By Mustill's reckoning, around 4.5 billion years ago, when our sun was a baby, solar gravity likely interfered with the orbit of a planet around a neighboring star. This idea isn't such a stretch; stars are often born in molecular clouds that form dense clusters of stars. As planets form around these stars, their orbits may become destabilized and flung into interstellar space or shared between stellar neighbors. The gravitational environment would have been a mess.
ANALYSIS: Could 'Planet X' Cause Comet Catastrophes on Earth?
As time went on, the young stars drifted apart and their planetary systems settled -- but these systems could easily contain planets that weren't formed around the star they currently call home. Did this happen to Planet 9? Computer simulations carried out by Mustill and his team certainly suggest it's a possibility.
"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," said Mustill.
Calculations of Planet 9's mass puts it at around 10 Earth masses -- so it could be a really big rocky world or a small Neptune-like ice giant. For now, it's composition is speculative at best. However, if this mysterious world wasn't born from our sun's protoplanetary disk, and is in fact a planet from another star, it may be a very alien world indeed. How would its composition differ from the other planets in the solar system?
ANALYSIS: The Hunt for Planet 9: What's It Made Of?
As noted by the researchers, it would be ironic if we do find Planet 9 to be from another star system -- it would be the closest (ex-)exoplanet to Earth and possible exploration destination for our spacecraft. The enthusiasm was high for sending NASA's New Horizons mission to Pluto, imagine the excitement surrounding the possibility of sending a robotic probe to Planet 9 if it was found to be a late addition to our solar system's planetary family?