Towards a dynamics-based estimate of the extent of HR 8799’s unresolved warm debris belt

Towards a dynamics-based estimate of the extent of HR 8799’s unresolved warm debris belt

In many ways, the HR 8799 planetary system resembles our Solar system more closely than any other discovered to date – albeit significantly younger, and on a larger and more dramatic scale. The system features four giant planets and two debris belts. The first of these belts lies beyond the orbit of the outermost planet, and mirrors the location of our Solar system’s Edgeworth-Kuiper belt. The second, which has yet to be fully observationally characterised, lies interior to the orbit of the innermost known planet, HR8799 e, and is an analogue to our Asteroid Belt. With such a similar architecture, the system is a valuable laboratory for examining exoplanetary dynamics, and the interaction between debris disks and giant planets.

In recent years, significant progress has been made in the characterisation of the outer of HR8799’s debris disks, primarily using the Herschel Space Observatory. In contrast, the inner disk, which lies too close to its host star to be spatially resolved by that instrument, remains poorly understood. This, in turn, leaves significant questions over both the location of the planetesimals responsible for producing the observed dust, and the physical properties of those grains.

We have performed the most extensive simulations to date of the inner, unresolved debris belt around HR 8799, using UNSW Australia's supercomputing facility, Katana. In this work, we present the results of integrations following the evolution of a belt of dynamically hot debris interior to the innermost planet, HR8799 e, for a period of 60 million years, using an initial population of 500,000 massless test particles. These simulations have enabled the characterisation of the extent and structure of the inner belt, revealing that its outer edge must lie interior to the 3:1 mean-motion resonance with HR8799 e, at approximately 7.5 au, and highlighting the presence of fine structure analogous to the Solar system’s Kirkwood gaps. In the future, our results will also allow us to calculate a first estimate of the small-body impact rate and water delivery prospects for any potential terrestrial planet(s) that might lurk, undetected, in the inner system.

Copyright © 2015 National Space Society of Australia Ltd.
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