Why is Pluto not a planet

The notion that a planet might exist beyond Neptune was suggested soon after the discovery of Neptune in 1846. At the beginning of the 20th century, Percival Lowell and William Pickering studied perturbations in the orbit of Uranus to predict the position of an unknown planet and soon after commenced a search. In 1929, Clyde Tombaugh started a systematic photographic search at the Lowell Observatory in Arizona and on February 18th, 1930, Tombaugh observed Pluto close to where Lowell’s calculations had suggested.

We now now that Pluto orbits the Sun every 248 years at a distance of up to nearly 7.5 billion km. It has one moon, Charon, discovered in 1978, and so similar in size to the mother planet that the two are often referred to as a double planet. Pluto’s diameter is approximately 2400 km., whereas Charon’s is about 1200 km. The distance between the two of about 20 000 km. would make Charon a very imposing sight from Pluto’s surface – about seven times the size of the moon as seen from Earth. Due to the vast distance between Earth and Pluto, it’s surface is visible only via the Hubble telescope. Despite a comparative lack of data, observers have been able to discern that the surface has a barren landscape comprising mostly rock and icy, and that the planet seems to have a light atmosphere. Pluto is so small that many astronomers originally thought it was in fact a former moon of Neptune which had escaped its orbit. In addition, some astronomers suggest that due to Pluto’s peculiar orbit which is tilted at about 17 degrees to the ecliptic (the plane in which the other planets orbit the Sun) it may have entered its present orbit a lot later than the other planets. Pluto also resembles Triton, a moon of Neptune, with ice volcanoes ejecting gas, ice and dust from the surface. However, the discovery of Charon in 1978 seemed to cast doubt on the theory that Pluto is an escaped moon of Neptune.

Whether to afford Pluto continued status as a planet has been a subject on continued debate for many decades. Many astronomers consider Pluto one of the larger examples of a class of objects seen to orbit the Sun beyond Neptune. These are known as TNOs, or Trans-Neptunian Objects. In 1999, the International Astronomical Union (IAU), the organization whose duty it is to classify astronomical objects, debated whether to designate TNOs, including Pluto, a catalog number, which was widely interpreted as revoking Pluto’s status as a planet. Pluto was not assigned a TNO number and, at least for a further decade, remained a planet. In 2006, the IAU reconsidered the issue and after intense debate voted to remove Pluto from the list of full planets, giving it the new classification of dwarf planet. This change mirrors the increasing recognition amongst the astronomical community that Pluto might be a larger object of the Kuiper belt, the large bands of rock, ice and debris left over from the formation of the solar system now orbiting the Sun beyond Neptune.

According to the IAU in 2006, to qualify as a planet, an object must:

1. be in orbit around the Sun;
2. have been shaped by its own gravity into a near-round object;
3. have cleared its orbital neighbourhood, meaning that its gravitational pull must be large enough to have removed surrounding rock, ice and other debris from its orbital vicinity.

Pluto failed on the requirement III because it orbits partially within, and is now considered to be part of, the Kuiper belt.