Pluto was discovered 80 years ago today. Most of us grew up thinking of Pluto as the ninth planet, but it was demoted from that status a couple of years ago as there was no sensible definition for the minimum size of a planet, and reasonable definitions either excluded Pluto, or included many other objects. Along with Eris and Ceres, Pluto is now classified as a dwarf planet. Pluto has an noticably eccentric orbit, it's orbit is elongated and elliptical. All planets and dwarf planets have elliptical orbits to some extent, but Pluto's is notieably more elliptical than the others. This means that though it is, on average, more distant to the sun than Neptune, it is often closer. Indeed, it only moved beyond Neptune in 1999. This does not mean that Pluto and Neptune will crash, the orbits do not cross as they are in different planes.
The outer planets were discovered (Pluto was originally called a planet) by looking at deviations in the motions of known planets from their theoretical predictions. E.g. Uranus was observed, and its motion was found not to agree with Newtonian mechanics. It was suggested that this could be due to another, more distant, object exerting a hitherto unaccounted for force. The calculations were made and Neptune was predicted. Then, when deviations in Neptune's orbit were observed, the process was repeated in an attempt to find Planet X.
Interestingly, when problems with mercury's orbit were noted, people repeated the same process, and expected to find a planet closer to the sun than mercury. This planet was called 'Vulcan'. It wasn't there. The solution came after Einstein's General Relativity showed that Newtonian mechanics wasn't really appropriate - the 'error' in mercury's orbit was due to Newtonian mechanics being wrong, rather than to some other object exerting pulls.
Pluto was originally considered to be 'Planet X', but it was too small to be the mystical 'planet X' - though there was a coincidence in Lowell's calculations for planet X in 1915 and Pluto's position at that time (though the photograph wasn't recognised then). There are other explanations for the discrepancies.
Though Pluto wasn't confirmed until March 1930, and it was photographed in 1915 (but missed at the time), the observations which gave rise to its discovery happened 80 years ago today. Happy Birthday, Pluto.