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

We report the Transiting ExoplanetSurvey Satellite ($TESS$) detection of a multi- planetsystem 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 planetcandidate. We also detect a candidate transiting super-Earth($1.4\ R_{\oplus}$) with an orbital periodof 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 planetsof similar size while controlling for age and environment. The ratio of orbital periodsbetween TOI 125 b and c ($P_c/P_b = 1.97$) is slightly smaller than an exact 2:1 commensurabilityand is atypical of multiple planetsystems 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 commensurateperiods, the system is unlikely to be in resonance.