If Planet 9 exists, then where did it come from?

Contributed by
Sep 15, 2017

With Pluto no longer having planetary status (you’ll always be a planet to me, Pluto), that ninth planet in our solar system remains hypothetical—but where did it emerge from if it actually exists?

The mysterious space rock otherwise known as Planet 9 either formed around the sun or was captured from another planetary system. Now a team of scientists at the University of Sheffield’s Department of Physics and Astronomy has gone further in demystifying its origins and suggest that it probably was born somewhere near the star we orbit.

 Pluto was demoted to a dwarf planet after astronomers found many other objects of comparable size in the Edgeworth-Kuiper belt that were just as far from the sun. Planet 9 was suspected to be floating around there somewhere in 2016 when astronomers were trying to explain the odd orbits of some of those objects. The theoretical planet has never been directly observed, but scientists are still determined to figure out how it ended up there, if it is what they think it is.

Planet 9 is unlikely to have been snatched up from outside the solar system because it being 10 times bigger then Earth means that it had to have formed much closer to the sun. Planets of that size don’t just materialize at such a distance.


An artist's impression of Planet 9 throwing shade at the Milky Way.

On the off chance that it came into being somewhere else, the capture could have happened while the sun still blazed in its birth star cluster, even though that scenario is improbable. Planetary systems and stars are created simultaneously. This is why stellar interactions within these clusters of celestial bodies happen often. Planetary systems are directly influenced by an environment that births stars, and such densely populated environments can often snatch up stars and planets.

That still isn’t convincing enough to the team who studied this phenomenon in depth.

“We have shown that - although capture is common - ensnaring planets onto the postulated orbit of Planet 9 is very improbable,” said team lead Dr. Richard Parker of the study recently published in Monthly Notices of the Royal Astronomical Society. “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 captured from another planetary system."

Blame the orbit of Planet 9. Free-floating planets (FFLOPs) on the edge of the solar system with orbital constraints similar to those of the hypothetical planet have hardly any chance of capture. When the team applied additional obstacles, chances plummeted to nearly zero.

Don’t worry, Planet 9. We still believe in you.


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