thinly.”

Future observations using high-resolution near-infrared spectroscopy will aim to investigate the atmospheric composition of TOI-5573 b, particularly looking for signatures of methane and water vapor. Such studies are anticipated to enhance understanding of the relationship between stellar metallicity and the frequency of giant planets.

The findings related to TOI-5573 b contribute significantly to the field of exoplanet research, challenging existing paradigms of planetary formation and offering new avenues for exploration in the quest to comprehend the diversity of planetary systems in our galaxy. The study detailing this discovery has been published in the journal Astronomy & Astrophysics.