@Juric2008], as has been suggested for Fomalhaut b [@Kalas2008; @Kalas2013; @Faramaz2015]. On the other hand, after the dispersal of gas and dust, planetesimals could continue growing to form icy planets at tens of au over 100 Myr timescales; however, numerical studies show that once a Pluto size object is formed at 30-150 au within a disc of planetesimals, these are inevitably stirred, stopping growth and the formation of higher mass planets through oligarchic growth [@Kenyon2002; @Kenyon2008; @Kenyon2010]. Thus, it is not yet clear how far from their stars planets can form. Moreover, the discovery of vast amounts of gas (possibly primordial) in systems with low, debris-like levels of dust [so-called “hybrid discs”, e.g. @Moor2017] has opened the possibility for long lived gaseous discs that could facilitate the formation of both ice and gas giant planets at tens of au.

Broad debris discs provide a unique tool to investigate planet formation at tens of au. Planets formed at large radii or evolved onto a wide orbit should leave an imprint in the parent planetesimal belt, and thus in the dust distribution around the system. Gaps have been tentatively identified in a few young debris discs using scattered light observations suggesting the 