TOI-712: An adolescent, multi-planet system hosting three transiting mini-Neptunes

Most of the significant changes in planetary evolution occur within the first billion years. Observing young planetary systems allows us to understand the physical mechanisms driving these changes in ways that observations of mature planets cannot. Transit surveys have begun to discover young planets, but mostly have been limited to faint, short-period systems. As an all-sky survey, TESS is able to detect the brightest and rarest types of transiting planetary systems. Here, we report the discovery of a young, multi-planet system orbiting the bright K4.5V star, TOI-712 (V = 10.8, M* = 0.73 Msun, R* = 0.67 Rsun, Teff = 4633 K). From the TESS light curve, we measure a rotation period of ~12.6 days, and an approximate age of 500 Myr. The photometric observations reveal three small transiting planets (Rb = 2.3 Re, Rc = 2.7 Re, Rd = 2.5 Re), with orbital periods of Pb = 9.5 days, Pc = 51.7 days, and Pd = 84.8 days. We calculate that the habitable zone falls between 83 and 325 days, placing TOI-712d near its inner edge. Among known planetary systems, TOI-712 stands out as young, long period, and bright, which will facilitate further characterization.