Welcome TESS followers to our latest news bulletin!
This week, we are looking at three recent papers from the archive. Enjoy!
First, we highlight the discovery and characterization of TOI-6109, a young planetary system in the Alpha Persei cluster consisting of two Neptune-sized planets orbiting a 75-million-year-old Sun-like star. Such systems are particularly valuable for understanding atmospheric evolution and testing models of planet formation, as they can provide snapshots of potential mass-loss processes during the early stages of planetary evolution. However, such systems remain rare and challenging to study due to high stellar activity that complicates both detection and mass measurements.
Next, we outline a paper presenting the publicly-available, fully automated exoplanet vetting system LEO-Vetter. Manual vetting of candidates for transiting planets is time-consuming and introduces subjective biases that make demographic studies of planetary populations difficult. Thus, automated vetting systems are essential for efficiently processing the enormous volume of TESS data and producing uniform planet catalogs suitable for robust occurrence rate calculations.
The third paper reports the discovery and characterization of TOI-2449 b (also known as NGTS-36 b), a Jupiter-sized exoplanet with an orbital period of 106 days around a Sun-like star. Such warm giant exoplanets are relatively rare, making them valuable for understanding planetary formation and migration. Compared to hot Jupiters, long-period planets like TOI-2449 b experience less stellar irradiation and tidal interactions, making their comparatively pristine state important for constraining how planetary systems evolve over time.
THYME XIII: Two young Neptunes orbiting a 75-Myr star in the Alpha Persei Cluster (Datillo et al. 2025) :
Datillo et al. (2025) present a detailed analysis of the TIC 384984325 system hosting at least two transiting planets. The target was observed by TESS in Sectors 18, 58, and 85, and the authors confirmed its membership of the Alpha Persei cluster through multiple independent lines of evidence. In particular, the star's Galactic position and kinematics match the cluster's spatial distribution and velocity, with Galactic coordinates of X = -125.77 ± 0.34 pc, Y = 72.40 ± 0.20 pc, and Z = -31.57 ± 0.09 pc, and space velocities of U = -14.27 ± 0.09 km/s, V = -22.91 ± 0.05 km/s, and W = -6.66 ± 0.03 km/s. TIC 384984325 exhibits a strong lithium absorption line with an equivalent width of 193 ± 6 milliangstroms, consistent with the cluster's lithium sequence and confirming its youth. Additionally, the measured rotation period of 3.02 ± 0.02 days aligns with the rotation-temperature relationship observed in Alpha Persei, and gyrochronological analysis yields an age consistent with the cluster's established age of 75 ± 5 million years. Combining TESS, CHEOPS, and ground-based follow-up observations, Datillo et al. (2025) obtained a detailed picture of the TIC 384984325 system. Their analysis of the stellar properties showed that the star has an effective temperature of 5660 ± 50 K, a stellar radius of 1.021 ± 0.038 solar radii, a mass of 1.03 ± 0.05 solar masses, and a luminosity of 0.882 ± 0.055 solar luminosities. The inner planet, TOI-6109 b, has an orbital period of 5.69 days and a radius of 4.87 Earth radii, while the outer planet, TOI-6109 c, orbits with a period of 8.54 days and has a radius of 4.83 Earth radii. The authors argue that the two planets are close to a 3:2 mean motion resonance, with dynamical simulations showing that approximately 29% of stable configurations place the system in resonance, while an additional 15% exhibit quasi-resonant states. Additionally, they found that the system exhibits significant transit timing variations (TTVs) reaching several hours. In TESS Sector 58, the TTVs of the two planets show clear anticorrelation, a signature that strongly indicates genuine planetary masses rather than false positives. The authors’ statistical analysis of the anticorrelation indicated that the TTV signal is 13 standard deviations larger than expected from photometric noise alone, with none of their 10,000 Monte Carlo simulations producing a comparable value by chance. Additionally, Datillo et al. (2025) statistically validated the outer planet TOI-6109 c, with a false positive probability of only 0.001 ± 0.000059 and a nearby false positive probability of 0.00 ± 0.001, both well below standard validation thresholds. The authors suggest that the planets show evidence of inflated atmospheres characteristic of young worlds, placed lower limits on the current hydrogen-helium envelope mass fraction for TOI-6109 b and TOI-6109 c of 7.94% and 9.55%, respectively, and argued that the possible core masses of the two planets range from 5 to 18 Earth masses. Thanks to TESS, Datillo et al. (2025) were able to discover and analyze in detail two new transiting exoplanets residing in the relatively sparse parameter space of young planetary systems.
LEO-Vetter: Fully Automated Flux- and Pixel-Level Vetting of TESS Planet Candidates to Support Occurrence Rates (Kunimoto et al. 2025) :
Kunimoto et al. (2025) present LEO-Vetter, a fully automated exoplanet vetting system designed to efficiently distinguish genuine planetary transits from instrumental artifacts and astrophysical false positives in TESS observations. To evaluate the performance and reliability of the code, the authors searched the lightcurves from ~200,000 M dwarfs, where the algorithm winnowed approximately ~20,000 potential signals down to 175 candidates meeting stringent quality criteria. Kunimoto et al. (2025) validated the results through artificial planet injections, demonstrating that the code correctly identifies 91% of genuine transits while successfully rejecting 97% of spurious detections caused by systematics. The authors’ classification framework incorporates thirteen flux-level false alarm diagnostics identifying instrumental and stellar noise sources, four flux-level astrophysical false positive diagnostics checking for common imposters such as binary star systems, and an imaging analysis examining pixel-level data to detect contamination from nearby sources. Kunimoto et al. (2025) report that when applied to previously-identified TESS Objects of Interest, LEO-Vetter recovered 125 out of 135 targets (~93%), consistent with the 91% completeness rate for the injected signals and in line with the human expert classifications. Overall, the authors found that stronger signals proved easier to classify correctly, with detection rates reaching 96% completeness and 99% reliability for candidates with SNR > 12 and showing at least 5 transits. Additionally, stars observed across different cadences showed ~93% completeness, ~99.9% effectiveness, and ~96% reliability rates, respectively, indicating that multiple visits enhance the code’s ability to discriminate between genuine signals and false positives. Kunimoto et al. (2025) note that the unphysical transit duration and SNR consistency tests were particularly effective at removing signals with physically implausible characteristics, eliminating ~63-64% of spurious signals. Applying the algorithm to ~300,000 FGK stars yielded comparable performance, with ~91% completeness and ~99% reliability rates, confirming the authors’ classification criteria is applicable across different stellar populations. Kunimoto et al. (2025) utilized TESS data to analyze in detail 127 previously known planet candidates, and identify 45 new ones, with 21 considered especially promising including 13 completely new discoveries.
Detection and characterisation of a 106-day transiting Jupiter : TOI-2449 b / NGTS-36 b (Ulmer-Moll et al. 2025) :
Ulmer-Moll et al. (2025) present a detailed analysis and characterization of TOI-2449 b / NGTS-36 b, a gas giant planet orbiting a Sun-like star every ~100 days. The target was observed by TESS in Sectors 4, 5, 31, and 32, and the planet was initially identified as a single transit event in data from sector 31. Ground-based follow-up observations with the Next Generation Transit Survey helped determine the orbital period by capturing one partial transit on Sep 14, 2021 and another on Nov 13, 2022. The authors combined the photometric observations with extensive radial velocity measurements from four high-resolution spectrographs: CHIRON, CORALIE, FEROS, and HARPS, covering a total baseline of 1136 days. Their joint analysis of all available data reveals that TOI-2449 b has a mass of 0.70 Jupiter masses and a radius of 1.001 Jupiter radii. The planet orbits at a semi-major axis of 0.45 AU with a slightly eccentric orbit having(e ~ 0.1). The transit duration is relatively long at 8.3 hours, and the planet has an estimated equilibrium temperature of approximately 400 K, assuming a Jupiter-like bond albedo. Ulmer-Moll et al. (2025) characterized the host star TOI-2449 as a G0/G1-type star with an effective temperature of ~600 Kelvin, a radius of ~1.1 solar radii, and a mass of ~1.1 solar masses. Their estimated stellar age is 2.5 billion years, with substantial uncertainties. TOI-2449 is located at a distance of about 155 parsecs from Earth and has a TESS magnitude of 9.9, making it relatively bright for detailed follow-up studies. Additionally, Ulmer-Moll et al. (2025) report the detection of a potential long-period signal in the measured radial velocities, with a periodicity of approximately 1140 days, which they attribute to the stellar magnetic activity cycle rather than an additional planetary companion. This interpretation is supported by activity indicators derived from the high-resolution spectra, particularly the H-alpha chromospheric line which shows variability on a similar timescale. The inferred stellar rotation period of about 16 days is consistent with the star's age and spectral type. The authors’ model of the interior structure of TOI-2449 b indicates that the planet contains 11 Earth masses of heavy elements (with relatively high uncertainties), distributed between a rocky core and metals mixed in the hydrogen-helium envelope. The planet’s envelope has a heavy element fraction of 0.03 (with uncertainties of ~0.03), corresponding to a marginal planet-to-star metal enrichment ratio of ~3.3. Ulmer-Moll et al. (2025) argue that the planet likely formed through core accretion at several AU from the star, which was followed by inward migration accompanied by hydrogen and helium accretion. Additionally, the authors note that TOI-2449 b occupies an important regime where methane and ammonia are the dominant carriers of carbon and nitrogen rather than carbon monoxide and molecular nitrogen as in hot Jupiters. This makes the planet particularly valuable for measuring the carbon-to-nitrogen-to-oxygen ratio, which provides strong constraints on formation location and migration history. Capitalizing on TESS data, the authors were able to discover and characterize TOI-2449 b – the latest addition to the small but growing population of well-characterized transiting warm Jupiters, where only 23 known planets have orbital periods exceeding 100 days and precise mass and radius measurements.
Fig. 1: Taken from Datillo et al. (2025). TESS data of TOI-6109, highlighting the detected transits of the two planets (red and purple, respectively). The upper/middle panels shows the original/detrended data, respectively, while the lower two panels show the phase-folded lightcurves for the two planets.
Fig. 2: Taken from Kunimoto et al. (2025). Completeness (upper panels), effectiveness (middle panels), and reliability (lower panels) for LEO-Vetter for simulated planets and false alarms in TESS data. The left/right columns show the results as function of the number of transits/orbital period, respectively.
Fig. 3: Taken from Ulmer-Moll et al. (2025). TESS (left) and NGTS (middle and right) transits of the 106-days transiting gas giant TOI-2449 b, along with the best-fit model. The lower panels show the corresponding residuals.