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Establishing the Formation of AF Lep b with NIRCam: The Lowest-Mass Imaged Exoplanet with a Dynamical Mass

Paper abstract: AF Lep b is a recently discovered young (24 Myr) giant planet with a dynamical mass of 3 Jupiter masses and semi-major axis of 8 au. Among about two-dozen imaged planets, AF Lep b is the lowest mass planet with a dynamical mass measurement and the closest analog to Jupiter. There are early signs that the dynamical mass is discrepant with predictions from evolutionary models, which may be caused by delayed formation in a disk and would make the planet younger than its host star. Confirming this discrepancy requires an accurate bolometric luminosity from photometry at thermal wavelengths (3-5 micron). Here we propose JWST/NIRCam imaging during a narrow window of observability in Cycle 2 to empirically constrain the key molecular absorbers (e.g., CH4, CO) and overall shape of AF Lep b's 3-5 micron emission. This will establish the bolometric luminosity of this planet and determine whether its metallicity and C/O ratios are enhanced relative to the host star. By precisely constraining its bolometric luminosity, this program will determine if there is robust evidence of a delayed formation for AF Lep b – the first such measurement for any exoplanet. Orbit fits of AF Lep b indicate that by Cycle 3, there is a high likelihood that the planet will have moved too close to its host star to be successfully recovered. Director's Discretionary Time is therefore required for this program to successfully observe this system within a narrow upcoming visibility window. This may be our only opportunity to obtain JWST imaging of this exciting planet during the JWST mission lifetime.