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

Kavli Affiliate: Roland Vanderspek

| First 5 Authors: Andrew W. Mann, Mackenna L. Wood, Stephen P. Schmidt, Madyson G. Barber, James E. Owen

| Summary:

Mature super-Earths and sub-Neptunes are predicted to be $simeq$Jovian
radius when younger than 10 Myr. Thus, we expect to find 5-15$R_oplus$ planets
around young stars even if their older counterparts harbor none. We report the
discovery and validation of TOI 1227 b, a $0.85pm0.05 R_J$ (9.5$R_oplus$)
planet transiting a very low-mass star ($0.170pm0.015 M_odot$) 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$pm$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 $simeq0.5 M_J$. 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 $<5R_oplus$ planets.

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