The best image to date of Pluto. Credit: NASA / ESA / M. Buie (Southwest Research Institute) The best image to date of Pluto. Credit: NASA / ESA / M. Buie (Southwest Research Institute)

Pluto and the Kuiper Belt

Sen—Pluto was long thought to be an outpost in our Solar System, beyond which was just empty space. But with the discovery of thousands of similar objects since 1992, it was realised that Pluto is no longer unique and that the Kuiper Belt holds a wealth of information about our Solar System that has yet to be revealed.

Pluto was discovered in 1930 by Clyde Tombaugh at the Flagstaff Observatory in Arizona. By flicking back and forth between photographic plates taken at different times, he noticed an object that moved against the background stars. Pluto was then named the ninth planet of the Solar System, despite its unusual characteristics.

While the eight terrestrial and gas giant planets exist in more or less the same plane around the Sun, Pluto is inclined from the ecliptic by almost 17 degrees. It also has a highly elliptical orbit, meaning that its distance from the Sun varies between 30 and 50 astronomical units. An astronomical unit is the distance between the Sun and the Earth, i.e. 150 million kilometres. When Pluto is closest to the Sun it is also inside the orbit of Neptune.

Pluto has a thin atmosphere comprised mainly of nitrogen, and the atmospheric pressure on the surface is only a tiny fraction of the Earth’s. The surface is made of volatile ices and is a reddish colour due to the presence of methane ice. There are ice caps at the poles, and the surface is most likely cratered. The interior is estimated to be comprised of 70% rock and 30% ice.

Pluto’s largest moon Charon was discovered in 1978 as a “bump” that moves around Pluto. It is has a diameter of around 1200 kilometres, making it nearly half the size of Pluto, which has a diameter of 2300 kilometres. This effectively makes the duo a “binary planet.”

Pluto and Charon share another connection. Pluto rotates once every 6.4 days, as does Charon, and Charon also orbits Pluto in 6.4 days. This makes the pair tidally locked, so that the same side of Pluto is always seen from Charon and vice versa.

Their sizes and masses can be measured as the two objects eclipse each other. The mass of Pluto was determined in 1985 for the first time and the results showed that it is only 0.002 Earth masses.

Charon has no detected atmosphere, and the fact that its mass is lower than Pluto’s mass means that it probably doesn’t have enough surface gravity to retain an atmosphere. The blue colour of Charon is most likely from water ice and its interior would likely have more ice than Pluto.

Two other moons of Pluto were discovered in 2005 by the Hubble Space Telescope and these have been named Nix and Hydra, which are estimated to be 40 and 160 kilometres in diameter respectively. A fourth moon, P4, was discovered in 2011 and was later named Kerberos. In 2012 the same team which discovered Kerberos found a fifth moon, P5, which was later named Styx.

Pluto and its five moons imaged by Hubble

The Hubble Space Telescope's image of Pluto and its five moons taken in July 2012. Credit: NASA

Pluto and Charon’s classification of being “unique” was shattered in 1992 with the discovery of the first Kuiper Belt Object (KBO). The search for these elusive objects had begun in 1986 by David Jewitt and Jane Luu, who wondered if the space beyond Pluto was really as empty as it appeared to be. They used both photographic plates and a CCD camera for their search, but eventually abandoned the photographic plates in favour of the ever improving CCD technology. In 1992 their patience was rewarded when they spotted a KBO clearly moving against the background stars.

The object was called 1992 QB1 and like Pluto it has a red colour which points to frozen methane on the surface. It is located at 41 astronomical units from the Sun and has a diameter of around 250 kilometres.

Since 1992, over 1000 KBOs have been discovered, and it is estimated that the Kuiper Belt contains hundreds of thousands of these icy objects. The Kuiper Belt is named for Gerald Kuiper, who postulated its existence in the 1950’s. It is also sometimes called the Edgeworth-Kuiper belt, including the name of Kenneth Edgeworth who also had the same idea in the 1940’s. The Kuiper Belt extends between 30 and 55 astronomical units, and it is the source of the short period comets.

Some of the newly discovered KBOs turned out to be quite large. In fact for some time it was thought that Eris was actually larger than Pluto. Later measurements showed that Pluto still reigns as king of the Kuiper Belt, but this revelation cast doubts on classing Pluto as a planet. At a meeting of the International Astronomical Union in 2006, Pluto was demoted to the class of dwarf planet, but made the prototype of this class.

Many KBOs have similarly eccentric orbits as Pluto, and over 30 are also in a binary system. Binary KBOs can only be detected if there is a large brightness difference between the two objects and it is likely that more than 10% of KBOs are binary objects.

The KBOs exist in their own particular neighbourhoods and are generally divided into three groups. The majority of the KBOs have low eccentricity orbits and are called Classical KBOs. The Resonant KBOs are so called as they exist in a 3:2 resonance with Neptune – as does Pluto. A 3:2 resonance means that for every three orbits of Neptune around the Sun, Pluto does two. There are also Scattered disc KBOs, which exist far from the Sun and are on highly elliptical and inclined orbits.

The Kuiper Belt holds a wide variety of objects, such as the rugby ball shaped Haumea. Haumea spins very rapidly as a result of a past impact, which is the most likely cause of its shape. It also has two small satellites called Hi'iaka and Namaka.

The farthest icy object from Earth ever discovered is at a distance so great that it is not even in the Kuiper Belt. Discovered in 2004, Sedna has a diameter of around 1500 kilometres. Sedna takes 10,500 years to orbit the Sun and its orbit ranges from 76 to 1000 astronomical units. It has been suggested that Sedna belongs to the inner Oort cloud, a hypothesised cloud of objects surround the Solar System at vast distances.

Sedna's orbit

Sedna's orbit stretches far beyond the Kuiper Belt. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

In 2006, NASA launched its New Horizons spacecraft which will reach Pluto and Charon in 2015. After its flyby, it will head further out into the unknown to investigate the Kuiper Belt Objects.


Pluto was discoverd in 1930 by Clyde Tombaugh.

Pluto's largest moon Charon was discovered in 1978.

Pluto has 4 moons - Charon, Nix, Hydra and P4

The first Kuiper Belt Object was found in 1992 and given the name 1992 QB1.

There have since been thousands of KBOs discovered, and it is thought that hundreds of thousands exist.


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