# Near-resonance in a system of sub-Neptunes from TESS

## TL;DR

This paper reports the discovery and validation of a multi-planet system around TOI 125 from TESS data, including sub-Neptunes and candidates, with analysis indicating the system is likely not in resonance despite near 2:1 period ratios.

## Contribution

First detailed characterization of a multi-planet system with sub-Neptunes around TOI 125, including validation and dynamical analysis of period ratios.

## Key findings

- Three validated sub-Neptune planets with radii around 2.8 R⊕.
- Detection of two low signal-to-noise planet candidates, including a super-Earth.
- The system's period ratios are close to, but not in, exact resonance, and are dynamically stable.

## Abstract

We report the Transiting Exoplanet Survey Satellite ($TESS$) detection of a multi-planet system orbiting the $V=10.9$ K0 dwarf TOI 125. We find evidence for up to five planets, with varying confidence. Three high signal-to-noise transit signals correspond to sub-Neptune-sized planets ($2.76$, $2.79$, and $2.94\ R_{\oplus}$), and we statistically validate the planetary nature of the two inner planets ($P_b = 4.65$ days, $P_c = 9.15$ days). With only two transits observed, we report the outer object ($P_{.03} = 19.98$ days) as a high signal-to-noise ratio planet candidate. We also detect a candidate transiting super-Earth ($1.4\ R_{\oplus}$) with an orbital period of only $12.7$ hours and a candidate Neptune-sized planet ($4.2\ R_{\oplus}$) with a period of $13.28$ days, both at low signal-to-noise. This system is amenable to mass determination via radial velocities and transit timing variations, and provides an opportunity to study planets of similar size while controlling for age and environment. The ratio of orbital periods between TOI 125 b and c ($P_c/P_b = 1.97$) is slightly smaller than an exact 2:1 commensurability and is atypical of multiple planet systems from $Kepler$, which show a preference for period ratios just $wide$ of first-order period ratios. A dynamical analysis refines the allowed parameter space through stability arguments and suggests that, despite the nearly commensurate periods, the system is unlikely to be in resonance.