An Abrupt Change in the Disk Fraction of Free-Floating Planets at the Deuterium-Burning Ignition Limit

Free-floating planets (FFPs) - planetary-mass objects that roam the galaxy unbound to any star - have been identified through both indirect microlensing detections in the field \citep{Mroz2017, Sumi2023} and direct observations within young, nearby star-forming regions \citep{Tamura1998, Zapatero2000, Lucas2000, PenaRamirez2012, Lodieu2018, Suarez2019, Luhman&Hapich2020, MiretRoig2022a, Pearson2023, Langeveld2024, Martin2024, Luhman2024, DeFurio2024}. Simulations propose that FFPs can arise either by ejection from planetary systems \citep{Chatterjee2008, Boley2012, Juric&Tremaine2008, Veras&Raymond2012} or through gravitational collapse of isolated gas clumps, similar to stars \citep{Bate2002, Padoan&Nordlund2004, Hennebelle&Chabrier2008}. Despite the theoretical framework, the relative contribution of these formation mechanisms remains unclear. One promising approach to distinguish formation scenarios involves examining the presence and characteristics of circumstellar disks around FFPs. However, limited sample sizes have hindered previous studies \citep{Allers2006, Zapatero2007, PenaRamirez2012, Esplin&Luhman2019, Scholz2023}. Here we present an exhaustive survey of disks around a large sample of FFPs and low-mass brown dwarfs (BDs) within the 5-10 Myr old Upper Scorpius association \citep{MiretRoig2022a}. By employing a refined analysis of data obtained with the Spitzer Space Telescope \citep{Werner2004} and the Wide-field Infrared Survey Explorer \citep[{\it WISE},]{Wright2010}, we extend previous disk fraction studies down to objects as low as 7 Jupiter masses (MJup). Our findings reveal a steep increase in disk fraction, from about 20-30% for objects with M>20 MJup to 60-80% at M<20 MJup over a very narrow mass range. Such an abrupt change in the disk fraction points toward a major fundamental change in the physics of either FFP and BD formation mechanisms and/or FFP and BD disk evolution. Strikingly, it occurs near the deuterium fusion ignition threshold at the age of Upper Scorpius \citep[≈20 MJup,]{Chabrier&Baraffe2000}. We speculate that the abrupt change in the disk fraction provides the first observational evidence of deuterium burning’s role in the formation and early evolution of BDs and FFPs, defining a new physical boundary. The extended disk lifetimes around FFPs and low-mass BD likely have significant implications for the formation of companions and moons, presenting exciting opportunities to investigate the formation and evolution of hierarchical isolated planetary systems.