A new study has suggested that Planet Nine, a hypothesized world lurking at the far reaches of the Solar System, may have been a rogue planet captured by our Sun.

The research was conducted by undergraduate student James Vesper and professor Maul Mason from New Mexico State University (NMSU), and presented last week at the 229th meeting of the American Astronomical Society (AAS) in Texas, reported Space.com.

Rogue planets are worlds that are unattached to a host star for one of a number of reasons, such as being flung out of their original solar system. They are thought to be hugely abundant, perhaps outnumbering planets around stars – of which there are billions in our galaxy. But finding them is difficult, as they emit little to no light as they wander the cosmos.

It’s not too farfetched to think, though, that our Solar System may encounter rogue planets in its lifetime. To find out what would happen in such a scenario, Vesper and Mason ran simulations of 156 different types of encounter.

In 40 percent of the events, the planet was captured by our Solar System, either gently entering orbit around our Sun via a “soft capture” method, or more violently flinging other planets out of our system. In 60 percent of the encounters, the rogue planet was briefly captured before being flung out again, reported Inverse.

^{Planet Nine is thought to have a highly elliptical orbit. Caltech/R. Hurt (IPAC)}

They also found that it’s unlikely any rogue planet larger than Neptune has had an encounter with our Solar System, else the inner regions would be more disordered than they are today. All this sits fairly well with the expected characteristics of Planet Nine.

The world is thought to be about 10 times the mass of Earth, or about half the mass of Neptune. Its orbit – inferred from the motion of objects in the outer Solar System – is also thought to be highly elliptical, taking 10,000 to 20,000 years to complete, and extending up to 1,000 AU from the Sun (1 AU, astronomical unit, is the distance from Earth to the Sun).

This isn’t the first study to suggest Planet Nine may be an intruder in our Solar System. In June last year, researchers at Lund University in Sweden suggested our Sun may have stolen it from another star. Other theories suggest it may have always been here.

Whatever the origin of Planet Nine, scientists think we are getting closer to confirming it is real. It’s hoped that this year, we may get direct evidence for its existence, via observation campaigns looking for it at the edge of the Solar System.