Initial Entropy and Potential Delayed Formation of the Directly Imaged Exoplanet AF Lep b

Abstract

AF Lep b is a rare directly imaged exoplanet with properties consistent with core-accretion evolution models. Using its precise dynamical mass and spectroscopically inferred bolometric luminosity alongside the Spiegel & Burrows evolution models, I determined an initial entropy >8.7 k B /baryon at 3σ and an age of 12 ± 4 Myr for AF Lep b. Comparing this planet’s age to the isochrone age (24 ± 3 Myr) of its host association, the β Pictoris moving group (BPMG), suggests AF Lep b formed 12 ± 5 Myr later than its host star. Alternatively, the BPMG’s updated kinematic age ( 16.3−2.1+3.4 Myr) implies that AF Lep b formed 5 ± 5 Myr after its host star’s formation, aligning more closely with protoplanetary disk dispersal timescales. The sensitivities of our findings to the planet’s mass and luminosity are discussed. AF Lep b uniquely facilitates the constraints of its initial entropy and formation epoch, paving the way for similar insights into forthcoming exoplanet discoveries.