Kuiper belt

The Kuiper belt is a region in the solar system stretching beyond the orbit of Neptune from 40 to 50 AU, which contains billions of frozen bodies. It was discovered in 1992 and since then about 1000 objects have been catalogued. Some of them are very large - the biggest has a diameter in excess of 1000 km.

Nature of 27 November 2003

Despite research by astronomers to explain the structure of this belt, it remains a mystery. Like most of the planets in the solar system, the large objects in the Kuiper belt must have formed from smaller objects which agglomerated when they collided with each other. For this process to operate in the remote regions beyond Neptune, a quantity of matter several tens of times greater than the mass of the Earth would have had to initially have been present in the region where the belt is now. And yet the observations of this region show that it now contains less than a tenth of the mass of the Earth. In the past few years scientists have therefore tried to find out how 99% of the initial material in the Kuiper belt was lost. In a new study, Harold Levison (SwRI) and Alessandro Morbidelli (OCA) showed using digital simulation that perhaps the Kuiper belt has not lost any mass at all!

Levison and Morbidelli suggest that the proto-planetary disk in which the planets, asteroids and comets were formed had an outer limit not previously accounted for, located in the present position of Neptune at 30 AU, and that the region now occupied by the Kuiper belt was empty. All the Kuiper objects that we now see beyond Neptune would then have formed at a distance much closer to the Sun and would have been transported outwards during the final stage of planet formation.

In one theory the giant planets migrated after their formation. Uranus and Neptune in particular were formed closer to the Sun and then migrated outwards. Levison & Morbidelli showed that while it was migrating, Neptune could have pushed all the observed Kuiper objects to distances further from the Sun.

"We haven't really solved the problem of mass loss in the Kuiper belt, we've just side-stepped it!" said Levison. "Space beyond Neptune was probably actually devoid of objects".

Thus, in this model, the inner region at 30 AU contains enough matter to form the objects in the Kuiper belt. The mechanisms by which Neptune pushed the belt outwards only affected a small fraction of the objects that have become what we see now. The rest were ejected from the solar system by Neptune. This new idea explains many of the features observed in the outer solar system, especially the properties of the orbits of Kuiper objects and the position of Neptune.

"One of the most disconcerting aspects of the migration of Neptune is why it stopped where it now is" said Morbidelli. "Our new model explains this, too. Neptune migrated until it reached the edge of the proto-planetary disk where it suddenly stopped.".

Reference: Levison, H.F. and Morbidelli, A., The formation of the Kuiper belt by the outward transfer of bodies during Neptune's migration. Nature Vol. 426, pp 419-421, 2003.