‘Blue Binaries’ Argue for Smooth Neptune Migration

We’re getting a few clues about the nature of planet migration in the early Solar System thanks to a class of objects being described as ‘blue binaries.’ Cold Classical Kuiper Belt Objects (CCKBOs) are generally reddish, but a population of widely separated binaries has now been identified that is thought to have originated in the inner edges of the Kuiper Belt.

The paper reporting on this work argues that these objects were pushed out a distance of over 4 AU to their present location among the CCKBOs as the result of gravitational interactions with Neptune billions of years ago, a movement induced by the migration of the planet from 20 to 30 AU. If so, we can draw some conclusions about that migration, and we’re reminded in the process of how rich the Kuiper Belt is in objects of different origins.

Led by Wes Fraser (Queen’s University, Belfast), the study used data drawn from the Gemini North instrument and the Canada-France-Hawaii Telescope, both on Mauna Kea, as part of a project called the Colors of the Outer Solar System Origins Survey. The team went to work on spectral data in a range of wavelengths from the ultraviolet to the optical and near-infrared. Its conclusion, reached by checking against models of Neptune’s migration, is that the blue binary pairs moved outward slowly enough that they were not disrupted into single objects.

Image: Artist’s conception of a loosely tethered binary planetoid pair like those studied by Fraser et al. in this work, which led to the conclusion that Neptune’s shepherding of them to the Kuiper Belt was gradual and gentle in nature. Credit: Gemini Observatory/AURA, artwork by Joy Pollard.

Let me quote from the paper re the team’s simulations:

The idea that the blue CCKBO binaries are contaminants that were pushed out into cold classical orbits during Neptune’s migration agrees well with the hypothesis that the blue–red bifurcation of the excited KBO colour distribution is a result of an object’s heliocentric formation distance. In our simulations, surviving binaries were pushed out by no more than 6 au, originating in the ∼38–40 au range. Similar simulations suggest that the red-coloured, widely separated binary 2007 TY430, which now resides in the 3:2 MMR [mean motion resonance], plausibly originated at ∼37–39 au, but could have originated even further out . This would require that the distance inside which blue binaries originated was only a handful of au inside the current inner edge of the cold classical region.

The Kuiper Belt contains a heterogeneous population indeed. Work by Alex Parker (University of Victoria, BC) and J.J. Kavelaars (Herzberg Institute of Astrophysics, Canada) tells us that the Cold Classical Kuiper Belt Objects are the only part of the Kuiper Belt population that formed in place. These objects share a reddish color, low inclinations and low eccentricities, a contrast to a population of dynamically excited objects that come in a range of colors.

Whereas this latter group is comprised of only about 10 percent binaries, the Cold Classical Kuiper Belt Objects exist as binary pairs about 30 percent of the time. The newly discovered blue binaries, though, distinguish themselves from both camps. In an email this morning, Dr. Fraser told me that while a few blue objects were previously known to exist in this region, his team had realized that the blue objects they were studying all came in binary pairs: “It was the discovery of blue+binary together that showed us we were looking at something special.”

Moreover, the binary nature of the objects as the products of Neptune’s ‘push out’ demands that they originally formed as multiple objects. From the paper again:

This push-out scenario would have the startling implication that virtually all planetesimals that formed in the region from which the blue binaries originated must either have formed as binaries or higher multiplicity systems, or attained high multiplicity before push-out occurred. This is required by the fact that all but one of the blue CCKBOs are found in binary pairs.

Thus we can see the blue binaries, originating at about 38 AU, as contaminants that in the paper’s language “…could provide a unique probe of the formative conditions in a region now nearly devoid of objects.” The paper explores various formation scenarios for these binaries that include pebble accretion and later binary production as well as binary formation from the collapse of a cloud of gas and solids that produces bound systems of two or more objects.

The paper is Fraser et al., “All planetesimals born near the Kuiper Belt formed as binaries,” published online by Nature Astronomy 4 April 2017 (abstract). The paper by Parker and Kavelaars cited above is “Destruction of binary minor planets during Neptune scattering,” Astrophysical Journal 722, L204–L208 (2010) (full text).