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 ?
A Solution ?
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