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

Abstract 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 . ″ 384 (8 au projected separation). TOI-2267 hosts two Earth-sized planets (TOI-2267 b, 1.00 ± 0.11 R ⊕ , and TOI-2267 c, 1.14 ± 0.13 R ⊕ , if orbiting the primary star; or 1.22 ± 0.29 R ⊕ and 1.36 ± 0.33 R ⊕ , 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 5.1 m 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 ± 0.09 R ⊕ if orbiting the primary star, or 1.77 ± 0.43 R ⊕ if orbiting the secondary star) using the updated TRICERATOPS+ pipeline. We attempt to determine the host star for TOI-2267 d using a transit shape stellar density analysis, but are unable to conclusively assign a host. Our validation of TOI-2267 d suggests that TOI-2267 either is the first known double-transiting M-dwarf binary system or hosts three planets in an extremely compact orbital configuration.