Figure 4.
We concluded that the blue color of these
objects was primordial. But how?
Artist’s concept of
a tenuously bound
blue binary pair.
Such loosely coupled
icy fragments may
have joined young
Neptune in its
smooth outward
migration from the
inner to the outer
Solar System.
Credit: Gemini
Observatory/AURA/
NSF, artwork by
Joy Pollard.
A Solution?
systems. But this would act to favor recolor-
ing of the impacted body more frequently
than the secondary. Moreover, single KBOs
would be affected in the same way as the
binaries, and yet we see no blue single cold
classical KBOs.
Figure 5.
Barycentric orbital
elements, eccentricity
(top) and inclination
(bottom) vs. semi-
major axis of KBOs.
The dashed lines
show the boundaries
of the cold classical
region we adopt;
colored points are
CCKBOs with well
measured colors. Grey
points are objects
with no reliable
color measurement.
Black triangles, red
circles, and blue stars
represent single, red
binary (s > 17%), and
blue binary (s < 17%)
CCKBOs.
The idea for a solution occurred upon a re-
view of work by Nesvorný (2015) who argued
for a Neptune migration scenario that in-
volved an early stage of smooth, gentle mi-
gration, followed by a late stage instability
or jump. N-body simulations demonstrated
that during Neptune’s smooth migration,
widely separated binaries could survive
sweep-up and push-out in the 2:1 MMR,
some of which were dropped into the cold
classical region during the later jump. The
key realization was of the gentle push-out
that occurs during smooth migration, and
not the violent scattering that populated all
hot KBO populations. This led us to conclude
that, unlike the red cold classicals, the blue
binaries are interlopers or contaminants that
survived this push-out process (Figures 5
and 6).
From the existence of these blue binaries,
we now know that Neptune must have un-
dergone an early phase of smooth
outward migration. Our simulations
suggest that the blue binaries could be
accounted for if Neptune migrated ~
7 AU over an exponential timescale of
~ 30 million years. It is still early days,
however, as much of the parameter
space around this migration needs to
be tested. How fast could Neptune
have migrated without disrupting the
blue binaries? How far did the binaries
likely get pushed out? These and other
important questions are yet to be de-
termined.
The astute reader will immediately see
the elephant in the room. Beyond what
the blue binaries have told us about
Neptune’s early days, we are faced
with the surprising result that before
14
GeminiFocus
January 2018 / 2017 Year in Review