Validation of a Third Earth-sized Planet in the TOI-2267 Binary System

We report the validation of a third terrestrial exoplanet in the nearby (22 pc) TOI-2267 system. TOI-2267 is a binary system with stellar components TOI-2267A (M5, 3030 K) and TOI-2267B (M6, 2930 K), with an on-sky separation of 0.$^{\prime\prime}$384 (8 au projected separation). TOI-2267 hosts two Earth-sized planets (TOI-2267 b, $1.00\pm0.11 R_{\oplus}$, and TOI-2267 c, $1.14\pm0.13 R_{\oplus}$, if orbiting the primary star; or $1.22\pm0.29 R_{\oplus}$ and $1.36\pm0.33 R_{\oplus}$, respectively, if orbiting the secondary star) with orbital periods of 2.3 and 3.5 days. This system also contains a third Earth-sized planet candidate with an orbital period of 2.0 days that was previously identified as a likely planet with a low false-positive probability, but could not be firmly validated due to the lack of independent observations beyond TESS data. We combine two new transit observations from the 5.1m Hale Telescope at Palomar Observatory with archival TESS data and high-resolution imaging to statistically validate the planetary nature of TOI-2267 d ($0.98\pm0.09 R_{\oplus}$ if orbiting the primary star, or $1.77\pm0.43 R_{\oplus}$ if orbiting the secondary star) using the updated TRICERATOPS+ pipeline. We attempt to determine the host star for TOI-2267 d using transit shape stellar density analysis, but are unable to conclusively assign a host. Our validation of TOI-2267 d suggests that TOI-2267 is either the first known double transiting M dwarf binary system, or hosts three planets in an extremely compact orbital configuration.

💡 Research Summary

The paper presents the statistical validation of a third terrestrial exoplanet, TOI‑2267 d, in the nearby (22 pc) M‑dwarf binary system TOI‑2267. The binary consists of a primary M5 star (TOI‑2267 A, T≈3030 K) and a secondary M6 star (TOI‑2267 B, T≈2930 K) separated by 0.384″, corresponding to a projected distance of ~8 au. Previously, two Earth‑sized planets (b and c) with orbital periods of 2.3 d and 3.5 d were identified from TESS data, but the host star for each remained ambiguous. A third transit signal with a period of ~2.0 d was also detected in the TESS light curves, yet without independent ground‑based confirmation its planetary nature could not be firmly established.

To address this, the authors obtained two high‑precision J‑band transit observations of the candidate using the Wide‑field Infrared Camera (WIRC) on the 5.1 m Hale telescope at Palomar Observatory. The observations employed a beam‑shaping diffuser and 4‑second exposures (co‑added 9×) to achieve stable photometry across full transits and ample baseline. Systematics were modeled with linear combinations of comparison‑star weights, sky background, and PSF width, and the optimal model was selected via Bayesian Information Criterion. Both nights yielded a >3σ detection of the transit depth, and the derived transit shape parameters (impact parameter, Rp/R★, a/R★) were consistent between nights and independent of the assumed host star.

Using the updated TRICERATOPS+ pipeline, the authors recomputed the false‑positive probability (FPP) for TOI‑2267 d, finding FPP < 0.01, well below the conventional validation threshold. Consequently, TOI‑2267 d is statistically validated as a planet. Its radius is 0.98 ± 0.09 R⊕ if it orbits the primary, or 1.77 ± 0.43 R⊕ if it orbits the secondary. Attempts to assign the host via transit‑derived stellar density profiling were inconclusive because the densities of the two M dwarfs are too similar to discriminate with the current data.

The validation leads to two possible interpretations. First, TOI‑2267 may be the first known M‑dwarf binary where transits are observed on both stellar components, implying that each star hosts at least one planet. Second, all three planets could orbit the same star, forming an extremely compact system with periods of 2.0, 2.3, and 3.5 days, near a 3:2 mean‑motion resonance. Either scenario provides a rare laboratory for studying planet formation and dynamical evolution in close (≲ 50 au) binary environments, where disk truncation, altered accretion, and enhanced X‑UV irradiation can affect planet growth and atmospheric retention.

The authors recommend follow‑up observations to resolve the host ambiguity. High‑resolution spectroscopy (e.g., with JWST/NIRSpec or ELT/HIRES) could detect radial‑velocity signals or atmospheric signatures that differentiate the two stars. Transit‑timing variations (TTVs) from continued photometric monitoring could reveal mutual gravitational interactions and confirm whether the planets share a common host. Long‑term RV monitoring would also yield planetary masses, enabling bulk density estimates and assessments of potential habitability. Overall, the discovery of TOI‑2267 d enriches the small but growing sample of planets in tight M‑dwarf binaries and underscores the importance of ground‑based follow‑up for validating TESS candidates in multi‑star systems.