The planets
[32] The Kuiper Belt lies
beyond the orbit of Neptune,
and comprises asteroids and
comets, and larger Kuiper Belt
objects such as Pluto and Eris
which have been classified as
minor planets.
The diagram also shows
the position of the asteroid
belt inside the orbit of Jupiter
(between Mars and Jupiter).32Kuiper Belt in honour of the Dutch astronomer Gerard Kuiper, one of the
first to suggest its existence.
A large member of the belt is Pluto, discovered by Clyde Tombaugh
at the Lowell observatory in 1930. When found, it was assumed to be
comparable in size to the Earth or at least Mars, and to be the only
body of that size moving in the far reaches of the Solar System. It had
a strange orbit, much more eccentric that other planets, and when at
perihelion (closest to the Sun), it came within the path of Neptune. Its
orbital tilt is 17 degrees so there is no fear of collision.
Also, Pluto was certainly not a giant. As better measurements were
made, the diameter went down and down, and someone suggested it
would soon vanish altogether. We now know its true diameter is less
than that of our Moon, and Pluto cannot rank as a primary planet. In
the 1990s other bodies were found – Eris is larger and more massive.
Pluto is merely a Kuiper Belt Object (KBO) and only appeared bright
because it is relatively near. Pluto has actually been mapped using the
world’s largest telescopes. It appears to have bright and dark patches on
its surface but that is all we know. A probe is on its way there now and
should pass Pluto in 2014, and we might then find out a great deal more
There have been major discussions about the status of Pluto: some
astronomers, particularly Americans, have been reluctant to demote it
from the dignity of a proper planet. However, facts cannot be denied and
finally a compromise was reached. Several of the Kuiper Belt Objects
have been reclassified as dwarf planets.
All are bitterly cold and we cannot expect any to retain an appreciable
atmosphere. The theory that they were due to the break-up of a larger
body is beset by mathematical problems. It is more likely that the objects
were simply left over when the main Solar System was formed from the
solar nebula.
There are some Kuiper Belt Objects that have long, narrow paths and
make orbital journeys of many centuries. They might even reach the Oort
Cloud, a hypothetical cloud of cometary bodies so far out that we cannot
see it. Its existence is inferred.
As far as we are concerned the Kuiper Belt marks the outpost of the
planetary system. There is always the chance of another major planet
beyond the Kuiper Belt, but this seems very improbable.