A Newly Tripled System: the Nearest, Smallest Exoplanet Younger than 500 Myr

Young planetary systems (<500 Myr) are incredibly invaluable for exploring the questions of planetary formation and the formation of stellar systems. They allow for critical constraints to theories of planetary evolution, and thus their long term evolution and development into late stage systems. Multiplanetary systems are especially invaluable for these analyses. We present the discovery of HD 63433 d, a transiting exoplanet orbiting a young (414±23 Myr), nearby (22.4035±0.0225 pc) solar analog (0.99 ± 0.03 M_⊙) and likely member of the Ursa Major co-moving group. After performing an analysis of the host star's TESS light curves and incorporating updated Gaia DR3 stellar parameters, we calculated a radius of 1.102 ± 0.045 R_⊕ and an orbital period of 4.209 days. HD 63433 d is the third planet detected in the HD 63433 (TOI 1726, TIC 130181866) system and has the smallest orbital separation of the three bodies. To date, HD 63433 d represents the nearest, smallest exoplanet with an age < 500 Myr, and given the brightness of the stellar host G_Gaia = 6.737 mag, this system is an excellent candidate for follow up observations of radial velocity and spectroscopic analysis.