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.

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?

Pluto's New Horizons

[EDIT - June 2014] This article is now also available as a video, thanks to a group of media students at Sheffield Hallam University who created an animated version of our article. For more information, read about how we partner with universities to create videos.

  

“Pluto is dead”, according to Mike Brown (in How I Killed Pluto and Why it Had it Coming), but that doesn’t mean we shouldn’t go there. In fact, this is one of the reasons why we are going there. Unsurprisingly, given the distances involved, we know very little about Pluto (“You may think it’s a long way to the shops, but that’s just peanuts to space,” to quote Douglas Adams in The Hitchhiker’s Guide to the Galaxy).

We're used to seeing detailed pictures of our neighbouring planets like Mars, but this is the best image we have of Pluto - taken by the Hubble Space Telescope in 1994. So what does Pluto really look like? Image credit: NASA/HST

Discovered in 1930, Pluto remains something of a mystery, as astronomers are a long way from understanding its origin. Pluto is one of many rocky and icy bodies which form the Kuiper Belt, in the outer region of the solar system. Pluto was discovered by Clyde Tombaugh in a systematic search for a planet beyond Neptune. Its high surface reflectivity initially made it appear larger than it actually is. Pluto was initially considered as a planet, before the International Astronomical Union officially defined what a planet was. At one point Pluto was thought to be a rogue moon of Neptune.