A new discovery is strengthening the idea that a large, mysterious planet — known as Planet 9 or Planet X — may be lurking unseen at the Solar System’s edge. Astronomers say they have found a tiny object orbiting far out from the Sun that fits with the Planet X theory. In fact, the object may have even been pushed onto the path it takes now by this hidden planet’s gravity.
The tiny rock — eloquently named TG387 and nicknamed “The Goblin” — was spotted by astronomers at the Carnegie Institution of Science using a giant Japanese observatory in Hawaii called Subaru. The Carnegie team first spotted the object in 2015 and then followed it on its journey around the Sun for the last four years. Those observations revealed an incredibly distant target. TG387 takes a whopping 40,000 years to complete just one orbit around the Sun. And it’s on a very elliptical path far from the inner Solar System; the closest it ever gets to the Sun is 65 Astronomical Units (AU), or 65 times the distance between the Sun and the Earth. For reference, Pluto only gets as far as 49 AUs from the Sun.
This orbit is particularly enticing since it puts TG387 in a select group of distant Solar System objects that all point to the possible existence of Planet X. Right now, there are 14 far-out space rocks that all share similar orbit patterns, suggesting that this planet is out there. Their paths are all super elongated, and they all cluster together in the same area when they approach the Sun. Plus, their orbits are all tilted alike, and they point in the same general direction, as if something big has pushed them into similar places. These objects are the strongest lines of evidence astronomers have for Planet X, and finding a new one that matches this pattern reinforces that idea that this planet is more than just a theory.
Plus, each new find helps astronomers narrow down where to look for Planet X. “Each time we find another one of these smaller objects, it will lead us to constrain where the bigger planet could be,” Scott Sheppard, an astronomer at Carnegie Science and the lead author of a study in The Astronomical Journal detailing the discovery, tells The Verge. “They’re all on very similar orbits, but their orbits are all slightly different, which [limits] where the planet could be.”
The idea that a giant planet is lurking beyond Neptune is an idea that astronomers have speculated for the last century. However, the hunt for this planet turned much more serious in 2012, when Sheppard and his team found a far-flung object that was truly unique. It was an object called VP113, and it currently holds the record for the most distant object orbiting the Sun. The closest it ever gets to the star is 80 AUs, or 80 times the Earth-Sun distance. Sheppard noticed that this object also followed a path similar to those of a few other distant space rocks, as well as a far-off dwarf planet called Sedna. “They all have this clustering, and so that suggested that something was pushing them into similar types of orbits,” says Sheppard.
Then, in 2016, a pair of researchers from Caltech, Mike Brown and Konstantin Batygin, did the math. Based on the orbits of six of these objects, they estimated that there’s a planet roughly 10 times the mass of Earth orbiting far beyond Neptune. Their calculations showed that it possibly takes 10,000 to 20,000 years to orbit the Sun. Brown and Batygin dubbed the phantom planet “Planet 9,” though others had been calling it Planet X years before.
Since then, more and more objects have been found that fit this orbit pattern. The idea is that these objects are in just the right orbits needed to survive Planet X’s gravitational wrath. If they followed any other path, they would likely collide with the big planet or the planet’s gravity would send them hurtling out of the Solar System. However, all of these very distant extreme objects orbit in such a way that they never get close to Planet X when it crosses their orbits. “Whenever the planet is crossing the orbit of one of these objects, these objects are on the other side of the Solar System. So they never get close to each other,” says Sheppard.
But not all of these objects are as reliable narrators as they could be. “Among these 14 objects, some tell a more precise story than others,” Batygin, who was not involved with today’s study, tells The Verge. For one thing, some of the objects cross the orbit of Neptune, and that planet’s gravity might have some influence on the objects and warp their routes. “Neptune has the effect of muddying things up, even if you have an orbit carefully sculpted by Planet 9.” That makes it hard to know whether or not the object is truly pushed about by this unseen planet.
But this new discovery, TG387, is special because its orbit is so distant. When it’s farthest from the Sun, the rock will be at an extreme 2,300 AUs away. In fact, it’s remarkable that astronomers found it all since it’s about seven times smaller than Pluto and so far off. But because of its extreme distance, TG387 is not influenced in any way by the large objects in the inner Solar System. Jupiter, Saturn, Uranus, and Neptune don’t have any effect on its orbit. That means if this object was truly batted around by Planet X, it might hold more information about the planet’s orbit than other objects do. And when the team ran simulations of the Solar System with a Planet X in it, they found that this object’s orbit isn’t subject to change. “This one joins an elite group of six objects that are stable,” says Batygin.
Of course, Planet X is nowhere near a done deal. There are only 14 objects that potentially support its existence. That’s a super low number by statistical standards. “We don’t have tens of these objects,” Michele Bannister, an astronomer studying distant small bodies at Queen’s University Belfast, who was not involved with this research, tells The Verge. “I’d be very happy if we had tens, but there’s barely even a handful.” Additionally, Bannister says it’s important to remember that astronomers still don’t have a comprehensive snapshot of the distant Solar System. The time of year, the weather, and the part of the sky a telescope observes all influence the kind of objects that are discovered out there, adding bias to the sample.
Plus, the objects that we find are typically on their closest approaches to the Sun on their super distant orbits, and that skews our discoveries a bit. For instance, TG387 was found when it was around 80 AUs away, not thousands of AUs away. That means we may not have a good idea of what all is out there because we can’t see the objects that are super far out on their orbits. “Each of these objects we detect is the tip of an iceberg for a larger population,” says Bannister. For every new discovery made, there must be hundreds of thousands of more objects that astronomers can’t see. And those objects could tell a different story than the Planet X one.
However, Bannister, who predicted that an object like TG387 could exist in the Solar System, does say this discovery is instrumental in helping to shape our understanding of the distant Solar System. We still don’t understand why there are objects like this one that are completely detached from the rest of the planets. “They’ve made a great discovery,” she says. “These are exactly the objects we need to be finding to understand the formation and history of our Solar System.”
Meanwhile, our best hope in the search for Planet X is to find more objects that corroborate its existence. “We don’t expect every object we find to fit this pattern, though that is what is happening right now,” says Sheppard. Better yet, finding Planet X would be pretty convincing, too. The problem is that there’s a lot of sky to scour and our telescopes don’t cover that much area at a time. The Subaru telescope in Hawaii is perhaps the best tool since it can observe the area of about six full Moons at a time. But it’s still tough to pinpoint the exact location of such a faraway, faint planet. “It’s a lot like looking for your target with a sniper rifle,” says Batygin. “You have to know where to look.”
But TG387 does help point astronomers in a slightly better direction. Before this discovery, there were about 30 orbits that Sheppard and his team thought Planet X could be on. Now, there are only 25 or so, he says. And the astronomers will be back at Subaru in mid-October to pick up the search. “We’ve covered about 30 percent of the prime area, and we hope by the end of this year, we’ll have covered 60 to 70 percent of that prime area,” says Sheppard.
If Planet X is found, then a whole new crop of questions will arise. Perhaps the biggest one of all is where did it come from? Most don’t think it’s possible for this planet to have formed where it is now. It likely formed in the inner Solar System and got flung outward, perhaps by Jupiter or Saturn. “That would suggest a lot of big things formed in our Solar System, and it was a very chaotic place in the formation era,” says Sheppard.
But before those questions can be answered, Planet X must be found. And those on the hunt are sure it will happen. “I’m really quite confident — about a 99 percent level of confidence — that Planet 9 is really out there,” says Batygin. “It might take on the order of a decade to find, but I’m quite confident it’s there.”