Why scientists think there is a hidden object in our solar system

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Far away in our solar system, out beyond Neptune and even further than that, there is a whole chunk of space that still remains largely mysterious. It can be tempting to think - in an era when we can examine planets that are thousands of light years away - that our own planetary neighbourhood is conquered and charted.

But it is not. Those "extreme trans-Neptunian objects", or ETNOs, lie 250 times as far away from the Sun as we are. (That's close by compared to the Oort Cloud, a theoretical shell of objects as much as 200,000 times as far away from the Sun as we are.) Those ETNOs are astonishingly distant, and we know very little about them.

What we do know, however, seems a little odd. They seem to have a tilted orbit, and exhibit other unexpected behaviour. They were all clustered together in a way that wouldn't normally be expected.