planetesimals can and do form at tens and hundreds of au in extrasolar systems [e.g. @Su2006; @Hillenbrand2008; @Wyatt2008; @Carpenter2009; @Eiroa2013; @Absil2013; @Matthews2014pp6; @Thureau2014; @Montesinos2016; @Hughes2018], although the exact planetesimal belt formation mechanism is a matter of debate [e.g. @Matra2018mmlaw].

It is natural then to wonder *how far out can planets form?* In situ formation of the imaged distant gas giants is challenging as the growth timescale of their cores can easily take longer than the protoplanetary gas-rich phase [@Pollack1996; @Rafikov2004; @Levison2010]. Gravitational instability was thought to be the only potential pathway towards in-situ formation at tens of au [@Boss1997; @Boley2009], but the revisited growth timescale of embryos through pebble accretion could be fast enough to form ice giants or the core of gas giants during the disc lifetime [@Johansen2010; @Ormel2010; @Lambrechts2012; @Morbidelli2012pa; @Bitsch2015; @Johansen2017]. Alternatively, the observed giant planets at tens of au may have formed closer in and evolved to their current orbits by migrating outward [@Crida2009], as could be the case for HR 8799 with four gas giants in mean motion resonances [@Marois2008; @Marois2010] surrounded by an outer debris disc [@Su2009; @Matthews2014hr8799; @Booth2016], or may have been scattered from closer in onto a highly eccentric orbit [@Ford2008; @Chatterjee2008; 