A sub-Earth-sized TESS planet candidate in the nearby M-dwarf binary GJ 774, most likely orbiting GJ 774 A (TIC 351339274)

Abstract

Small planets transiting the nearest M dwarfs are the terrestrial planets most accessible to detailed characterisation. We report a sub-Earth-sized transiting planet candidate in the wide (17.6") M-dwarf binary GJ 774 at 12.86 pc, most likely orbiting the M2.5 primary GJ 774 A (TIC 351339274). The signal was a SPOC multi-sector threshold-crossing event (MES = 7.95) and is not a TESS Object of Interest. In eight TESS sectors (2019-2026) a 1.28-h transit recurs every 4.225138 +/- 0.000010 d with a box-mean depth of 365 +/- 32 ppm in A's light. The 2026 data, which entered neither the SPOC search nor our selection, show it at the ephemeris predicted from 2019-2023 data (304 +/- 43 ppm; none of 10^5 phase-scrambled versions of the 2026 data is as deep). The per-target false-alarm probability is < 0.03 at 95%. Odd/even, secondary-eclipse, archival-plate and Gaia radial-velocity tests exclude unrelated blended eclipsing binaries and eclipsing companions on 4.2-d orbits, and hierarchical eclipsing binaries are improbable. The stars are 0.85 TESS pixel apart, so the TRICERATOPS false-positive probability (0.26) is set by a planet on GJ 774 B. B is disfavoured because its own light does not dim enough: PRF-deblended photometry gives 109 +/- 115 ppm in B's light, against 1089 ppm required if B were the host. Conditional on a point source at A or at B, the joint host model gives P(A)=0.59-0.97 (0.97 for the native target-pixel baseline). Over all sectors the PRF photometry is consistent with A, but in the out-of-sample 2026 data 40% of the deficit appears at B's position, an unexplained 2-3 sigma (PSF-model and depth dependent) tension with host A. On A the planet has R_p=0.70 +/- 0.06 R_Earth, S=15.0 +/- 1.8 S_Earth and T_eq=548 +/- 16 K. It is not validated; resolved ground-based photometry of the pair would test host B directly.