A new classification devised by Jonathan Horner of Oxford University, with collaborators Dr Wyn Evans (of Oxford and Cambridge Universities), and Professor Mark Bailey and Dr David Asher (both of Armagh Observatory) promises a semblance of order to replace much of the confusion about the variety of objects coming under the titles comets, Centaurs, trans-Neptunian objects, Kuiper Belt Objects etc.

These bodies give important clues as to how our planetary system formed but many of them have changed orbits significantly over the lifetime of the solar system due to the gravitational influence of the four giant planets -- Jupiter, Saturn, Uranus and Neptune.

As is the case with plants and animals, a good classification scheme is the first step in understanding how groups with different characteristics relate to each other, and to tracing their evolutionary paths back to their origins.

"Minor bodies between Saturn and Neptune are often described simply as 'Centaurs' and those beyond Neptune simply as 'Kuiper Belt Objects' -- but this is not very enlightening as their histories and fates may be very different, regardless of where they happen to orbit now", says Jonathan Horner. In the same way, the traditional arbitrary labeling of comets as 'long-period' or 'short-period' has not been very consistent or helpful.

The group known as Centaurs are a particular puzzle because they are like hybrids with some comet-like and some asteroid-like characteristics. Many of them are over 100 km across, much larger than the nucleus of a typical comet, yet several have been seen surrounded by a cloud of gas and dust like the coma of a comet. The first Centaur, called Chiron, was discovered in 1977. Since then more than 100 roughly similar objects have been found.

Building on previous research by a number of different astronomers, Horner and colleagues base their scheme on the fact that the long-term changes to the orbit of a body in the outer solar system are controlled by one or two of the four giant planets.

The important factors are the object's distance from the Sun at its closest and farthest points (perihelion and aphelion). These main classes are then subdivided into four types (I, II, III and IV) to take account of the angle the body's orbit makes with the main plane of the solar system.

For example, Chiron is an SU IV object, with its perihelion in Saturn's zone of control and its aphelion in Uranus's. By contrast, the Centaur Pholus is SN III. (N stands for Neptune.)

Professor Mark Bailey comments, "One of the first things our new scheme has done is to help us understand how the very diverse set of objects we are calling Centaurs can be divided into groups with orbits that have broadly similar characteristics. For instance, we can see at a glance that the orbits of Chiron and Pholus evolve in different ways and over different periods of time.

"Now we can more effectively trace back the history and predict the likely future for the different groups of Centaurs. We can examine the chances that one could cross into the inner part of the solar system and, if so, on what time-scale. The enormous amount of dust that would come off such an object as it approached the Sun would be an environmental hazard for Earth."

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