TESS 2-minute data of 25,490 M dwarfs give two new transit candidates at 4.11 h and 5.51 h and limit 2-4 Earth-radius planets at 0.15-0.5 d to below 0.04% of stars

Question: Do public TESS 2-minute light curves of M dwarfs contain transiting planets with periods of 0.15 d to 0.6 d that are not known, and how common are such planets? Most TESS searches start at 0.5 d. Approach: We searched one sector (the most recent) of each of 25,490 TIC M dwarfs with TESS magnitude 12.5 or brighter (sectors 1 to 107) with a box least-squares search. Thresholds were SNR 9 and SDE 9. For the 948 stars above the thresholds, automated tests used up to six sectors (odd-even, secondary, shape, sector and transit depth consistency, size, repetition). We inspected the 17 survivors with transit fits, stellar-density checks, difference images, depth-aperture tests, Gaia neighbours and independent full-frame-image data. 3,000 injections measured the completeness of the search and automated tests. Results: The search keeps 8 of 10 known confirmed planets in this period range. We report two new candidates. TIC 67502817.01 has P = 0.171172940 +/- 0.000000192 d (4.108 h), depth 598 ppm and radius 1.67 Earth radii around an early M dwarf at 93 pc. It is in five sectors (2020 to 2023, three cameras) with one linear ephemeris over 7,943 orbits. The transit-shape density agrees with the host (3.3 against 3.5 g/cm3), the shape is U-like, and the pixel data put the source on the target. If it is a planet, it has the shortest known period for a transiting planet around a main-sequence star and needs a density above about 9 g/cm3. TIC 63562537.01 has P = 0.2297 d (5.51 h), depth 260 ppm and radius 1.00 Earth radius around an early M dwarf at 42 pc. A blind search of independent 2024 full-frame data finds it again. We reject 5 other new signals as binaries, blends or pulsation and cannot resolve 1. For 0.15 d to 0.5 d, the rate of 1-2 Earth-radius planets is 0.17% per star (68% interval 0.12% to 0.25%), and the 95% upper limit for 2-4 Earth-radius planets is 0.035% at 88% completeness. Limitations: One sector per star limits sensitivity to small planets. Completeness excludes the inspection step. TIC stellar parameters are approximate. The candidates need radial-velocity, high-resolution imaging and ground-based follow-up; for TIC 67502817.01 the Roche limit predicts a radial-velocity semi-amplitude of at least about 13 m/s.

TESS · M dwarfs · ultra-short-period planets · transit search · box least squares · planet candidates · TIC 67502817 · TIC 63562537 · occurrence rate · injection-recovery · difference imaging · eclipsing binaries · Roche limit · false positives