Physical Vetting of the Ultra-Short-Period Sub-Earth TOI 864.01

TL;DR

TOI 864.01 is an ultra-short-period ($P = 0.52067 d$) sub-Earth-radius candidate ($R_p ≈ 0.55 R_Earth$) orbiting an M-dwarf host TIC 231728511. The authors combine 54 sectors of TESS photometry with Gaia DR3 astrometry and physical stability arguments to vet the signal, addressing false positives even when standard metrics (FPP, Δln Z) are inconclusive due to shallow transit depth. They demonstrate that the lack of ellipsoidal variations (<200 ppm) and a transit-derived density consistent with the host star argue against stellar companions and background blends, while centroid stability and NFPP = 0 further support a planetary origin. The study provides a robust physical vetting pathway for USP sub-Earths and discusses prospects for mass constraints via radial velocities, which are challenging but potentially informative.

Abstract

We present a comprehensive analysis of TOI 864.01, a transit-like signal associated with the M-dwarf TIC 231728511. Utilizing the full baseline of TESS photometry (54 sectors), we recover a periodic signal with $P = 0.52067$ d and a shallow depth of $\sim$158 ppm. To assess the planetary nature of the candidate, we performed a rigorous vetting process combining centroid analysis, Bayesian model comparison, and false-positive probability calculations. While the low signal-to-noise ratio of the sub-Earth candidate yielded inconclusive formal statistical validation metrics (FPP) and Bayesian evidence ($Δ\ln Z$), we demonstrate the planetary nature of the system through physical exclusion of false positive scenarios. The TRICERATOPS Nearby False Positive Probability (NFPP) of 0.0000, combined with centroid stability, rules out background contamination. Furthermore, we calculate that a stellar-mass companion at the derived orbital separation ($a \approx 5 R_\star$) would induce ellipsoidal variations of order $\gtrsim 5000$ ppm. The absence of such variations in the TESS photometry ($< 200$ ppm limit) physically precludes stellar binary scenarios. We derive a planetary radius of $R_p \approx 0.55 R_\oplus$, confirming TOI 864.01 as a physically vetted ultra-short-period sub-Earth.

Figures (3)

• Figure 1: Phase-folded TESS light curve utilizing all 54 available sectors, binned to 5 minutes (blue points with error bars). The clear U-shape morphology and shallow depth ($\sim 158$ ppm) are consistent with a transiting planet. Note that the absence of secondary eclipses is further analyzed in Figure \ref{['fig:oddeven']}.
• Figure 2: Centroid motion analysis showing stability in both column and row coordinates, ruling out background false positives (referenced in Section 4.1).
• Figure 3: Odd-Even transit depth comparison. The even (blue circles) and odd (orange squares) transits show consistent depths within $1\sigma$, ruling out blended binary scenarios with twice the orbital period.