Making Sense Of Centaurs And Their Kin

Royal Astronomical Society Press Notice

Issued by Jacqueline Mitton, RAS Press Officer

tel: +44 (0)1223-564914

CONTACTS:

Jonathan Horner
Theoretical Physics, University of Oxford
Phone: (+44) (0)1865 273977 (not 18 - 31 August)
Mobile phone: (18 - 31 August) 07900 988145
E-mail:

Professor Mark Bailey
Armagh Observatory
Phone: (+44) (0)28 3752 2928
E-mail

Date: 5 August 2003

PN03-31:

MAKING SENSE OF CENTAURS AND THEIR KIN

It's time for astronomers to abandon the traditional ways they've categorised
comets and distant asteroids and for them to look at all the small bodies
populating the outer solar system in a fresh light, according to four
researchers writing in the 21 August issue of the Monthly Notices of the Royal
Astronomical Society.

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 labelling 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."