Why Is Pluto No Longer a Planet?

Pluto used to be the ninth planet in our Solar System. Then in 2006 everything changed. Dr Matt Agnew explains why... and it's a better story than you think.

In this episode of Why Though?, we travel back to 1930 when astronomer Clyde Tombaugh spotted a tiny moving dot in the night sky and made one of the biggest discoveries in space history. We find out what happened when scientists discovered Eris... a Pluto-like world that was even heavier than Pluto. And we learn the three-rule checklist that decides what actually counts as a planet.

Of course Matt's dog Pluto makes a cameo.

What you'll learn:

- Pluto was discovered in 1930 and taught as the ninth planet for over 70 years

- Better telescopes revealed a whole region of icy worlds beyond Neptune called the Kuiper Belt

- Scientists found Eris... similar to Pluto but even heavier, which forced a big question

- To be a planet, a world must orbit the Sun, be round, and clear its neighbourhood

- Pluto does two out of three... but it hasn't cleared its neighbourhood

- Pluto became a dwarf planet in 2006... and it's still completely incredible

Key Science Ideas:

- Kuiper Belt: A region beyond Neptune full of icy objects