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The Edge of the Solar System


For almost as long as history has been recorded, six planets have been known: Earth, Mars, Venus, Mercury, Jupiter, and Saturn, and they orbited Earth in that order outwards. This view persisted for thousands 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 small group of people. Some people mark the edge with Pluto, and others believe that the edge exists at Planet X.

The whole concept of an "edge" is somewhat inaccurate as far as the solar system is concerned, for there is no physical boundary to it - there is no wall past which there's a sign that says, "Solar System Ends Here." There are, however, specific regions of space that include outlying members of our solar system, and a region beyond-which the Sun can no longer hold any influence. These locations are what will be discussed here.

Kuiper BeltKuiper Belt Object (KBO) 1992 QB1 Discovery Images

G. Kuiper proposed that there exists a large belt of comets approximately 30-50 A.U. from the Sun (starting near Pluto's orbit). This region is believed to be the source of many short-period comets.

Modern views of the Kuiper Belt hold that it is the source of short-period comets, and that they are nudged from their normally stable orbits by the outer solar system's planets, collisions with each other, or by passing stars. The Kuiper Belt is believed to hold many small objects, but also many large objects - several larger than Pluto.

This is a mixed bag for Planet X proponents. If a giant Planet X exists, then there should be no Kuiper Belt because the Kuiper Belt would have prevented a giant planet from forming due to impacts and gravitational dispersion. But, if Pluto is considered a planet, and anything larger than Pluto is considered a planet, then the Kuiper Belt may hold many Planet Xs.

The first direct evidence that the Kuiper Belt exists was the discovery of the first Kuiper Belt Object (KBO). It was found in 1992, and is called 1992 QB1. Since then, nearly 1000 KBOs have been found, and many of them several hundred kilometers in diameter. A table of some of the larger ones appears below.

KBOs 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 moves, 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 by subsequent observations, it became the first KBO to be discovered. This image is used with permission by its original author; please do not use it without contacting him first.

In the summer of 2005, the KBO 2003 UB313 was announced to have a diameter larger than Pluto's. It has unofficially become the first planet to be discovered in this millennium, and the tenth planet of the solar system. Very little is known about it at this point. Its orbit carries it far into the outer solar system for most of its 560-year orbit. It has not been officially named, nor has it been decided whether it will be granted status as a major planet. If it becomes an official planet, its name will probably follow the traditional Greco-Roman names of the other planets, though it will not be called "Vulcan" because that has been reserved in case a planet is found between the Sun and Mercury. Its spectrum is Pluto-like, so it is believed that the body's albedo - the amount of light it reflects - is about 0.6, which is how its diameter is determined. It has also been discovered that this body has a moon (it was directly imaged using the Keck telescopes in Hawaii). In fact, 3 of the 4 brightest KBOs to date (if Pluto is included) posses a moon, and this means that there must be some common mechanism that provides moons for KBOs. Additionally, with a moon, Kepler's Third Law can be used to determine the mass of the object.


Date Discovered
Diameter (km)
Distance from Sun (A.U.)
2005 FY9 2005 ~1800 39-52
2003 UB313 2003; announced July 2005 2860 38-97 (highly elliptical)
2003 EL61 2003 ~1800 35-52
Quaoar (2002 LM60) June 4, 2002 1200 ± 200 43.249
Ixion (2001 KX76) May 22, 2001 1065 ± 165 39.387
Varuna (2000 WR106) November 28, 2000 900 ± 140 43.233
2002 AW197 January 10, 2002 890 ± 120 47.520

Oort CloudLocation of Sedna

Another outer region 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, and it is the current origin of long-period comets. The way they enter the inner solar system is by gravitational pushes usually caused by a passing star.

The Oort Cloud is a generally accepted theory, though no member of the Oort cloud has been conclusively found. The object "Sedna" that was announced on March 15, 2004, is a possible member of the inner Oort Cloud. Its orbit takes it to within 90 A.U., but as far out as about 900 A.U. from the Sun. It is believed to be about 75% Pluto's size, with a radius of 950±150 km. It is the coldest object in the solar system with a surface temperature of -240 °C (-400 °F), and it is also a very red object - the reddest in the solar system other than Mars. It was originally believed that Sedna had a moon, but that was based upon an erroneous measurement of its "day," which was more accurately determined in 2005 to be about 10 hours instead of 20 Earth days.

Sedna is the farthest known solar system object, and it has a period of 10,500 years. It is still, however, not representative of the Oort Cloud. As shown in the lower left panel of the diagram on the right, Sedna is still well within what was believed to be the inner boundary of the Oort Cloud. For this reason, the Oort Cloud is still hypothetical, with no direct evidence that it exists. But the search is still on.


Another valid boundary region for the "edge" of the solar system is the heliopause. This is the region of space where the sun's solar wind meets that of other stars. It is a fluctuating boundary that is estimated to be approximately 17.6 billion miles (120 A.U.) away. Note that this is within the Oort Cloud.

The two Voyager space craft should cross this region within the next years as they "leave" the solar system to become the first terran interstellar travelers.

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