For the majority of the twentieth century, the celestial body known as PLUTO was considered the ninth planet in the solar system. Mid-way through the first decade of the twenty-first century, it was re-classified as a ‘dwarf planet’ in a controversial decision leaving the number of planet in our solar system at eight instead of nine.
Originally discovered at the Lowell Observatory in Arizona in 1930, it was named after the Roman god of the underworld. Unfortunately, not much else was seen or observed from the vicinity of Pluto for several decades because it took that long to develop technology allowing astronomers to discern details about this mysterious planet.
In 1978, the moon ‘Charon’ was discovered in and around Pluto’s orbit. Scientists were able to use this body to gauge the size of Pluto at approximately 1500 miles. More importantly, however, was that with this finding came the discovery of hundreds of stellar bodies in the same orbit, some of which were as big, if not bigger than Pluto. Pluto, the ninth planet, seemed to be less of a planet by itself and more of a body that was in Neptune’s orbit, in the Kuiper Belt, along with thousands of other objects.
The decision was made in 2006 to come up with a definition of a ‘planet’ that would settle the question of how many planets were in our solar system. A planet is defined as something that must meet three criteria:
1) It needs to be in orbit around the sun
2) It needs to possess enough gravity to make it into a round shape
3) It needs to be the dominant gravitational body in its orbit
Pluto, unfortunately, does not dominate the pull in its orbit and has therefore been relegated to being a ‘dwarf planet’.
Research and debate will continue about this decision, especially amongst the scholars who grew up with the notion that there were nine planets, not eight.