The Spacewatch 0.9-m telescope on Kitt Peak. When Clyde Tombaugh discovered the planet Pluto 70 years ago, the picture of the solar system seemed clear and orderly. It showed our central star, its nine planets, the asteroid belt, and comets visiting occasionally from the dark and chilly fringes of the solar system. Today, thanks to the pioneering asteroid survey Spacewatch and similar projects, our planetary system appears as a humming hive populated with countless asteroids circling the sun like a swarm of bees. Larsen talked about it yesterday at the 24th General Assembly of the International Astronomical Union in Manchester, England.
Centaurs also seem to be related to comets.
From the point of view of their orbits, you cannot tell the difference between the two. Comets get comas and tails as they get closer to the sun, Centaurs never do. Trans-Neptunian Objects are even more remote. They circle the sun at a distance of at least 38 AU in circular orbits comparable with Pluto's orbit. There are at least as many of them in Pluto's vicinity as have been found in the main asteroid belt, a region between the orbits of Mars and Jupiter.