Newly identified planet is a rare 'miniature Saturn' orbiting a red dwarf

The recent discovery of exoplanet TOI‑5573 b, a miniature Saturn-like planet, gives astronomers a fresh look at how planets can arise around small stars.

The newly identified world measures 0.87 times Jupiter’s radius yet weighs just 0.35 Jupiter masses, yielding a Saturn‑like bulk density.

It orbits an M dwarf that is about 606 light‑years away, every 8.79 days, and at a distance of roughly 0.07 of an astronomical unit from its star. This keeps the planet’s dayside near 528 K, which is cool for a gas giant so close in.

Rachel B. Fernandes of Pennsylvania State University, and colleagues, confirmed the planet’s existence using NASA’s TESS photometry plus radial‑velocity work from the Habitable‑zone Planet Finder and NEID spectrographs.

Why TOI-5573 matters

The find adds to the sparse catalog of Giant Exoplanets around M‑dwarf Stars (GEMS), a class with members having a radius of eight to fifteen times Earth’s radius.

Hot Jupiters turn up around early‑type M dwarfs only about 0.27 ± 0.09 percent of the time, according to a TESS‑based census.

Rare systems such as TOI-5573 test planet‑formation theory because low‑mass disks surrounding cool stars should contain too little material to grow such bodies quickly.

Each new example helps refine models that tie disk mass, metallicity, and migration history into one picture.

Snapshot of TOI-5573 b’s neighborhood

The planet lies in the constellation Lynx, a quiet stretch of sky without any bright stars nearby. Its host, TOI‑5573, is faint to the naked eye, with an apparent magnitude near 15.6 in visible light.

The star is roughly 40 percent smaller and less massive than the Sun, and shows no signs of strong flares or rotation, making it an excellent candidate for future follow-up.

Its relative calmness improves the chances of detecting atmospheric features on the planet during transits.

How telescopes caught it

“The planet was initially discovered by TESS and confirmed using a combination of 11 transits … achieving a 5-sigma precision on the planet’s mass,” wrote Fernandes and co‑authors. TESS logged eleven dips in starlight across four observing sectors.

Ground‑based photometry from Wyoming’s Red Buttes Observatory nailed down timing, while high‑precision radial velocity curves delivered a 5‑sigma mass. 

Speckle imaging ruled out close stellar impostors, and the resulting light‑curve–RV fit showed a low‑eccentricity orbit, hinting that the planet drifted inward through the disk rather than being flung by violent scattering.

TOI-5573 b is cooler than most giants

Despite its tight orbit, TOI‑5573 b is one of the coolest Saturn analogs known around an M dwarf.

Its relatively mild equilibrium temperature means molecules such as methane and water vapor could persist high in the atmosphere, making it a tempting target for transmission spectroscopy, once the James Webb Space Telescope (JWST) shifts to fainter red dwarfs.

The host star’s metallicity comes in at roughly +0.4 dex, comfortably above solar.

Metal‑rich disks radiate heat less efficiently; that higher opacity slows gas accretion and can leave a core capped with a limited envelope instead of a Jupiter‑mass bulk.

M-dwarfs are hard places to form giants

Forming giant planets around M dwarfs remains a puzzle because their protoplanetary disks are smaller and less massive than those of Sun-like stars.

Most models suggest that building a solid core big enough to pull in gas has to happen quickly, before the disk disappears entirely within a few million years.

The odds are stacked against this happening in a low-mass disk, where the solid material is spread out thinly. The situation is even tougher for stars under 0.5 solar masses, like TOI‑5573, which sits just above that line.

What we learned from TOI‑5573 b

Core‑accretion models that include pebble drift reproduce Saturn‑mass outcomes in metal‑rich disks around stars as small as 0.2 solar masses.

In those simulations, a growing core reaches ten Earth masses quickly, but elevated dust content throttles cooling, delaying runaway gas capture until the disk disperses.

TOI‑5573 b fits that script: it is big enough to be a gas giant, small enough to be a “failed” Jupiter.

Gravitational instability could, in principle, spawn giants in situ, yet that pathway demands a disk nearly ten percent as massive as the star – an unlikely scenario for a modest red dwarf. The low eccentricity and close separation likewise favor migration over collapse at distance.

Future near‑infrared spectra should pin down atmospheric metallicity and search for hazes that could betray ongoing photochemistry.

High‑resolution optical spectra will also refine the star’s iron content, trimming the error bars that blur trends between stellar metal supply and giant‑planet frequency.

The study is published in Astronomy & Astrophysics.

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