Scattered Disk objects have very eccentric orbits, taking
them far beyond Neptune. Thus asteroid 1996 TL 66 ranges
between 35 and 135 astronomical units, with a period of
around 800 years. It may be as much as 500 kilometres (300
miles) across. There are also the Damocloids, referred to
earlier, with perihelia in the inner part of the Solar System.
Some of these trans-Neptunian objects are large. Pride
of place goes to 50000 Quaoar, discovered by observers
using the Oschin 48-inch telescope at Palomar and named
after the creation deity of the Tongva tribe, the original
inhabitants of the Los Angeles basin. Quaoar has a diame-
ter of 1250 kilometres (776 miles), more than half that of
Pluto; it has an almost circular orbit, at a distance of 6460
million kilometres (4014 million miles) from the Sun. The
orbital period is 284 years. Other large trans-Neptunians
are 28978 Ixion (1200 kilometres, 750 miles) and 20000
Varuna (900 kilometres, 560 miles). Only their remoteness
makes them faint. Their identification does make us con-
sider the status of Pluto; can it be regarded as a planet, or
as merely the senior member of the Kuiper swarm?
Moreover, there is always the chance that a new large
body exists in the remote reaches of the Solar System.
Searches for it have been made from time to time, but
whether it actually exists is by no means certain.THE SOLAR SYSTEM
Boundaries of the Solar System
Pholus.Asteroid 5145,
discovered in 1992 by D. L.
Rabinowitz from Kitt Peak
Observatory in Arizona. The
magnitude was then 17. The
diameter may be around
150 km (below 100 miles).
The revolution period is 93
years. The distance from the
Sun ranges between 1305
million and 4800 million km
(810 million and 2980 million
miles). In colour it is red. It
may well be a planetesimal
from the Kuiper Belt.Quaoar.This large
asteroid was discovered in
June 2002 by Chad Trujillo
and Mike Brown at Palomar,
California. It is likely to be
made of rock and ice. 1992 QB 1 .These three
images were taken by Alain
Smette and Christian
Vanderriest, using the 3.5-m
(138-inch) New Technology
Telescope at La Silla
Observatory. The magnitudewas 23; the faint image of
the object is circled. The
distance was then more
than 6000 million km
(3700 million miles) from
the Sun, beyond the orbit
of Pluto.I
t is not easy to define where the Solar System ‘ends’. The
nearest star beyond the Sun is over four light-years away,
corresponding to a distance of over 40 million million kilo-
metres (25 million million miles), so that the region where
the Sun’s influence is dominant may extend out to roughly
half this amount, but there can be no sharp boundary.
Pluto is the largest object in the far reaches of the
Solar System, but in recent years many asteroid-sized bod-
ies have been detected. In 1943 K. Edgeworth suggested
the possibility of a swarm of minor bodies orbiting there,
and the same suggestion was made independently in 1951
by G. Kuiper. The swarm does exist, and is known gener-
ally as the Kuiper Belt. The bodies are of various kinds.
The Centaurs are named after asteroid 2060 Chiron,
found in 1977 by C. Kowal; it is about 180 kilometres
(112 miles) in diameter, and moves mainly between the
orbits of Saturn and Uranus in a period of 50 years. Near
perihelion it develops a coma, though it seems to be much
too large to be classed as a comet. Other Centaurs have
been found – Pholus, Nessus and Asbolus, for example –
though only Chiron develops a coma.
The Cubewanos are called after 1992 QB 1 , the first to be
discovered – in 1992 by D. Jewitt and J. Luu, from Mauna
Kea. Their orbits are of low eccentricity, and most of them
lie in a band around 41 to 47 astronomical units from the
Sun; the orbital inclinations may be high. Their diameters are
of the order of 100 to 300 kilometres (60 to 190 miles).
Plutinos move ‘in resonance’ with Pluto, completing
two orbits against Pluto’s three. Most of them move at a
distance of around 39 astronomical units.D108- 151 UNIVERSE UK 2003CB 7/4/03 5:17 pm Page 137