Ciaran and Dan look at what makes a planet and whether there's a mysterious extra planet – Planet X – just beyond Neptune, lurking in our cosmic neighbourhood. 

Mysteries of Science is a fortnightly podcast created by The Week Junior's Science+Nature magazine and Fun Kids.

Tap follow or subscribe wherever you're listening to this to make sure you never miss an episode, and get ready for a summer of fascinating mysteries, including Planet X and massive treasure hunts.

Get three copies of The Week Junior's Science+Nature magazine for just £5. sciencenature.theweekjunior.co.uk/podcast is the place to claim yours.

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