According to the experts from Lund, the planet might have been ‘captured’ by our Sun in the distant past and remained undetected until now. It would be ironic that while astronomers are looking for exoplanets hundreds of light years away in other solar systems, there's probably one hiding in our own backyard. There is still no image of Planet 9, not even a point of light - we don't know if it is made up of rock, ice, or gas. All we know is that its mass is around ten times that of Earth.
As exciting as the discovery may be, it will take a lot more research to establish whether Planet 9 is indeed the first exoplanet found in our solar system - and what it is like. Stayed tuned!
Reference: "Is there an exoplanet in the Solar System?" - Alexander J. Mustill1, Sean N. Raymond, and Melvyn B. Davies - Monthly Notices Letters of the Royal Astronomical Society (2016) - doi: 10.1093/mnrasl/slw075
Credit for Featured image at top of page: Lund University