Outer space has been a lifelong interest to me, and I recently decided to take on some short science courses in planets and astronomy at Britain's Open University. Faced with the complexities of cosmology and gamma-ray, infrared, radio, ultraviolet and X-ray astronomy, as well as the sheer vastness of the universe, with its billions of stars, galaxies, nebulae and other objects, I soon decided my main focus would be the solar system itself: Earth's back yard, so to speak.
I thought I would be on fairly safe ground in memorising the nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and of course, Pluto. As my studies continued, however, I began to discover about trans-Neptunian objects (TNO)s, which orbit the Sun at a greater average distance than Neptune. If Pluto had been discovered today, it would have been classed as a TNO, not a planet. Since 1992, over a thousand such objects have been found, and Pluto came under increasing pressure to forego its planetary status, saved only by the fact that it was larger than any then-discovered TNOs.
In January, 2005, Michael Brown discovered 2003 UB313, which had been overlooked in routine observations in 2003. He gave it the informal nickname "Xena", although it was later named Eris by the International Astronomical Union (IAU). The Hubble Space Telescope measured its diameter as 2400km, larger than Pluto, in April 2006, and suddenly Pluto's status was under threat. Either 2003 UB313 became a tenth planet, or Pluto lost its own status. The IAU was forced to decide upon a formal definition of the term "planet" to clear up the controversy and confusion, and the decision was announced on 24th August, 2006. The definition meant that Pluto was no longer a planet, and that the eight other planets will be considered the "classical planets". Pluto, 2003 UB313, and 1 Ceres (previously classified as an asteroid) will be known as "dwarf planets".