Observations support the hypothesis that thousands of comets are present in a belt on the periphery of our Solar System
THE Solar System is usually thought of as
the Sun and the 9 planets (with their
moons) revolving around it. In this picture, the outer limit of the Solar System
is obviously the orbit of the farthest
planet Pluto at about 40 astronomical
units (I AU is the distance from the Sun
to the Earth, which is about 150 million
lon). Now, astronomers have found evidence for the existence of a part of the
Solar System beyond this limit in the form of comets.
Comets, the mythological messengers of the gods, are comprised of a solid
nucleus composed of ice mixed with
dust grains as small as a millionth of a
metre. As the nucleus approaches the
Sun, the temperature rises and the ice
evaporates. The resulting gas escapes
with the dust particles trailing in a
gigantic tail millions of km long.
Two possible collections of comets
were suggested more than 4 decades
ago. In 1950, astronomer
Ian Oort proposed that.
there exists a spherical
cloud (the Oort cloud)
containing some 100-1,000 billion comets out-side of the Solar System airid yet bound to it gravitationally. This cloud is at
near-interstellar distances
- about 10,000 AU - and
its comets are thought
to be icy planetesimals
(minute planet pieces) which have been
ejected by growing 'proto-planets
(Nature, Vol 374, No 6525).
Around the same time as the Oort
cloud was proposed, K E Edgeworth and
G P Kuiper suggested the existence of
& belt of comets at distances of about
35 AU (beyond Neptune) to 1,000 ALI.
Called the Kuiper belt, this is believed to
be the source for the observed short-
period comets - which complete a'
cycle in less than 200 years.
After computer simulations confirmed the validity of Kuiper's hypothesis, there was a growing interest in
wovw observing these distant
comets. Among the pioneers in this field were
David Jewitt of the University of Hawaii and
Jaue Lou, now at Harvard University.
After many years of intense observations, these researchers in 1992 reported
the discovery of a faint object, designated 1992 QBI and about 41 AU from the
Sun. This is just beyond Pluto's mean
distance from the Sun. This object is
extremely faint (it i's 6 million times
fainter than the faintest star visible to
the naked eye) and is assumed to be
about 250 km in diameter.
After the discovery of 1992 QBI, 23
similar objects have been discovered, 15
of them by Jewitt and Lou. The largest
object discovered is around 360 km in
diameter and the most distant one is
about 46 AU from the Sun.
With these discoveries, astronomers
are now conjecturing about the total
population and composition of the
Kuiper belt. It is estimated that there are
at least 35,000 Kuiper belt objects with
sizes greater than 100 km between 30
and 50 AU from the Sun. The total mass
is inferred to be around 0.003 times the
mass of the Earth. Interestingly, many
astronomers are now of the view that
Pluto and its icy moon Charon are the
largest objects to have accreted in the
Kuiper bell. These may have been accretional runaways, objects that grew big
enough to start eating up their smaller neighbours.
Our Solar System is not unique in
having a Kuiper belt. According to some
observations, Kuiper belts are a corn -
mon feature of star formation. Between
1983 and 1984, the Infrared Astronomy
Satellite discovered extended dust disks
around what are called p73 main-
sequence stars. The dust disks are
thought to have originated in the
comets in orbit around the main-
sequence stars. In fact, subsequent
searches have found evidences of such
disks in nearly half of all main-sequence
stars, which account for about 90 per
cent of all the stars we observe.
With the Hubble Space Telescope
(Hsr) now repaired, there has been a
renewed interest in the Kuiper belt
objects. The better resolution of the
HST has already revealed some 50
objects of around 10-20 kin diameter
in the belt. Though detailed analysis
of these images is yet to be done, it is
evident that the Kuiper belt will provide us with clues on the many unanswered questions about star and planet formation.
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