TOI-4616 b: Earth-Sized Rocky Exoplanet

• TOI-4616 b is an Earth-sized rocky exoplanet transiting a mid-M dwarf, validated through combined TESS photometry and extensive ground-based follow-up.
• The planet exhibits a 1.55-day orbital period, a radius of 1.22 R⊕, and a transit depth of approximately 0.36%, placing it in a highly irradiated regime.
• Its benchmark status and high Transmission Spectroscopy Metric (TSM ≈ 21.4) underscore its potential for future atmospheric characterization and studies on terrestrial planets.

TOI-4616 b is an Earth-sized rocky exoplanet transiting the nearby mid-M dwarf TOI-4616, also identified as LP 466-156 and TIC 258796169. It was discovered in Transiting Exoplanet Survey Satellite (TESS) observations and statistically validated through a combined analysis of space-based photometry, ground-based multi-band transit follow-up, high-resolution imaging, and optical and near-infrared spectroscopy. The system has been presented as a benchmark for comparative studies of terrestrial planets around mid-M dwarfs and for future atmospheric investigations because of its proximity to Earth, well-constrained stellar properties, and extensive follow-up data (Lang et al., 11 Mar 2026).

1. Discovery and statistical validation

The signature of TOI-4616 b first emerged in the TESS 2-min cadence PDCSAP light curves from Sectors 17, 42, 43, and 70. After stitching the sectoral light curves, removing outliers, and retaining only data within $\pm 10\,\mathrm{h}$ of each predicted transit, the signal was modeled with a joint Gaussian-process plus transit framework using a SHOTerm kernel in celerite for stellar rotation and instrumental variability and batman for the transit shapes. The fit employed an affine-invariant MCMC and recovered a clear $1.55$-day periodic dip of depth $\simeq 0.36\%$ (Lang et al., 11 Mar 2026).

Ground-based follow-up was used to confirm the TESS signal and to exclude false-positive scenarios. Multi-band transit photometry was obtained with SAINT-EX in $I+z$ and $i^\prime$, with the MuSCAT2 and MuSCAT3 imagers in $g^\prime$, $r^\prime$, $i^\prime$, and $z_s$, with LCOGT/SINISTRO, and with SPECULOOS-North/Artemis in $z^\prime$. Across these observations, no statistically significant chromatic variation in transit depth was detected. This is significant because wavelength-dependent transit depths are commonly used to diagnose blended eclipsing binaries or other contaminating astrophysical configurations.

The statistical validation relied on TRICERATOPS, which evaluates the relative probabilities of planetary and false-positive scenarios within a hierarchical Bayesian framework. Following detrending with a biweight filter in wōtan, the TESS-only analysis initially yielded $1.55$0 and $1.55$1, dominated by scenarios involving the $1.55$2 neighbor TIC 258796170. Incorporation of the high-resolution imaging contrast curve, uncontaminated Sloan-$1.55$3 ground photometry, and exclusion of the neighbor’s flux reduced the nearby false-positive probability to $1.55$4 and the overall false-positive probability to

$1.55$5

which is below the recommended validation threshold of $1.55$6. On that basis, TOI-4616 b was statistically validated as a bona fide planet.

2. Host star TOI-4616

TOI-4616 is classified as an M4 V star. Shane/Kast spectra on the $1.55$7 Shane telescope matched an M4 V template, and IRTF/SpeX SXD observations yielded a near-infrared spectral type of M4.0 $1.55$8 0.5. The host was described as a nearby mid-M dwarf, and the detailed summary adopted a Gaia DR3 distance of $1.55$9, while the abstract reported a distance of $\simeq 0.36\%$0 (Lang et al., 11 Mar 2026).

The stellar parameters were derived from an empirical spectral-energy-distribution fit, Gaia DR3 parallax, the Stefan–Boltzmann law, and Mann et al. empirical relations. Using broadband photometry from Gaia BP and RP, 2MASS $\simeq 0.36\%$1, and WISE $\simeq 0.36\%$2–$\simeq 0.36\%$3, together with unreddened PHOENIX atmospheric models, the analysis obtained a bolometric flux

$\simeq 0.36\%$4

an effective temperature of

$\simeq 0.36\%$5

and a stellar radius

$\simeq 0.36\%$6

The luminosity and radius were connected through

$\simeq 0.36\%$7

and the stellar mass was estimated from the Mann et al. (2019) $\simeq 0.36\%$8–mass relation as

$\simeq 0.36\%$9

These values imply a stellar density of approximately $I+z$0 and a luminosity of

$I+z$1

Spectroscopic activity diagnostics further constrained the host. The optical spectrum showed strong H$I+z$2 emission with $I+z$3 and $I+z$4, while the absence of Li i absorption indicated an age $I+z$5. From $I+z$6-band indices and the Mann et al. relations, the metallicity was estimated as

$I+z$7

3. Observational constraints on contamination

A central element of the system’s validation was the exclusion of contaminating sources. Speckle imaging with NESSI/WIYN on UT 2021 Dec 21 ruled out bound or background companions within $I+z$8 down to $I+z$9 at $i^\prime$0 (Lang et al., 11 Mar 2026).

Archival imaging from 1954 to 2025 provided an additional constraint. Because TOI-4616 has a high proper motion of approximately $i^\prime$1, its present-day sky position can be checked against historical images to determine whether a background object could now be blended with the target. The archival data excluded contaminating background stars at the current position of TOI-4616.

The multi-band transit photometry also constrained false positives through chromaticity tests. Despite varying filter wavelengths and instrumental systematics, the transit depth did not show a statistically significant chromatic dependence. In combination with the speckle contrast curve and the treatment of the nearby source TIC 258796170 in the TRICERATOPS analysis, these observations sharply reduced the probability that the TESS signal arose from a background eclipsing binary or a nearby transiting planet rather than from a planet transiting TOI-4616 itself.

4. Transit geometry and orbital solution

The joint transit analysis returned a planet-to-star radius ratio of

$i^\prime$2

from which, using the stellar radius, the planetary radius was determined to be

$i^\prime$3

(Lang et al., 11 Mar 2026).

The orbital ephemeris was reported as

$i^\prime$4

The observed transit depth follows directly from

$i^\prime$5

Assuming a circular orbit, Kepler’s third law gives a semimajor axis of approximately

$i^\prime$6

The short orbital period and small semimajor axis place the planet in a highly irradiated inner orbit around a low-mass star. This suggests that TOI-4616 b is dynamically and radiatively distinct from more mildly irradiated terrestrial planets around earlier-type M dwarfs, while still orbiting a host warmer and larger than the latest M and ultracool dwarf primaries.

5. Physical properties of the planet

TOI-4616 b has a radius of $i^\prime$7, and the source characterized it as an Earth-sized planet. The incident stellar flux is approximately

$i^\prime$8

and, assuming zero Bond albedo ($i^\prime$9) and full heat redistribution, the equilibrium temperature is

$g^\prime$0

(Lang et al., 11 Mar 2026).

These values place TOI-4616 b in what the source describes as a regime intermediate between Earth-sized planets orbiting early M dwarfs and those around ultra-cool hosts. In the mass-radius plane of Earth-sized planets around M dwarfs, it was noted as one of the most highly irradiated yet truly Earth-sized transiting planets known. Because no planetary mass was reported in the source, bulk density and interior composition were not directly measured; any stronger compositional inference would therefore require future mass determinations.

The combination of Earth-sized radius, short period, and substantial irradiation is central to the planet’s scientific interest. A plausible implication is that TOI-4616 b occupies a parameter space well suited to testing how terrestrial planets around mid-M dwarfs respond to intense high-energy stellar forcing while remaining accessible to transit-based characterization.

6. Benchmark status and future study

The system was explicitly described as a valuable benchmark for comparative studies of terrestrial planets around mid-M dwarfs and for future atmospheric investigations. That characterization rests on three stated attributes: proximity to Earth, well-constrained stellar properties, and extensive multi-band follow-up (Lang et al., 11 Mar 2026).

The source further reported a Transmission Spectroscopy Metric of $g^\prime$1, exceeding the nominal threshold of $g^\prime$2 for terrestrial planets. It therefore suggested that JWST transmission spectroscopy could detect atmospheric features if a high-mean-molecular-weight atmosphere persists after extreme XUV-driven escape, with cumulative XUV fluence quoted as $g^\prime$3. The stated interpretation is that TOI-4616 b offers a stringent test of atmospheric survival under conditions where primordial envelopes are expected to be stripped, and of secondary atmosphere formation and retention.

This framing links TOI-4616 b to broader questions in terrestrial exoplanet science around M dwarfs: atmospheric escape, the persistence or loss of volatile envelopes, and interior–atmosphere coupling in the terrestrial regime. The source accordingly identified it as a prime target for future mass determinations, atmospheric characterization, and studies of atmospheric escape and interior–atmosphere coupling. A plausible implication is that the system can serve as a comparative anchor between less irradiated Earth-sized planets around earlier M dwarfs and the more extreme terrestrial planets orbiting late-M and ultracool dwarf hosts.