Since the beginning of the 20th century many scientists were searching for “Planet X”. It was on 18th Feb 1930 that Clyde W. Tombaugh discovered a new celestial body at the Lowell observatory in Arizona. At the very edge of the solar system (furthest away) is Pluto; a new planet (at the time). Pluto is 5,913,520,00km away from the sun and has an average temperature of 223 degrees Celsius. Therefore Pluto is a very cold (mainly icy and frozen) place. The size of Pluto is 2/3 that of our moon. Pluto has 3 moons in its orbit; Charon (1/2 the size of Pluto), Nix and Hydra. Pluto is a relatively small terrestrial body with a mass of 0.0125*10^24kg. Compared to Earths mass (5.9736*10^24kg) Pluto appears significantly smaller.
The declassification of Pluto as a planet led to its subsequent re-categorisation to a ‘dwarf’ planet. Since the discovery of Pluto, it is now known that it (and its moons) resides in and is part of a collection of objects called Kupiers belt. Kupiers belt is a region in space extending 55 astronomical units (55 times the distance between the Sun and Earth) beyond the orbit of Neptune.
There are several attributes that make Pluto ‘different’ to the (other) planets. The first attribute is that because Pluto is the furthest from the Sun it means that it has a 248 year orbital period – this is much greater than the orbital of ‘proper’ (planets that satisfy the 3 criterion) planets. Another attribute is that Pluto’s orbit has the highest eccentricity meaning that the distance of Pluto from the Sun varies much more than that of ‘proper’ planets. The pattern in which the ‘proper’ planets follow in their order is also another factor as to why Pluto is ‘different’ to ‘proper’ planets. The first 4 planets (Mercury, Venus, Earth & Mars) are known as the terrestrial planets due to their smaller size and the possession of a solid surface. The next 4 planets (Jupiter, Saturn, Uranus & Neptune) are then known as the gas giants because they are larger than the terrestrial planets, they have larger satellites and appear to have no solid surface hence gas giants. Pluto being the 9th ‘planet’ breaks this pattern as it has the characteristics of a terrestrial planet (smaller and has a solid surface) and not a gas giant as would be expected.
Pluto is known as a ‘dwarf’ planet and since its reclassification, by the IAU, other celestial bodies have been discovered that would have equal rights to be called a planet or at least a ‘dwarf’ planet.
For example in the Kupier belt, beside Pluto is Eris. Eris was discovered on January 5th 2005 from images taken on October 21st 2003. The formal designation of Eris is ‘2003 UB313’. Like Pluto, Eris is a great distance from the Sun (c. 96.7 astronomical units) and also like Pluto; Eris has an extremely long orbital period. The orbital period of Eris is 557 years. Eris is larger than Pluto by 27%. Eris is the largest known ‘dwarf’ planet. Due to the fact that Eris is larger than Pluto it was initially described as the 10th planet in the solar system and so therefore had just as equal rights as Pluto to be known as a planet. On 24th August 2006, the IAU came up with and introduced a new definition for the classification of a planet – with these new rules it meant that Eris (and also Pluto) were no longer classed as ‘proper’ planets and thus became ‘dwarf’ planets.
Due to the advancements of growing knowledge and the invention of new equipment for the astronomical research field it might be possible to reach into further distances in our and then other solar systems. It is going to be necessary to distinguish between bigger planets like Earth and smaller ones like Pluto which have different characteristics.
Sources;
http://nssdc.gsfc.nasa.gov/planetary/planets/plutopage.html
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