TESS Hunt for Young and Maturing Exoplanets (THYME). VI. An 11 Myr Giant Planet Transiting a Very-low-mass Star in Lower Centaurus Crux

<jats:title>Abstract</jats:title> <jats:p>Mature super-Earths and sub-Neptunes are predicted to be ≃ Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15<jats:italic> R</jats:italic> <jats:sub>⊕</jats:sub> planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05 <jats:italic>R</jats:italic> <jats:sub>J</jats:sub> (9.5 <jats:italic>R</jats:italic> <jats:sub>⊕</jats:sub>) planet transiting a very-low-mass star (0.170 ± 0.015 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup 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 follow-up 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 ≃0.5 <jats:italic>M</jats:italic> <jats:sub>J</jats:sub>. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5 <jats:italic>R</jats:italic> <jats:sub>⊕</jats:sub> planets.</jats:p>