The 'Edge'

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> The 'Edge'

For almost as long as history has been recorded, six planets have been known: Earth, Mars, Venus, Mercury, Jupiter, and Saturn. This view persisted for hundreds of years, and people thought that Saturn was the edge of the solar system. The discovery of Uranus in modern times pushed the edge even further, and then Neptune and Pluto expanded the width of the solar system to twice its previously "known" size.

The discovery of Pluto prompted more speculation that there could be other planets out there, and the existence of a "Planet X" is still passionately believed to exist by a minority group of scientists. Some people mark the edge with Pluto, and others still believe that the edge exists at Planet X.

Kuiper Belt

More recent theory postulates that there exists a very large belt of asteroids and comets beyond Pluto that extends outwards for several A.U., that is also suspected of being the source of many short period comets. Most planetary models show that if the Kuiper belt exists, then there is no Planet X, for it would not have been able to form, much less stay intact with all of the debris in the Kuiper Belt.

Unfortunately for the Planet X proponents, many Kuiper belt objects (KBO's), have been discovered in recent years. The first was found in 1992, and is called 1992 QB1. Since then, over 500 KBO's have been found, many of them several hundred kilometers in diameter. A table of the larger ones appears below.

KBO's are very difficult to detect due to their vast distances from the sun and very low surface reflectivity. Once identified, it is also very difficult to determine anything about them, even their size. The sizes in the table below are estimates that are based upon guesses as to how much light they reflect.

The image to the right shows how 1992 QB1 was discovered. By photographing the same patch of sky over the course of several hours or days, and then looking for anything that you don't expect, you can find asteroids, comets, or other objects. In this case, it was determined that the unexpected object (arrows) was at a distance that would put it in the Kuiper Belt. Confirmed and extrapolated, it became the first KBO to be discovered.

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Oort Cloud

Another strong theory for defining the edge of the solar system is the Oort Cloud, a theoretical cloud of comets that surrounds the solar system out to a distance of two light-years (11.36 trillion miles). The Oort Cloud was first proposed by Jan Oort in 1950. This "land of comets" contains comet-forming nuclei left over from the formation of the solar system. It is currently thought that this is the location where all comets originate. The way they enter the inner solar system is by gravitational pushes, usually by a passing star.

The Oort Cloud is believed by most scientists to exist, but it is still only theoretical. There have been no direct sightings of any Oort Cloud members, unlike the Kuiper Belt.

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Heliopause

Most people mark the edge of the solar system where the sun's solar wind meets that of other stars. This is a fluctuating boundary called the heliopause. The heliopause is estimated to be approximately 17.6 billion miles away.

The two Voyager space craft should cross this within 12 years.

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