Accreting planetary-mass companions offer an unprecedented window into substellar formation pathways and planetary accretion physics. However, accretion rate diagnostics are not well calibrated in this regime. Plausible (but unconfirmed) theoretical arguments suggest that different physical processes dominate at planetary vs. stellar masses, challenging the validity of extrapolating T Tauri-derived scaling relationships to planets and brown dwarfs. Observationally, accretion rate estimates for substellar objects show a scatter of more than five orders of magnitude around the canonical object mass-mass accretion rate relation. This spread probably results from a combination of selection effects, invalid scaling relations in certain mass regimes, heterogeneous diagnostics probing different parts of the accretion flow, and true physical effects such as accretion variability and age or surface density variation. I will present our group's efforts to disentangle systematics from true physical effects and to develop methods for reliably estimating mass accretion rates for substellar objects. These efforts include the assembly of the Comprehensive Archive of Substellar and Planetary Accretion Rates (CASPAR) Database and new simulation tools to understand the origins (physical and observational) of observed deviations in accretion rates among objects with similar physical properties. I will highlight preliminary results from observational campaigns on Keck, SOAR, and APO designed to create new empirical accretion templates and scaling relations for low mass objects. I will also outline the unique potential of JWST to inform the physics of accretion onto planets and brown dwarfs. The ultimate goal of these efforts is to establish robust accretion diagnostics for substellar objects, which are needed in order to derive accurate accretion rates for protoplanets and to make inferences about formation pathways for bound and isolated planetary-mass companions.

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