Tuesday

The cold classical Kuiper Belt objects (KBOs) have low inclinations and eccentricities, and are the only Kuiper Belt population thought to have formed in-situ. Those majority posses red colours, with (g-r)>0.8, and ~30% are found in binary pairs. Here we report the discovery of a new population of blue coloured objects residing amongst the red cold classical KBOs. Critically, all of these blue coloured objects are found to be in tenuously bound, binary pairs. We show that widely separated binaries like these can survive moderate push-out into the cold classical region during the early phases of Neptune’s migration, moving out by as much as 6 AU. The idea that the push-out survivors are predominantly binary requires that all push-out survivors formed as high multiplicity systems. This implies that KBOs did not form through classical hierarchical accretion, but rather must have formed through some process like gravitational collapse of a cloud of solids, a process that naturally produces high multiplicity systems. Alternatively, if planetesimal growth initially progressed via pebble accretion, it is plausible that binary formation through the L2s process could produce ~100% binary fraction.