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Thursday 22 September 2016

Is there a ninth planet in our Solar System?

By now, you’ve probably heard the hubbub in the news about the hypothetical “Ninth Planet” in our Solar System, and, unfortunately for those of us who studied astronomy before 2006, no, it’s not Pluto. There’s a new Planet Nine on the block, although no-one has ever seen it and we don’t actually know if it exists.

Yes, some scientists think there may be another, unseen planet in our Solar System. How can they think that?

Why haven’t scientists seen Planet Nine yet?

In our solar system, planets are generally considered to be visible things - get yourself a 12-inch telescope and you can see Pluto, and that’s not even a planet any more! So yes, it sounds daft to say that there’s a whole other planet in our Solar System that we haven’t seen yet, but, in our defence, it’s very far away.

Planet Nine is thought to be a trans-Neptunian object - a minor planet that orbits the Sun at a distance further out than Neptune. Both the Kuiper belt and the Oort cloud are included in this region, and Planet Nine is thought to orbit somewhere between the two.

An artist’s rendering of the relationship between the Kuiper belt and the Oort cloud
The Oort cloud is a spherical region of icy debris thought to surround the Solar System. Although it was thought to extend from 5,000 to 100,000 Earth-Sun distances, it may start much closer to the Sun. Image credit: NASA and A. Feild (Space Telescope Science Institute)

Why do scientists think Planet Nine exists?