Figure 4 . 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 : Joy Pollard
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 .
blue ice and recoat the surfaces of only a few systems . But this would act to favor recoloring 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 .
We concluded that the blue color of these objects was primordial . But how ?
The idea for a solution occurred upon a review of work by Nesvorný ( 2015 ) who argued for a Neptune migration scenario that involved an early stage of smooth , gentle migration , 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 undergone 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 determined .
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 pushout , the majority of planetesimals near ~ 35 AU were binary . We know this from the simple fact that no blue singles have been found in the cold classical region .
6 GeminiFocus July 2017
Figure 4.
A Solution?
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: Joy Pollard
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 dur-
ing smooth migration, and not the violent
scattering that populated all hot KBO popu-
lations. 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).
blue ice and recoat the surfaces of only a few
systems. But this would act to favor recolor-
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