An extrasolar planet, or exoplanet, is by definition a planet located outside our solar system. Now it appears that this definition is no longer viable. According to astronomers in Lund, there is a lot to indicate that Planet 9 was captured by the young sun and has been a part of our solar system completely undetected ever since.

“It is almost ironic that while astronomers often find exoplanets hundreds of light years away in other solar systems, there’s probably one hiding in our own backyard”, says Alexander Mustill, astronomer at Lund University.

Stars are born in clusters and often pass by one another. It is during these encounters that a star can “steal” one or more planets in orbit around another star. This is probably what happened when our own sun captured Planet 9.

In a computer-simulated model, Alexander together with astronomers in Lund and Bordeaux has shown that Planet 9 was probably captured by the sun when coming in close contact while orbiting another star.

“Planet 9 may very well have been ‘shoved’ by other planets, and when it ended up in an orbit that was too wide around its own star, our sun may have taken the opportunity to steal and capture Planet 9 from its original star. When the sun later departed from the stellar cluster in which it was born, Planet 9 was stuck in an orbit around the sun”, says Alexander Mustill.