TOI-5349b: Warm Saturn-like Exoplanet

• TOI-5349b is a transiting, warm Saturn-like exoplanet orbiting a metal-rich early M-dwarf with a 3.3-day period.
• It was confirmed via coordinated TESS photometry, ground-based transit follow-ups, and precise radial-velocity measurements, yielding a mass of 0.40 MJ and a radius of 0.91 RJ.
• Its Saturn-like density and moderate equilibrium temperature (~720 K) make it an ideal benchmark for atmospheric characterization and studies of exoplanet formation.

Searching arXiv for the focal paper and closely related GEMS context papers. TOI-5349b is a transiting, warm, Saturn-like exoplanet orbiting the metal-rich early M-dwarf TOI-5349 with an orbital period of approximately 3.3 days. It was confirmed by the Searching for GEMS (Giant Exoplanets around M-dwarf Stars) survey through a coordinated photometric and radial-velocity campaign that combined TESS discovery photometry, ground-based transit follow-up, and precise spectroscopy from HPF and MAROON-X. The system is notable because the planet’s mass, radius, and bulk density place it squarely in the Saturn-like regime, while the host star’s super-solar metallicity and early-M spectral type align with an emerging pattern among transiting GEMS planets: short-period gas giants around cool dwarfs often have Saturn-like masses and densities and preferentially orbit metal-rich stars (Sandoval et al., 23 Sep 2025).

1. Discovery and confirmation

TOI-5349b was initially identified in photometry from NASA’s Transiting Exoplanet Survey Satellite. The signal was found in TESS full-frame images from Sectors 42–44 at 600 s cadence and re-observed in Sectors 70–71 at 200 s cadence. Light curves were extracted using the PSF-based tglc pipeline, which employs Gaia priors to mitigate contamination. The best-fit transit model to the phase-folded TESS data is shown in Figure 1 of the discovery paper (Sandoval et al., 23 Sep 2025).

Ground-based transit observations were then used to secure the ephemeris. Two partial transits were observed in Bessel I with the 0.6-m Red Buttes Observatory in January 2023, and three transits were obtained in Sloan $i'$ with Pomona College’s 1.0-m Table Mountain telescope in January 2025. These follow-up data supplemented the space-based photometry and anchored the transit timing solution (Sandoval et al., 23 Sep 2025).

The planetary interpretation was confirmed with precise radial velocities. The dataset comprised 13 HPF near-infrared visits between 2022 October and 2024 January and six MAROON-X red-optical visits in 2023 October. These measurements yielded a coherent Keplerian signal. Additional false-positive checks strengthened the validation: high-resolution speckle imaging with NESSI on WIYN excluded nearby bright companions at separations of 0.15–1.2 arcsec, and a cross-match with the Gaia wide-binary catalog found no evidence for a resolved wide companion. Spectroscopic line-profile diagnostics from HPF-SERVAL showed no correlations with the radial velocities and no secondary spectral features, ruling out an eclipsing binary or blended-spectrum false positive. The stellar density implied by the transit also matched the spectroscopic/SED-derived density within $1\sigma$, supporting a planet transiting the M dwarf (Sandoval et al., 23 Sep 2025).

2. Host star TOI-5349

TOI-5349 is characterized as a metal-rich M1-type dwarf. Stellar properties were derived from HPF spectra using HPF-SpecMatch and from an EXOFASTv2 SED+isochrone fit based on MIST grids, incorporating broadband photometry and Gaia DR3 parallax. The star has spectral type M1 $\pm 0.5$, effective temperature $T_{\rm eff} = 3751 \pm 59$ K, and surface gravity $\log g_* = 4.72 \pm 0.04$ in cgs units (Sandoval et al., 23 Sep 2025).

The metallicity is reported as $[{\rm Fe/H}] = +0.50 \pm 0.16$ dex from HPF-SpecMatch. LAMOST DR10 low-resolution optical spectra independently classify the star as M1 $\pm 0.5$ with consistent $T_{\rm eff}$ and $\log g$. Because of library limitations for M dwarfs, the metallicity is interpreted in the source as best understood as confidently super-solar rather than as a precision abundance benchmark (Sandoval et al., 23 Sep 2025).

The stellar mass and radius are $0.61 \pm 0.02\,M_\odot$ and $1\sigma$0, respectively. Additional reported parameters include luminosity $1\sigma$1, density $1\sigma$2, distance $1\sigma$3 pc from Gaia DR3 parallax $1\sigma$4 mas, age $1\sigma$5 Gyr, and systemic radial velocity $1\sigma$6 km s$1\sigma$7 (Sandoval et al., 23 Sep 2025).

The star appears relatively inactive. The projected rotation is constrained as $1\sigma$8 km s$1\sigma$9, corresponding to the HPF instrumental resolution limit. No significant rotational modulation or flares were detected in the TESS photometry, and HPF and MAROON-X line-profile indicators show no correlations with radial velocity. This combination of properties is consistent with a relatively inactive early M dwarf and is relevant for both RV interpretation and atmospheric follow-up (Sandoval et al., 23 Sep 2025).

3. Orbital architecture and transit geometry

The joint photometry and radial-velocity fit yields an orbital period of $\pm 0.5$0 days and a reference transit epoch of $\pm 0.5$1 BJD$\pm 0.5$2. The scaled planetary radius is $\pm 0.5$3, which corresponds to a transit depth of $\pm 0.5$4, or $\pm 0.5$5. The measured impact parameter is $\pm 0.5$6, the orbital inclination is $\pm 0.5$7 degrees, and the total transit duration is $\pm 0.5$8 hours (Sandoval et al., 23 Sep 2025).

The semi-major axis is $\pm 0.5$9 au, with $T_{\rm eff} = 3751 \pm 59$0. The radial-velocity semi-amplitude is $T_{\rm eff} = 3751 \pm 59$1 m s$T_{\rm eff} = 3751 \pm 59$2. The eccentricity is reported as $T_{\rm eff} = 3751 \pm 59$3, with a $T_{\rm eff} = 3751 \pm 59$4 upper limit below 0.12, and the argument of periastron is $T_{\rm eff} = 3751 \pm 59$5 degrees. The orbit is therefore consistent with circular within current uncertainties (Sandoval et al., 23 Sep 2025).

The eccentricity parameterization also yielded $T_{\rm eff} = 3751 \pm 59$6 and $T_{\rm eff} = 3751 \pm 59$7. The source interprets these values as reinforcing the near-circular solution. The geometry is likewise described as grazing-free and moderately central, based on the consistency of $T_{\rm eff} = 3751 \pm 59$8 and $T_{\rm eff} = 3751 \pm 59$9 with the measured impact parameter (Sandoval et al., 23 Sep 2025).

No evidence was found for a long-term radial-velocity acceleration, additional periodic signals, or transit timing variations in the present data. This indicates that the current observations are well described by a single-planet solution (Sandoval et al., 23 Sep 2025).

4. Planetary physical properties

From the joint fit, TOI-5349b has a mass of $\log g_* = 4.72 \pm 0.04$0, equivalent to $\log g_* = 4.72 \pm 0.04$1, and a radius of $\log g_* = 4.72 \pm 0.04$2, equivalent to $\log g_* = 4.72 \pm 0.04$3. The resulting mean density is $\log g_* = 4.72 \pm 0.04$4, described as essentially Saturn-like and approximately $\log g_* = 4.72 \pm 0.04$5 Saturn’s density of $\log g_* = 4.72 \pm 0.04$6 (Sandoval et al., 23 Sep 2025).

The planet’s surface gravity is $\log g_* = 4.72 \pm 0.04$7 in cgs units, corresponding to $\log g_* = 4.72 \pm 0.04$8 m s$\log g_* = 4.72 \pm 0.04$9. Its incident flux is $[{\rm Fe/H}] = +0.50 \pm 0.16$0, consistent with the relation

$[{\rm Fe/H}] = +0.50 \pm 0.16$1

Assuming a blackbody planet with Bond albedo $[{\rm Fe/H}] = +0.50 \pm 0.16$2, the equilibrium temperature is $[{\rm Fe/H}] = +0.50 \pm 0.16$3 K, evaluated via

$[{\rm Fe/H}] = +0.50 \pm 0.16$4

These parameters place TOI-5349b in the warm Saturn regime rather than among strongly irradiated hot Jupiters (Sandoval et al., 23 Sep 2025).

The bulk properties are summarized in the source as placing the planet squarely in the Saturn-like regime: $[{\rm Fe/H}] = +0.50 \pm 0.16$5, $[{\rm Fe/H}] = +0.50 \pm 0.16$6, $[{\rm Fe/H}] = +0.50 \pm 0.16$7, and $[{\rm Fe/H}] = +0.50 \pm 0.16$8 K. This combination is central to its role within the GEMS sample, where several short-period gas giants around cool dwarfs occupy a similar density locus (Sandoval et al., 23 Sep 2025).

A plausible implication is that TOI-5349b serves as a comparatively clean benchmark for the structural study of Saturn-like planets around low-mass stars because its radius is near Jupiter-like while its mass remains substantially sub-Jovian, yielding a low bulk density without requiring the extreme irradiation conditions characteristic of many inflated hot Jupiters. This inference follows from the reported mass, radius, density, and equilibrium temperature, but the detailed interior interpretation is not explicitly quantified in the source.

5. Observational methodology and statistical modeling

The analysis used a joint photometry-plus-radial-velocity fit implemented with the exoplanet package, with posterior inference performed using PyMC3’s No-U-Turn Sampler. Transit photometry was modeled with starry under a quadratic limb-darkening law. For each photometric dataset, the limb-darkening coefficients were reparameterized following Kipping so that they could be sampled efficiently and independently by bandpass (Sandoval et al., 23 Sep 2025).

Because the TESS light curves showed no significant out-of-transit variability, no Gaussian process was included for photometric activity. The radial velocities were modeled with a single-planet Keplerian allowing small eccentricity, together with per-instrument radial-velocity offsets and jitter terms for HPF and for the blue and red MAROON-X channels. The priors were Normal on the period and reference transit time and Uniform on the other parameters, including $[{\rm Fe/H}] = +0.50 \pm 0.16$9 and $\pm 0.5$0 for the eccentricity parameterization (Sandoval et al., 23 Sep 2025).

Posterior sampling used four chains with 6,000 tuning steps, and all parameters had Gelman-Rubin $\pm 0.5$1. This indicates well-mixed chains within the adopted inference setup. The analysis is therefore statistically anchored in a standard joint-fit framework for transiting exoplanet systems, while the specific choice to omit a photometric Gaussian process reflects the absence of significant out-of-transit variability in the TESS data (Sandoval et al., 23 Sep 2025).

The radial-velocity dataset also carries instrument-specific information. HPF, operating at $\pm 0.5$2 over 808–1278 nm, obtained 13 visits, each consisting of two 945 s exposures, with median per-pixel S/N $\pm 0.5$3 at 1070 nm. HPF RVs were derived with HPF-SERVAL, and the fitted HPF jitter term is $\pm 0.5$4 m s$\pm 0.5$5, interpreted in the source as indicative of combined instrumental and astrophysical white noise for a faint M dwarf in the NIR. MAROON-X, operating at $\pm 0.5$6 with blue coverage 500–670 nm and red coverage 650–900 nm, obtained six simultaneous blue/red visits of 1800 s with median peak S/N $\pm 0.5$7 and 51.5, respectively. The MAROON-X RVs were measured with SERVAL and fit as independent instruments with small jitters of $\pm 0.5$8 m s$\pm 0.5$9 and consistent offsets of approximately $T_{\rm eff}$0 m s$T_{\rm eff}$1 (Sandoval et al., 23 Sep 2025).

6. Position within the GEMS population and formation implications

The source presents TOI-5349b as emblematic of a broader GEMS pattern revealed by TESS and RV follow-up: many transiting giants around cool dwarfs with $T_{\rm eff}$2 K cluster at Saturn-like masses and densities while orbiting metal-rich hosts. Within the compiled GEMS sample with well-measured masses and radii, TOI-5349b lies in the core of the Saturn-density locus, defined in the paper as $T_{\rm eff}$3–$T_{\rm eff}$4, and has parameters similar to TOI-5344b, TOI-5688 Ab, TOI-5573b, and TOI-6158b (Sandoval et al., 23 Sep 2025).

Its host metallicity, approximately $T_{\rm eff}$5 dex, is described as among the highest in the GEMS sample and as reinforcing the planet-metallicity correlation seen for hot and warm Jupiters around FGK stars. The paper argues that metal-rich disks boost solid surface density and opacity, accelerating core formation and potentially delaying radiative cooling. According to that interpretation, both effects can promote giant planet formation by core accretion and may stall growth at Saturn masses if runaway gas accretion is quenched by disk dispersal or opacity effects, in line with “failed gas giant” scenarios discussed in the literature (Sandoval et al., 23 Sep 2025).

The source contrasts this with the possibility that gravitational instability contributes to the most massive GEMS systems, identified there as those with masses $T_{\rm eff}$6. For TOI-5349b specifically, the combination of Saturn-like mass, low eccentricity, and moderate irradiation is taken to favor core accretion plus inward migration. The paper also notes a comparative result that Jupiter-radius planets around M dwarfs have similar masses to those around FGK hosts, when super-Jupiters are excluded, and interprets this as pointing to broadly similar formation physics across stellar mass, with metallicity and minimum disk-mass thresholds playing key roles (Sandoval et al., 23 Sep 2025).

This suggests that TOI-5349b is important less as an isolated outlier than as part of a statistically emerging subpopulation of short-period, Saturn-density giants around low-mass stars. A plausible implication is that such systems may provide a useful comparative baseline for testing whether the bulk composition and migration history of giant planets around M dwarfs differ systematically from those around Sun-like stars. That comparative program is explicitly motivated by the source, although its detailed realization remains prospective (Sandoval et al., 23 Sep 2025).

7. Atmospheric characterization and future observational prospects

TOI-5349b is identified as a feasible transmission-spectroscopy target. The host star has $T_{\rm eff}$7, and for an H$T_{\rm eff}$8-dominated atmosphere the atmospheric scale height is estimated from

$T_{\rm eff}$9

with $\log g$0, $\log g$1 K, and $\log g$2 m s$\log g$3, yielding $\log g$4 m, or about 210–220 km. A single scale height corresponds to an expected transmission signal amplitude of roughly

$\log g$5

per scale height, so several-scale-height molecular features of a few hundred ppm are described as plausible (Sandoval et al., 23 Sep 2025).

The reported Transmission Spectroscopy Metric for TOI-5349b is $\log g$6, near the sample median and below JWST/NIRSpec PRISM saturation limits of $\log g$7, enabling full 0.6–5.3 $\log g$8 coverage. The paper therefore identifies JWST and high-throughput ground-based facilities as suitable instruments for detecting H$\log g$9O and CH$0.61 \pm 0.02\,M_\odot$0, provided that clouds, hazes, and stellar contamination are modest. Because the star appears relatively inactive and the RV/activity indicators are quiet, TOI-5349b is characterized as a cleaner atmospheric target than some mid-M GEMS systems affected by spot-crossing or contamination (Sandoval et al., 23 Sep 2025).

The recommended future observations are explicitly prioritized in four categories. First, additional precise radial velocities with HPF, MAROON-X, or similar facilities could tighten the eccentricity constraint and search for low-amplitude long-term trends indicating outer companions. Second, continued transit timing monitoring could probe TTVs. Third, high-resolution imaging at multiple wavelengths could further constrain faint companions beyond the existing NESSI contrast limits. Fourth, JWST transmission spectroscopy could measure atmospheric composition and metallicity and test whether the low atmospheric metallicities found for some M-dwarf giants generalize to the Saturn-like GEMS population. The source further notes that secondary eclipse detection would be challenging at $0.61 \pm 0.02\,M_\odot$1 K but could constrain thermal structure if feasible (Sandoval et al., 23 Sep 2025).