T hink about the far reaches of the solar system, where Uranus, Neptune and Pluto orbit. Is there any reason why it should suddenly stop? Why shouldn't there be other planets - even if only small ones - at the periphery of the solar system?
Ever since Pluto was spotted in 1930 people have speculated about a Planet X, a tenth planet awaiting identification. Part of the reasoning was that Uranus and Neptune, discovered in 1781 and 1846 respectively, seemed to have slight wobbles in their motions around the Sun. Perhaps they were being perturbed by the gravitational tugs of one or more large unseen planets. We knew Pluto was too small to be the culprit, leading to a widespread belief in Planet X.
This idea was based on a false premise. When Nasa's Voyager 2 spacecraft flew past Uranus in 1986, and then Neptune in 1989, accurate radio tracking of how its path was deviated by the gravity of those planets enabled space researchers to derive more precise values for their masses. It turned out that the previous evaluations were out by a few parts in a thousand, and the apparent wobbles disappeared from the theoretical modelling.
A massive Planet X was ruled out in 1992. But late in the same year a small planet was found beyond Pluto. The discovery, by a team led by David Jewitt of the University of Hawaii, was not unexpected.
If our models for the formation of the planets is correct, with a gradual build up of massive bodies from collisions and amalgamations between smaller lumps, then we would expect residual building blocks to remain beyond Neptune.
For decades astronomers had argued that Pluto was simply the largest and consequently the brightest of the lumps left at the edge. In the early 1950s the Dutch-Ameri can astronomer Gerard Kuiper had hypothesised that there should be thousands of these in a band at the periphery of the solar system. This was dubbed the Kuiper belt even before the first member was found.
Recently it was recognised that Irish astronomer Kenneth Edgeworth independently suggested this in the late 1940s, so researchers started to call it the Edgeworth-Kuiper belt. Now, historical digging has shown that in 1930, just a few months after Pluto was spotted, University of Chicago astronomer Frederick Leonard published a paper in which he suggested that Pluto was not alone.
It is not clear whether Leonard understood just how crowded the region happens to be. Right now - less than eight years since Jewitt found the first trans-Neptunian object - the box score stands at 288 minor planets way out there in the belt, with more being added every week. These bodies are so far away that it takes light several hours to reach us after being reflected by their surfaces.
Pluto is small by planetary standards, only about 1,500 miles across. It has a natural satellite, Charon, which was discovered in 1978 and is about half Pluto's size. The horde of independent bodies now known range in diameter from about 60 up to several hundred miles, although those are simply educated guesses based upon how much sunlight they reflect.
Astronomers have been amazed at the different patterns demonstrated by these new-found minor planets. For example, many seem to have orbital periods (the time taken to complete a circuit about the Sun) which are simple multiples of that of Neptune. They complete two orbits for every three of Neptune, or three for every four of Neptune, and so on. This was not unexpected (a similar effect ensures that Pluto cannot collide with Neptune although their orbits cross), but it does indicate that something systematic is going on.
A different population of objects in the outer solar system is called the Centaurs. The first Chiron, was found in 1977. Since then several dozen have been found, ranging up to a couple of hundred miles in size. Some show signs of cometary activity, such as gaseous constituents being released as they come slightly closer to the Sun. Thus it is thought that these are, in common with the trans-Neptunian objects, really giant comets - bundles of ice and rock - largely inert simply because the environment is so cold where they orbit.
The Centaurs have trajectories that make close approaches to the outer planets feasible. Such approaches may result in an impact, but more likely would be an alteration of the Centaur orbit around the Sun. Because of this they are unstable, with gross dynamical changes being inevitable. It is even possible that one could fall into the inner solar system.
That would produce a phenomenal comet. Hale-Bopp, seen by many people four years ago, had a solid core about 25 miles across deep within its gaseous cloud and tail. A good-sized Centaur coming in to cross the Earth's orbit would be much, much brighter, and there are historical records that seem to speak of such events.
Using the new generation of large telescopes on mountain s in Hawaii, Chile and the Canary Islands astronomers are finding that space is far from empty. Pluto may be far away, but it is not alone. It is accompanied by thousands of little Plutinos, the existence of which we are only just beginning to map out.
• Duncan Steel researches asteroids and comets at the University of Salford .
