“But…I’m still a planet, aren’t I?”

To be or not to be a planet… That is the question for Pluto for a long time. Practically everyone alive today grew up learning about the nine planets in our Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Since its discovery in 1930, Pluto was considered a planet, just like the others. But in 2005, the astronomer Mike Brown announced that he had discovered a new object which was more distant, but larger in the Solar System.

This object was originally named 2003 UB 313 but then was given the official name of ‘Eris’, after the Greek God of strife and discord. Before that, its nickname was Xena. It came from Xena: Warrior Princess, which was a popular television show around the time it was found.

With the discovery of Eris, astronomers had to reconsider their definition of a planet. Since Eris is larger than Pluto, the number of planets in the Solar System would need to be expanded to 10. And who knows how much larger it would become with future discoveries. The International Astronomical Union met in Prague in 2006 to make a final decision. They decided that a planet must fulfil three criteria:

§  It must orbit the Sun. – Yes, so maybe Pluto is a planet.

§  It must have enough gravity to pull itself into a spherical shape. – Pluto… check!

§  It must have cleared out the other objects in its orbit.  – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

See? It’s this 3rd part where Pluto falls down. It has not yet cleared the neighbourhood of its orbit in space, while the rest of the planets have essentially cleared theirs out completely. So, sorry Pluto, but you aren’t a planet anymore! But don't feel too sad, though! You have a new title: “dwarf planet." Any object that doesn’t meet this 3rd criterion is considered a dwarf planet. And you’re in good company! There are currently four other dwarf planets — Makemake, Ceres, Haumea and Eris — and scientists expect more will be discovered over time. Dear Pluto, enjoy your dwarf planet status!