tokens: objects typically have red optical colours ( Gulbis et al. 2006 ) with ≈30% of the population found in binaries ( Grundy et al. 2011 ). The cold component comprises ≈50% of the CKB total population ( Petit et al. 2011 ) and is believed to have formed in situ because of the presence of extremely wide binaries that otherwise would have been destroyed by close encounters with Neptune ( Parker Kavelaars 2010 ). (2) The hot classicals (HCs), with a wider range of inclinations and eccentricities compared to CCs. These represent the other 50% of the total CKB population ( Petit et al. 2011 ) 1 . Contrary to the CCs, HCs were probably implanted in their current location, evidenced by the fact that the two populations are very different from each other. Indeed, in addition to having more excited orbits, the HCs show a wider range of photometric colours and a lower binary fraction than CCs ( Noll et al. 2008 ). Resonant KBOs These are objects in mean motion resonance with Neptune, such as Plutinos (3:2), Twotinos (2:1), and 5:3, 7:4, and 5:2 populations, and so on. These resonant KBOs are estimated to make up about 25% of the total Kuiper belt population ( Gladman et al. 2012 ). Gulbis et al. (2006) found that objects in the 3:2 resonance span the full range of KBO colours. The objects in the 4:3, 5:3, 7:4, and 2:1 resonances are predominantly red and the 5:2 resonant objects are grey or neutral in colour. Moreover, the 2:1 resonance inclination distribution is much colder than the 5:2 or 3:2 resonances ( Volk et al. 2016 ). Fig. 1 Eccentricities ( upper figure ) and inclinations ( bottom figure ) of TNOs. The data are from the IAU Minor Planet Center database 2 . The dashed line and the dash-dotted line represent respectively q = 30 and q = 40 au perihelion distances. Vertical solid coloured lines indicate the main mean-motion resonances with Neptune. Scattered disc objects (SDOs) These are the objects found in the scattered disc, which extends from 30 au to roughly 1000 au and interacts with Neptune by close encounters. The SDOs have perihelion distances in between 30 au and roughly 40 au and are mainly grey (or neutral) in colour ( Sheppard 2010 ). Detached objects These have slightly larger perihelion distances than the SDOs and are no longer affected by close encounters with Neptune. The first hypothesis for the very peculiar structure of the Kuiper belt was proposed by Malhotra (1993 , 1995 ) who predicted the existence of objects in external mean-motion resonances with Neptune. In the model, these objects are trapped via adiabatic resonance sweeping during a phase of outward planetesimal-driven migration of the ice giant ( Fernandez Ip 1984 ). Indeed, resonance capture can occur when the orbital frequency of a planetesimal and/or a planet changes gradually and the resonance ‘sweeps’ across the orbit of the planetesimal. Resonance sweeping may result in the capture of the planetesimal in the resonance (if the migration direction is convergent
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