Implications of the Discovery of AF Lep B: the Mass-Luminosity Relation for Planets in the Β Pic Moving Group and the L-T Transition for Young Companions and Free-Floating Planets

Dynamical masses of young planets aged between 10 and 200 Myr detected inimaging play a crucial role in shaping models of giant planet formation.Regrettably, only a few such objects possess these characteristics.Furthermore, the evolutionary pattern of young sub-stellar companions innear-infrared colour-magnitude diagrams might diverge from free-floatingobjects, possibly due to differing formation processes. The recentidentification of a giant planet around AF Lep, part of the beta Pic movinggroup (BPMG), encouraged us to re-examine these points. We considered updateddynamical masses and luminosities for the sub-stellar objects in the BPMG. Inaddition, we compared the properties of sub-stellar companions andfree-floating objects in the BPMG and other young associations remapping thepositions of the objects in the colour-magnitude diagram into adustiness-temperature plane. We found that cold-start evolutionary models donot reproduce the mass-luminosity relation for sub-stellar companions in theBPMG. This aligns rather closely with predictions from 'hot start' scenariosand is consistent with recent planet formation models. We obtain rather goodagreement with masses from photometry and the remapping approach compared toactual dynamical masses. We also found a strong suggestion that thenear-infrared colour-magnitude diagram for young companions is different fromthat of free-floating objects belonging to the same young associations. Ifconfirmed by further data, this last result would imply that cloud settling -which likely causes the transition between L and T spectral type - occurs at alower effective temperature in young companions than in free-floating objects.This might tentatively be explained with a different chemical composition.