Is there a rogue planet in the Solar System?

Astronomers in the Faculty of Science have shown that 'Planet 9' – an unseen planet on the edge of our Solar System – probably formed closer to home than previously thought.

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.p9

However, 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. It isn't possible to directly observe Planet 9, but that hasn't stopped theorists from trying to work out how it got there.

Planet 9 is at least ten times as massive as Earth, making it unlikely that it formed at such a large distance from the Sun. Instead, it must have 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.

A team led by Dr Richard Parker from the Department of Physics and Astronomy have shown that the capture scenario is extremely unlikely. They 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.

Dr Parker said: "We're not ruling out the idea of Planet 9, but instead we're saying that it must have formed around the Sun, rather than being captured from another planetary system."

The study was published in Monthly Notices of the Royal Astronomical Society.