Fomalhaut b as a cloud of dust: Testing aspects of planet formation theory

Abstract

We consider the ability of three models - impacts, captures, and collisional cascades - to account for a bright cloud of dust in Fomalhaut b. Our analysis is based on a novel approach to the power-law size distribution of solid particles central to each model. When impacts produce debris with (1) little material in the largest remnant and (2) a steep size distribution, the debris has enough cross-sectional area to match observations of Fomalhaut b. However, published numerical experiments of impacts between 100 km objects suggest this outcome is unlikely. If collisional processes maintain a steep size distribution over a broad range of particle sizes (300 μm to 10 km), Earth-mass planets can capture enough material over 1-100 Myr to produce a detectable cloud of dust. Otherwise, capture fails. When young planets are surrounded by massive clouds or disks of satellites, a collisional cascade is the simplest mechanism for dust production in Fomalhaut b. Several tests using Hubble Space Telescope or James Webb Space Telescope data - including measuring the expansion/elongation of Fomalhaut b, looking for trails of small particles along Fomalhaut b's orbit, and obtaining low resolution spectroscopy - can discriminate among these models. © 2014. The American Astronomical Society. All rights reserved.