Episode
Why Is Pluto No Longer a Planet?
- Published
- May 16, 2026
- Duration seconds
- 264
- Processing state
not_requested- Canonical source
- https://shows.acast.com/why-though-with-dr-matt-agnew/episodes/why-is-pluto-no-longer-a-planet
- Audio
- https://sphinx.acast.com/p/open/s/69be1b0c3bbfcfe8dba05019/e/6a06d18dcc4f860b9a508f7f/media.mp3
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Summary
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 - Planet definition: The three-rule checklist astronomers use to classify worlds - Clear its neighbourhood: Being the gravitational boss of your part of space - Dwarf planet: Orbits the Sun, is round, but hasn't cleared its neighbourhood - Scientific change: Science updates its ideas when new evidence appears Fun Experiment: The Planet Checklist Challenge Scientists use a three-rule checklist to decide what counts as a planet. Now it's your turn to be the astronomer. Grab five objects from around the house... a footy, a mandarin, a marble, a 50 cent coin, and a tin of baked beans. These are your space object…