Why do some scientists consider Pluto to not be a planet?

Pluto was discovered around 1930 and, during that time, the telescopes were not as good as they are now and they thought that Pluto was the only planet out in the vast outer area of our solar system. Pluto was always considered to be a strange planet but it was a favorite among the planetary scientists. It was named after a mythological Greek god (not the Disney character) When there was a decision made to change the definition of a planet, that meant Pluto dropped off as one of the planets in our solar system.

After Pluto was originally discovered, scientists also found out that Pluto had moons around it, in the same way that earth has a single moon. Later, it was realized that Pluto was actually one of many objects in an area of the solar system called the asteroid belt (also known as the Kuiper Belt) and that there were around 70,000 objects in that area; many were larger than Pluto. This placed a question in the minds of the astronomers as to whether Pluto should have ever been considered a planet at all.

In 2006, astronomers met at the XXVIth General Assembly of the International Astronomical Union to make a final definition of what a planet is and then to vote on it. At this meeting, they created the definition of a planet as follows: It needs to orbit around the sun. It needs to have enough gravity to pull itself into a spherical shape. It needs to have ‘cleared the neighborhood’ of its orbit. When scientists voted and agreed, the little planet of Pluto did not make it, mainly because the last requirement. Pluto has not ‘cleared the neighborhood’, but instead was surrounded by thousands of other objects. A true planet would either have absorbed those objects or cast them out of close location to the home planet.

Not everyone was happy about this decision. Letters from children all over the world were sent, objecting to the fact that little Pluto had been demoted. Every school science book and library book had to be changed and various astronomers found themselves in a position of having to explain why Pluto was no longer a planet. Adults grew up with Pluto – and kids loved the littlest planet. Who would have thought that this would have caused such an uproar? Who would have thought that Pluto would be so popular?

And so, even though Pluto did meet the requirements of the first two parts of the definition, it was excluded as an official planet in our solar system and, reclassified as a ‘dwarf planet’.

While it might no longer be a planet, there has always been a lot of interest in Pluto. Due to the interest, the NASA mission called New Horizons has plans to visit near Pluto and take pictures of the surface. The mission will reach Pluto in 2015, and at that time all of the fans of Pluto will be able to see what it really looks like.