Planet formation regulated by galactic-scale interstellar turbulence

Abstract

Planet formation occurs over a few Myr within protoplanetary discs of dust and gas, which are often assumed to evolve in isolation. However, extended gaseous structures have been uncovered around many protoplanetary discs, suggestive of late-stage in-fall from the interstellar medium (ISM). To quantify the prevalence of late-stage in-fall, we apply an excursion set formalism to track the local density and relative velocity of the ISM over the disc lifetime. We then combine the theoretical Bondi-Hoyle-Lyttleton (BHL) accretion rate with a simple disc evolution model, anchoring stellar accretion time-scales to observational constraints. Disc lifetimes, masses, stellar accretion rates and gaseous outer radii as a function of stellar mass and age are remarkably well-reproduced by our simple model that includes only ISM accretion. We estimate $20{-}70$ percent of discs may be mostly composed of material accreted in the most recent half of their lifetime, suggesting disc properties are not a direct test of isolated evolution models. Our calculations indicate that BHL accretion can also supply sufficient energy to drive turbulence in the outer regions of protoplanetary discs with viscous $\alpha_\mathrm{SS} \sim 10^{-5}- 10^{-1}$, although we emphasise that angular momentum transport and particularly accretion onto the star may still be driven by internal processes. Our simple approach can be easily applied to semi-analytic models. Our results represent a compelling case for regulation of planet formation by large-scale turbulence, with broad consequences for planet formation theory. This possibility urgently motivates deep observational surveys to confirm or refute our findings.