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.