Many of us grew up knowing that there were nine planets. We all had some mnemonic device to remember them, my personal favorite being “My Very Ecstatic Mother Just Sent Us Ninety Pizzas.” And then, the last “planet,” Pluto, was deemed too small to be a planet and was demoted to a dwarf planet, leaving our solar system with eight planets. Then, Pluto was very briefly added to the roster and then demoted again; for a while our solar system stabilized at eight planets.
Now, scientists have determined that there is likely another planet beyond Pluto’s orbit, creatively nicknamed “Planet Nine.”
Originally proposed in 2014, Planet Nine was the hypothetical solution to a particular space mystery. Beyond Pluto lies the Kuiper belt, a large collection of rocky bodies similar to the asteroid belt. The Kuiper belt, plus a little bit more beyond, are collectively part of a zone called the “Trans-Neptunian region,” which is basically an arbitrary boundary of things that orbit the sun outside of the Neptune’s orbit. One would expect that this Trans-Neptunian zone would be fairly spread out, much like the asteroid belt; however, there’s an anomaly: there is a massive collection of objects bunched up on one side of the zone’s orbit, causing one side of the Trans-Neptunian zone to be denser than the other end at any given time. This shouldn’t be happening unless there’s a massive body exerting a gravitational pull on these objects, which was the basis for the 2014 proposal that a large planet exists way out there, Planet Nine.
It was only now, in 2016, that researchers Konstantin Batygin and Mike Brown proved its existence using math and computer simulations. As of now, there are no actual observations of the planet, but the physics and math hold up. The original project started when two colleagues, Chad Trujillo and Scott Shappard, published a paper about Kuiper Belt objects that showed 13 bodies that seemed to move in tandem, held together by a small planet. While Brown didn’t think the small planet idea was likely, he was interested and sought out Batygin’s help.
The two approach the problem of “is there something out there” very differently, with Brown taking a more observational method while Batygin sports a more challenging metagame. In the end, the discovery of Planet Nine was an accident, with Batygin inputting ridiculously large numbers and almost random positions that yielded a suitable answer to the space anomaly.
On top of mathematically proving that Planet Nine (or something similar) exists out there, they also determined that it’s not in the middle of the dense collection of objects, but rather on the opposite end of the orbital. Essentially, Planet Nine and the dense object cluster orbit the sun at different rates; on the occasions that they do pass each other, Planet Nine’s gravity acts as a slingshot, slightly pulling the object cluster towards it while maintaining enough distance that they won’t collide.
Assuming that we find a planet where Planet Nine is, our Solar System will revert back to its original nine-planet line up. At least until we discover something else.