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    The search for Pluto (the Roman God of the Underworld), which began shortly after the discovery of Neptune, was most associated with the astronomer Percival Lowell, a man more famous for his ideas about Mars than discovery of distant planets. Still, the idea of a 'Planet X' fascinated him and soon he began searching the heavens for this planet with a team of researchers, using his private observatory in Flagstaff, Arizona. After many years of trawling through photographs nothing was found until, 13 years after Lowell's death, a young amateur astronomer named Clyde Tombaugh finally discovered Pluto in February 1930.

    Since then this icy planet has fascinated many people the world over, yet not that much is actually known about it, as its sheer distance from us has so far prohibited any space missions to send probes near it, and makes viewing it from the ground harder too. However, this situation is changing - ever bigger and more powerful telescopes are helping us to understand this planet, and as we keep learning more details about this amazing world, it becomes ever more intriguing.

    One of the reasons that Pluto is so fascinating for me, is that it seems a bit like a seagull in the desert - totally out of place amongst its cosmic neighbours. All of the other 4 outer planets leading to Pluto are massive gas-giants yet Pluto is small and solid. Due to this, and the fact that Pluto is considered to be a very large KBO - Kuiper Belt Object, there has been some discussion as to whether Pluto should be officially classified as a planet.

Some Facts

    Regardless, it has an average Density of between 1.8 and 2.1 g/cm³, and is estimated to be composed of 50-75% rock mixed with ice. Proof of Pluto's icy surface temperature comes with the knowledge that it's surface is composed 98% of Nitrogen, with traces of Carbon Monoxide and Methane, which can only be present as a solid under 70K.

    Also noteworthy is Pluto's highly elliptical orbit, because although it's usually the farthest planet from the Sun, there is an occasion during its orbit that it's actually inside Neptune's orbit! This last happened in 1979 and serves to demonstrate just how much the planet's distance from the Sun varies. At the moment it has recently passed its closest point and is slowly increasing its distance from the Sun. Pluto also has a thin atmosphere of Carbon Dioxide and Methane. This atmosphere is not permanent though and when Pluto gets too far from the Sun, it collapses. This is next expected to happen in 2020 or so, although we can't be sure as it has never actually been observed.

Charon

    As I already mentioned, Pluto is a planet of superlatives - the smallest, farthest and coldest 'planet' orbiting the Sun. Its diameter is a mere 2,320 kilometers (about two thirds that of our moon), and yet, rather surprisingly, Pluto has a moon of its own; Charon. Charon is 1,210 kilometers wide, more than half the diameter of Pluto itself, and becuse of this, Pluto is often referred to as a 'Double Planet'. Charon thereofore holds the record of being the largest Moon relative to its planet in the Solar System.

    It is considered very likely that Pluto and Charon were formed independantly of each other, unlike the Earth and our moon, due to the fact that its surface composition is thought to be very different (mainly water ice) and also because it has a much lighter density (approx 1.2g/cm³ to Pluto's 1.8-2.1).

    Strangely, Charon circles Pluto in a slightly eccentric orbit (0.0076), probably due to the recent collision of a sizeable object. Another peculiar feature had by Pluto and Charon though is that Charon orbits Pluto in the same 6.387 days that Pluto takes to rotate, keeping the two locked in synchronous rotation. Charon therefore behaves like our moon in this respect - it always keeps the same face towards Pluto; the difference in this case however is that Pluto also always keeps the same face towards Charon!

    Well, not much to say here, except that it was discovered in 1930 by american astronomer Clyde Tombaugh and frozen methane was discovered on Pluto's surface in 1976. Then in 1978 astronomers James Christy and Robert Harrington discovered its moon Charon.