TIC 166321785.01: a 1.9 Earth-radius planet candidate on a 2.48-day orbit around a nearby K dwarf, found in TESS full-frame images

We report a transiting planet candidate around TIC 166321785 (Gaia DR3 6114787304875186176), a T = 11.1 K dwarf at 96.3 pc, found by Tessa, an open search of TESS full-frame-image light curves of nearby K and M dwarfs that were mostly never observed at 2-minute cadence. Data from 5 sectors show 41 transits with period P = 2.480126 ± 0.000003 d, depth 643 ppm and duration 1.37 h (SNR 14.1), implying a planet radius of 1.87+0.15−0.08 R⊕. The signal passes every test available to us: odd and even transits agree (0.4σ) and there is no significant secondary eclipse (SNR 3.3); the depth is consistent across sectors (p = 0.51); the stellar density implied by the transit shape matches the catalogue value (ratio 2.58, 68% interval 0.64–2.92); difference imaging places the source on the target (5.1″ ± 2.2″, 3 sectors) and on none of the 52 Gaia neighbours bright enough to fake it; Gaia DR3 astrometry (RUWE 1.26) and radial velocities show no sign that the target is a binary; in a leave-one-season-out test, each of 4 seasons is predicted from the others and shows the dip at the predicted times; the identical search on this star's upside-down light curve finds nothing stronger than SNR 8.5, against 14.1 for the dip; TRICERATOPS gives FPP = 0.083 and NFPP = 8.3e-04: a likely planet (FPP < 0.5 and NFPP < 0.001; Giacalone et al. 2021), short of the statistical-validation bar (FPP < 0.015, NFPP < 0.001). The candidate matches no known planet, TOI, CTOI, catalogued eclipsing binary or variable star. Other searches have also reported it (eumemic/exoplanets (GitHub)); Tessa found it independently, and this note adds its own vetting and predictions. We give an ephemeris and predicted transit times (timing uncertainty ≈2 min) for ground-based observations, and set out in advance the criterion by which new data will test the prediction. Files: the preprint (PDF); every predicted transit to 2027-06-30, with ground-site observability (CSV); and the candidate parameters (JSON).