TOI-6981 b has been validated as a transiting planet, according to a new analysis of an M4 dwarf-star system. The cold sub-Neptune was checked with ground-based transit photometry, statistical validation and high-resolution spectroscopy. Because the work covers one astronomical system, it supports the planetary interpretation of this signal rather than showing how common such worlds are.
The reported solution gives TOI-6981 b a radius of 2.40 ± 0.10 Earth radii and an orbital period of 60.778839 days. Assuming an Earth-like albedo of 0.3, it has an equilibrium temperature of 170.0 ± 4.5 kelvin and receives 0.20 ± 0.03 times Earth's instellation, the energy arriving from the Sun at Earth.
A period hidden among aliases
The period was not obvious at first because the TESS signal allowed several possible aliases. An additional Sector 88 transit selected the seventh alias, giving an orbital period of 60.7787 days. The 120-second TESS data and a KeplerCam observation ruled out the other aliases.
All available transits were then fit simultaneously with allesfitter's nested-sampling mode. The team compared a circular orbit with a model that allowed eccentricity. The circular model was favored over free eccentricity by 5.01 in log model evidence and adopted for the analysis. That is a model preference, not a direct measurement that the orbit is circular. The authors say precise radial-velocity data are still needed to constrain eccentricity.
Checks against false alarms
Several checks addressed the possibility that a nearby object was mimicking the signal. Speckle imaging found no companion between 20 milliarcseconds and 1.2 arcseconds within the reported contrast limits. At the system's distance of 25.5 parsecs, those separations correspond to roughly 0.5 to 31 astronomical units, and the imaging was sensitive to companions from five to nearly nine magnitudes fainter than the target. Very faint or long-period companions were not completely excluded.
Three TRES radial-velocity observations likewise showed no statistically significant variation. The individual velocities were 38.368, 38.371 and 38.383 kilometers per second, with relative uncertainties of 20 meters per second. That result was a vetting check, but no sufficiently precise radial-velocity mass measurement was available.
Statistical validation added a further check. Using the LCOGT i-prime transit, the reported false-positive probability, or FPP, and nearby false-positive probability, or NFPP, were each no more than 4.2 times 10 to the minus fifth power. Those values were below the authors' required limits of 0.015 for FPP and 10 to the minus third power for NFPP. The reported tests therefore met the paper's thresholds for validating the planetary interpretation, but they do not measure the planet's mass or atmosphere.
The measurements still missing
The host star was characterized as an M4 star, with a radius of 0.304 ± 0.012 times the Sun's radius, a mass of 0.296 ± 0.026 solar masses and an effective temperature of 3,215 ± 58 kelvin.
A direct mass measurement was not available. Instead, the paper used a Chen-Kipping mass-radius relation to estimate a mass of 6.36 ± 0.46 Earth masses and a density of 2.5 ± 0.4 grams per cubic centimeter. From the same model-based estimate, it predicted a radial-velocity semi-amplitude of 2.4 ± 0.3 meters per second, the size of the star's motion that future RV work would seek.
Those assumptions also feed into the planet's atmospheric prospects. The paper reports a transmission spectroscopy metric, a predicted proxy for the strength of a transmission signal, of 51 ± 9. In its comparison of cold transiting planets with equilibrium temperatures no higher than 255 kelvin, TOI-6981 b ranked third. The metric is not an atmospheric detection, and its value depends on the assumed mass.
Taken together, the paper offers a validated target with a measured radius and period, but not a complete physical portrait. Its mass and density are model-derived, the orbit's eccentricity remains unsettled, and the transmission spectroscopy metric is a forecast rather than an atmospheric detection. The study's conclusions are limited to one system, leaving follow-up observations to determine whether its mass and atmosphere match these expectations.
Paper data and sources
Original title: TOI-6981 b: a transiting sub-Neptune at the water ice line
Authors: Madison G. Scott, Amaury H. M. J Triaud, Emily K. Pass et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-09-29
DOI: Not available
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