Pluto is no longer the last outpost of the Solar System, but rather the gateway to the Kuiper Belt. The Kuiper Belt is a much bigger and colder version of the asteroid belt, being also composed of left-overs from the formation of the Solar System.

It’s difficult to imagine how large and distant the Kuiper Belt is, but this diagram shows the Solar System to scale. In order to fit the Kuiper Belt in, the "Sun" covers the orbits of Mars, Venus and Earth, and neither Mars nor the asteroid belt is shown.

Neptune, whose orbit is the inner boundary of the Kuiper Belt, is about 4.5 billion km (2.8 billion miles) from the Sun. Such distances are unmanageable in kilometers so we give them in astronomical units (AU) – 1 AU is the distance from the Earth to the Sun. Since Neptune is 30 AU from the Sun, it’s thirty times farther from the Sun than we are. The belt then extends a further 20 AU.

The objects beyond Neptune’s orbit are known as trans-Neptunian objects (TNOs). The ones that are in the Kuiper Belt are also Kuiper Belt Objects (KBOs). Three KBOs have already been designated as dwarf planets: Pluto, the largest, plus Haumea and Makemake (pronounced MAH kay MAH kay).

These three dwarf planets are unusually large for the Kuiper Belt, but are still smaller than the Moon. The main materials of the Kuiper Belt are various “icy volatiles” and rock. Icy volatiles are substances such as water, ammonia and methane that are liquids or gases on Earth, but frozen in the outer Solar System.

It’s estimated that there are at least 70,000 KBOs of 100 km (62 miles) or more across, and millions of smaller ones. Yet although the Kuiper Belt is large and its contents are numerous, the KBOs are small, widely spaced and low in density. If you could gather them all up and make one object of them, you’d need another nine or more Kuiper Belts to get an object of the mass of the Earth.

The first object to be discovered beyond Pluto and Charon is known as 1992 QB1 – it hasn’t been given a name. After five years of searching, English astronomer David Jewitt and Vietnamese-born American astronomer Jane Luu found it in August 1992. The date of this discovery is often given as the discovery date of the Kuiper Belt.

The existence of such a region had been under discussion for many decades. In fact, it was soon after Pluto’s discovery in 1930 that American astronomer Frederick C. Leonard (1896-1960) suggested that Pluto might not be a new planet, but the first of a new class of objects. In 1943 Irish astronomer Kenneth Edgeworth (1880-1972) wrote that the outer region of the Solar System should contain a large number of small bodies and he also thought it would be a source of comets.

Interestingly, Dutch-American astronomer Gerard Kuiper (1905-1973) independently proposed in 1951 that a disk of small icy bodies would be left over from the formation of the Solar System. Despite his conclusion that the disk no longer existed, it was named for him, though it’s also sometimes called the Edgeworth-Kuiper Belt.

Pluto and Charon, of course, are trans-Neptunian objects. For twenty years of their 248-year orbit they are closer to the Sun than Neptune, but they travel well beyond this during the remainder of the orbit. Pluto was the first KBO discovered, but it took six decades to confirm that it wasn't unique. Since 1992 over 1300 trans-Neptunian objects have been discovered.

Astronomers have found two distinct populations of objects in the Kuiper Belt. They are even distinguished by different colors, though the reason for this is still not understood.

The "classical" KBOs are in nearly-circular orbits close to the ecliptic, the plane of the Solar System. They probably formed where they are now.

The second group of objects have more elongated orbits which are tilted at an angle to the ecliptic. They probably formed closer to the Sun, maybe near Jupiter, but moved outwards when Uranus and Neptune did.

The Kuiper Belt seems to be the back of beyond. It’s so far away from us that there the Sun would simply look like a particularly bright star. Yet there’s more. Even farther away than the Kuiper Belt is the Scattered Disk - it stretches out another 50 AU. Its objects (SDOs) are in very elongated orbits crossing the plane of the Solar System at steep angles.

The Scattered Disk is where Eris was discovered in 2005. This discovery finally pushed the International Astronomical Union (IAU) into defining what a planet was. Eris is more massive than Pluto, but may be slightly smaller in diameter. On this diagram you can see the classical KBOs as white dots, and the orbits of Pluto and Eris. Eris’s orbit is at angle of more than forty degrees to the ecliptic.

Scattered Disk objects come in close enough to be affected by Neptune's gravity and the orbits are unstable. This is where some short-period comets are expected to originate. Most comets however come from the Oort Cloud which astronomers think surrounds the Solar System starting at 20,000 AU.

NASA’s New Horizons probe is on its way to the outer Solar System. It’s expected to arrive at Pluto in 2015 and then move on to other Kuiper Belt objects. No doubt for every question it answers, there will be many new ones.

References:

(1) Karen C. Fox, “Kuiper Belt of Many Colors” http://www.nasa.gov/topics/solarsystem/sunearthsystem/main/kuiper-colors.html
(2) NASA, “Kuiper Belt and Oort cloud Lithograph” http://www.nasa.gov/audience/foreducators/topnav/materials/listbytype/Kuiper_Belt_Lithograph.html
(3) The Planetary Society, “Types of Trans-Neptunian Objects” http://planetary.org/explore/topics/trans_neptunian_objects/facts.html (accessed 2011-07-24)