Solution to the debris disc mass problem: planetesimals are born small?

Abstract

Debris belts on the periphery of planetary systems, encompassing the region occupied by planetary orbits, are massive analogues of the Solar system's Kuiper belt. They are detected by thermal emission of dust released in collisions amongst directly unobservable larger bodies that carry most of the debris disc mass. We estimate the total mass of the discs by extrapolating up the mass of emitting dust with the help of collisional cascade models. The resulting mass of bright debris discs appears to be unrealistically large, exceeding the mass of solids available in the systems at the preceding protoplanetary stage. We discuss this "mass problem" in detail and investigate possible solutions to it. These include uncertainties in the dust opacity and planetesimal strength, variation of the bulk density with size, steepening of the size distribution by damping processes, the role of the unknown "collisional age" of the discs, and dust production in recent giant impacts. While we cannot rule out the possibility that a combination of these might help, we argue that the easiest solution would be to assume that planetesimals in systems with bright debris discs were "born small", with sizes in the kilometre range, especially at large distances from the stars. This conclusion would necessitate revisions to the existing planetesimal formation models, and may have a range of implications for planet formation. We also discuss potential tests to constrain the largest planetesimal sizes and debris disc masses.

Understanding the trans-Neptunian Solar system; Reconciling the results of serendipitous stellar occultations and the inferences from the cratering record.

“…Klahr & Schreiber (2020) find that for their assumptions about disk evolution, the formation size slowly increases with distance, but turns over and quickly decreases at 10s of au. Krivov & Wyatt (2020) suggested that large populations of small (∼ km) without accompanying large (∼ 100 km) bodies might be able to resolve some issues with debris disk models that otherwise infer implausibly large masses. If this interpretation is correct, it would be natural to expect a similar population could exist around the Sun.…”