TESS Hunt for Young and Maturing Exoplanets (THYME) VI: an 11 Myr giant planet transiting a very low-mass star in Lower Centaurus Crux

Mature super-Earths and sub-Neptunes are predicted to be ≃Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15R⊕ planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227 b, a 0.85 ± 0.05 RJ (9.5R⊕) planet transiting a very low-mass star (0.170 ± 0.015 M⊙ ) every 27.4 days. TOI 1227’s kinematics and strong lithium absorption confirm it is a member of a previously discovered sub-group in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11±2 Myr for Musca, based on lithium, rotation, and the color-magnitude diagram of Musca members. The TESS data and ground-based followup show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5MJ. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227 b is still contracting and will eventually turn into one of the more common < 5R⊕ planets.