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Neptune not guilty of
moving Kuiper Belt

Posted: 11 October 2010

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New research presented at the American Astronomical Society's Division of Planetary Sciences meeting last week challenges popular theory regarding Neptune's role in shaping the Kuiper Belt.

Diagram illustrating the process that destroys binaries during close encounters. Image via Alex Parker.

Using computer simulations, PhD student Alex Parker from the University of Victoria, with thesis supervisor K Kavelaars from the Herzberg Institute of Astrophysics, found that the surprising number of known loosely bound binary pairs in the Kuiper Belt could not exist if Neptune had played a role in shunting them to the outer reaches of the Solar System – the disruption of such a movement would have torn them apart.

"Our work is the first that uses these binaries to constrain the migration and early evolution of the Cold Classical Kuiper Belt by showing that they could never have been subjected to a period of close encounters with Neptune," Parker tells Astronomy Now. The Cold Classical Kuiper Belt lies in a very flat ring between 6 and 7 billion kilometres from the Sun, and hosts thousands of bodies larger than 100 kilometres across, roughly one-third of which may be in binary pairs. Some of the binary pairs are so loosely bound that they take up to 17 years to complete one orbit around each other.

Illustration of a primordial Kuiper Belt binary during a close approach with the planet Neptune. Image via Alex Parker.

The finding threatens the popular theory of the Nice model (named for Nice, France, where it was established), which proposes that the migration of the gas giants into their present orbital configurations after the initial proto-planetary disc had dissipated explains events such as the Late Heavy Bombardment Era and the existence of small bodies in regions such as the Kuiper Belt and the Oort Cloud, for example. The theory suggests that the inner edge of the Kuiper Belt was initially just beyond the orbits of Uranus and Neptune, which existed much closer to the Sun than they are today, and as Neptune migrated outwards, the Kuiper Belt Objects were flung out to the far reaches of the Solar System.

But the Nice model is not accepted by all planetary scientists, and Parker's work suggests that the Kuiper Belt instead formed near its present location and remained undisturbed over the age of the Solar System. "This work strongly suggests that the low-inclination component of the Kuiper Belt formed in-situ, representing a fossil sample of the debris disc left over from the formation of the Solar System," he says. "Other outer Solar System populations have almost certainly experienced interactions with Neptune (including other parts of the Kuiper Belt), and since these populations do not have the same wide binaries as the Cold Classical Kuiper Belt our work supports this past interaction, since any original wide binaries would naturally be removed by Neptune interactions."

The finding will force Nice model advocates back to the drawing board to account for a Kuiper Belt that forms in-situ. Understanding the history of the Kuiper Belt will provide a better understanding of how our Solar System formed and evolved, and how planets around other stars may be forming today.