GeminiFocus July 2017 | Page 6

nances (MMRs) with Jupiter, the Kuiper Belt’s external MMRs with Neptune are just teem- ing with objects — including the famously demoted Pluto. At first glance this is an odd state of affairs, because the action of plac- ing an object into a MMR requires overcom- ing a potential barrier. Once inside one of Neptune’s MMRs, resonant KBOs experience a restoring force keeping them at nearly the same semi-major axis, and are gener- ally protected from other disturbing forces that might cause them to drift through the region. How so many objects got into reso- nance in the first place, however, was per- plexing. Figure 2. Gemini North and the Canada-France- Hawai‘i Telescope (left, background). Both telescopes played a critical role in this research on blue binary Kuiper Belt objects. Photo Credit: Joy Pollard The overabundance of KBOs in resonances with Neptune is the best evidence we have that, at one point, Neptune moved outward to its current position. We have Renu Mal- hotra and her excellent work to thank for that (Malhotra, 1993). Malhotra recognized that if Neptune migrated outward, so too would the locations of Neptune’s MMRs. Those moving MMRs would have a sweep- ing effect on any planetesimal populations they pass over, picking up many of those objects into resonance as the MMRs moved past. This breakthrough realization reflects the mindsets of many KBO scientists; the in- terest is not so much in the KBOs, but what the KBOs tell us about the Solar System’s early formation and evolution. Since Malhotra’s original work, a number of competing theories about Neptune’s out- ward migration have been put forth, which generally fall into two categories: 1) smooth migration, as originally envisioned; or 2) a violent outward jump. The latter is best typified by the Nice model: the gas-giant planets, originally in a more compact con- figuration, through mutual gravitational in- teraction, hopped from their primordial lo- cations to nearly their present-day locations in an explosive outward jump (Tsiganis et al., 2005 ; Levison et al., 2008 ). If this idea is true, Neptune moved outward quite rapidly, by as much as ~ 10 AU. It now appears that some combination of smooth migration and dynamical instability-driven migration is responsible. Investigating the Remnant Populations The picture of the Kuiper Belt we now have is of two remnant populations: the dynami- cally excited objects, and the cold classicals. The dynamically excited objects, or hot ob- jects, which include objects in MMRs, are a remnant that survived the reorganization of the gas-giants, and were scattered into the general Ku