TESS hunt for young and maturing exoplanets (THYME). X. A two-planet system in the 210 Myr MELANGE-5 association

Abstract

Young (<500 Myr) planets are critical to studying how planets form and evolve. Among these young planetary systems, multiplanet configurations are particularly useful, as they provide a means to control for variables within a system. Here, we report the discovery and characterization of a young planetary system, TOI-1224. We show that the planet host resides within a young population we denote as MELANGE-5. By employing a range of age-dating methods—isochrone fitting, lithium abundance analysis, gyrochronology, and Gaia excess variability—we estimate the age of MELANGE-5 to be 210 ± 27 Myr. MELANGE-5 is situated in close proximity to previously identified younger (80–110 Myr) associations, Crius 221 and Theia 424/Volans-Carina, motivating further work to map out the group boundaries. In addition to a planet candidate detected by the TESS pipeline and alerted as a TESS object of interest, TOI-1224 b, we identify a second planet, TOI-1224 c, using custom search tools optimized for young stars (Notch and LOCoR). We find that the planets are 2.10 ± 0.09 R⊕ and 2.88 ± 0.10 R⊕ and orbit their host star every 4.18 and 17.95 days, respectively. With their bright (K = 9.1 mag), small (R* = 0.44 R⊙), and cool (Teff = 3326 K) host star, these planets represent excellent candidates for atmospheric characterization with JWST.