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.
The outskirts of the solar system have always been something of an enigma, with astronomers in the late 19th and early 20th centuries searching for a giant planet that wasn't there, and the subsequent discovery of Pluto in 1930.
Pluto was downgraded in status to a 'dwarf planet' because astronomers discovered many other small objects – so-called Edgeworth-Kuiper Belt objects – at similar distances from the sun.
Researchers simulated the sun's stellar nursery where interactions are common and found that even in conditions optimised to capture free-floating planets, only five-to-10 out of 10,000 planets are captured onto an orbit like Planet 9's.
The Daily Galaxy via University of Sheffield
Image credit: UC Berkeley