In 2016 astronomers working in the USA postulated the presence of 'Planet 9' to explain the strange orbital properties of some Edgeworth-Kuiper Belt objects. But while it isn't possible to directly observe Planet 9, it hasn't stopped theorists from trying to work out how it got there. Planet 9 is at least 10 times bigger than Earth, making it unlikely that it formed at such a large distance from the sun. Instead, it has been suggested it either moved there from the inner regions of the solar system, or it could have been captured when the sun was still in its birth star cluster.
But a team, led by astronomers at the University of Sheffield with colleagues from ETH Zurich, has shown that the capture scenario is extremely unlikely. The astronomers have shown that 'Planet 9' – an unseen planet on the edge of our solar system – probably formed closer to home than previously thought. The team led by Dr Richard Parker from the University of Sheffield's Department of Physics and Astronomy has found that Planet 9 is 'unlikely' to have been captured from another planetary system, as has previously been suggested, and must have formed around the sun.