TOI-4616 b: a benchmark Earth-sized planet transiting a nearby M4 dwarf

Abstract

Rocky exoplanets are particularly abundant around M-type stars. Their small radii and low luminosities provide favourable conditions for detecting transiting terrestrial planets and probing their atmospheric properties. We report the discovery and statistical validation of TOI-4616 b, an Earth-sized planet transiting a nearby mid-M dwarf observed by the Transiting Exoplanet Survey Satellite (TESS). We confirm the planetary nature of the signal and determine the system parameters by combining TESS photometry with ground-based multi-band transit observations, high-resolution imaging, and optical and near-infrared spectroscopy. The host star lies at a distance of 28.10 +(-) 0.07 pc and has a radius of 0.1889 +(-)0.0096 solar radii, a mass of 0.1881 +(-) 0.0094 solar masses, and an effective temperature of 3150 +(-) 75 K. TOI-4616 b has a radius of 1.22 Earth radii and an orbital period of 1.55 days. The planet receives an incident flux of approximately 40 times that of Earth, corresponding to an equilibrium temperature of about 525 K. This places TOI-4616 b in a regime intermediate between Earth-sized planets orbiting early M dwarfs and those around ultra-cool hosts. Statistical validation with the TRICERATOPS framework, supported by high-resolution imaging and chromatic transit constraints, yields a false-positive probability of 0.0135, below the recommended validation threshold of 0.015, confirming TOI-4616 b as a validated planet. Owing to its proximity to Earth, well-constrained stellar properties, and extensive multi-band follow-up, TOI-4616 b constitutes a valuable benchmark system for comparative studies of terrestrial planets around mid-M dwarfs and for future atmospheric investigations.

Figures (24)

• Figure 1: Target Pixel File (TPF) of TOI-4616 from TESS Sector 17. The color scale shows the median pixel flux. The red squares indicate the optimal photometric aperture used to extract the light curve. Nearby sources are marked and scaled according to their TESS magnitude difference relative to the target ($\Delta m$). A neighbouring star falls within the photometric aperture and contributes a small amount of contaminating flux, which is accounted for in the analysis. The figure was produced using TPFPLOTTERaller2020planetary.
• Figure 2: TESS PDCSAP light curves of TOI-4616 from Sectors 17, 42, 43, and 70. Each panel shows the flux time series for one sector, illustrating the sector-to-sector variation in photometric scatter and long-term systematics. Data gaps reflect the two-orbit observing strategy of TESS.
• Figure 3: Section of the TESS light curve illustrating the intrinsic variability of the host star. Blue points show the photometric measurements, while the black curve represents the Gaussian Process model used to capture correlated variability.
• Figure 4: Phase-folded TESS light curve after variability modeling. The grey points show the GP-detrended photometry. The black points show the data binned into 300 uniform bins in orbital phase from the full set of 54 270 measurements. The red curve shows the transit model.
• Figure 5: Recovery of the transit signal from highly variable SAINT-EX $i^\prime$-band photometry obtained on UTC 2024 October 09 (left) and UTC 2024 October 20 (right). In each panel, the top sub-panel shows the raw light curve (black points) and the GP model (blue curve) capturing correlated variability and systematics. The middle sub-panel shows the GP-detrended light curve with the best-fit transit model overplotted (red). The bottom sub-panel shows residuals after removing both GP and transit components. Despite strong time-correlated variability in the raw photometry, the joint GP+transit model robustly isolates the transit signal. The GP modeling was performed within the juliet framework.