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