Bob the Alien's Tour of the Solar System
Bob the Alien's Tour of the Solar System

Welcome to Pluto

Pluto (image credit: NASA/New Horizons)

Pluto is a dwarf planet orbiting the Sun many millions of miles away from it in a region of the Solar System known as the Kuiper Belt beyond the orbit of Neptune. With a diameter of 2,374 kilometres (1,475 miles), Pluto is smaller than all of the eight regular planets in the solar system, and also smaller than seven of their moons, including Earth's moon. Even so, Pluto is the largest of the five dwarf planets. Pluto is so far from the Sun that it takes 248 years to complete an orbit. A day on Pluto, the length of time it takes to spin once on its axis, is just under six and a half Earth days (6.4 days).

Pluto was discovered on 18th February 1930 by American astronomer Clyde Tombaugh at Lowell Observatory in Arizona. He was looking for an object beyond Neptune that might be affecting its orbit. He spotted Pluto when he noticed an object appearing in two different places when comparing two images of the same part of space. Although is has the same name as Mickey Mouse's dog, Pluto is actually named after the Roman God of the Underworld who has the power of invisibility. Its name was suggested by Venetia Burney, an 11 year old English schoolgirl, when hearing the news of the discovery. Her grandfather passed the name to an British astronomer who contacted the Lowell Observatory with it. Pluto officially received its name on 1st May 1930.

Charon (image credit:
NASA/New Horizons)

Pluto has five moons known to orbit it. These moons are Charon, Styx, Nix, Kerberos and Hydra. Charon was discovered in 1978 and is just over half the size of Pluto at 1,212 km diameter (753 miles). The two objects appear to spin around each other, taking 6 days to complete a full orbit. If Pluto was still a planet, the two objects would probably be classed as "binary planets". Two smaller, irregularly-shaped moons, were also discovered in October 2005. These moons, named Hydra and Nix take 38 days and 25 days respectively to complete their orbits. A fourth moon, now named Kerberos, was discovered in 2011, and a fifth moon, Styx, was discovered in 2012.

In 2015, a space craft called New Horizons flew by Pluto and its moons. It sent back detailed pictures and lots of data about these worlds which had never been visited before. It revealed perhaps some of the most diverse surfaces in the solar system. Pluto has mountains made of water-ice, frozen nitrogen glaciers, cratered parts, completely smooth parts suggesting recent geological acticity, possible volcanoes and a thin blue atmosphere.

Pluto's orbit
Diagram of the orbits of the eight planets and Pluto

From its discovery in 1930, Pluto became recognised as the ninth planet in the Solar System. It remained this way until August 2006 when a group of astronomers voted for it to be reclassified as a dwarf planet. This was mostly down to the fact that it orbits in a region of similar objects in the Kuiper Belt. The discovery of Eris in 2003, an object slightly smaller than Pluto but orbiting much further out, was a key reason for astronomers wanting to create a new classification of object. Pluto has always been a bit of an odd-one-out even when it was known as a planet. Unlike the eight regular planets which orbit in fairly circular orbits and on the same plane as each other, Pluto's orbit is elliptical and tilted. Its elliptical orbit means that, for a few years during each of its 248 year long journeys around the Sun, it is actually closer to it than Neptune, although there is very little chance that they'll ever collide with each other. At its closest, Pluto is a whopping 4,436,820,000 kilometres (2,756,902,000 miles) from the Sun. At its most distance, it is 7,375,930,000 kilometres (4,583,190,000 miles) away from it. At its average distance of 5,906,380 km (3,670,050,000 miles), light from the Suntakes five and a half hours to reach it. In comparison, it only takes eight minutes for light from the Sun to reach Earth.

Although Pluto might be something of a space oddity, it's one that has fascinated people for decades and will do for decades more.

Bob the Alien's Tour of the Solar System
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