Two scientists claim that they have found the best evidence yet that a ninth planet exists at the outer limits of the Solar System. It’s called Planet X, and it’s roughly the size of Neptune, the researchers say. The reason we haven’t seen it yet is because it orbits far out from the Sun on a highly elliptical path, which takes 10,000 to 20,000 years to make one full circuit. But no one’s ready to say the Sun has nine planets again — there's no direct evidence that this object actually exists.

The entire argument about the existence of Planet X is a theoretical one, based on mathematical models made from six Kuiper Belt objects. It’s based on the movements of these objects, which show signs they’ve been affected by something large, the researchers wrote in The Astronomical Journal. Essentially, the objects all have orbits that take them within the same area near the Sun, as if they’ve been pushed there by something bigger. And if Planet X — also sometimes called Planet Nine — exists, it’s way out there; it’s somewhere between 200 and 1,200 times the distance from the Sun to Earth.

It's not the first time people have claimed to have evidence of such a planet lurking on the edge of our galactic neighborhood. The debate about this world's existence has been going on for the last century. In 1905, Percival Lowell predicted a Planet X existed beyond Neptune; his calculations actually led to the discovery of Pluto, which was too small to be what Lowell suspected. Now a century later, researchers at the California Institute of Technology say they have the best argument yet for Planet X. "If you say, ‘We have evidence for Planet X,’ almost any astronomer will say, ‘This again? These guys are clearly crazy.’ I would, too," study author Mike Brown, a planetary scientist at Caltech, told Science. "Why is this different? This is different because this time we’re right."

However, experts argue that the only way anyone can be certain is to observe the planet itself. "The paper is interesting, but until somebody’s observed it, there’s nothing," Dave Jewitt, a planetary scientist at UCLA. And so far, no surveys have observed this mysterious planet in action.

Nothing has ruled out the idea of such a planet, either. NASA searched for distant objects in the Solar System with its Widefield Infrared Survey Explorer (WISE). In 2013, WISE ruled out the possibility of planets larger than Saturn as far out as 10,000 times the distance between the Sun to Earth, according to a study published in the Astrophysical Journal. The study actually disproved the existence of two theorized Planet X candidates at the time. But Brown and his co-author Konstantin Batygin, also a planetary scientist at Caltech, say Planet X is only 10 times the size of Earth — smaller, in other words, than WISE could detect — so if it's out there, WISE might have missed it. Brown also makes the claim that other surveys, such as the Outer Solar System Origins Survey, aren't sensitive enough to find the planet.

Jewitt also cautions that six is a small sample size for such a big claim. Originally Brown and Batygin had 13 objects they were observing, but had to eliminate over half because seven had been strongly disturbed by Neptune. "So the sample size got smaller," says Jewitt. "But then they make the claim that based on this sample of six, their result is statistically significant. So then it becomes a statistical argument."

The statistics do sound promising, at first. The researchers say there's a 1 in 15,000 chance that the movements of these objects are coincidental and don't indicate a planetary presence at all. That’s the equivalent of a certain event happening about twice in a lifetime. "When we usually consider something as clinched and air tight, it usually has odds with a much lower probability of failure than what they have," says Sara Seager, a planetary scientist at MIT. For a study to be a slam dunk, the odds of failure are usually 1 in 1,744,278. That’s like something happening once in recorded history. But researchers often publish before they get the slam-dunk odds, in order to avoid getting scooped by a competing team, Seager says.

Most outside experts agree that the researchers' models are strong. And Neptune was originally detected in a similar fashion — by researching observing anomalies in the movement of Uranus. Additionally, the idea of a large planet at such a distance from the Sun isn't actually that unlikely, according to Bruce Macintosh, a planetary scientist at Stanford University. In fact, he says it's stranger for the Solar System not to have an object like this. Thanks to observations from the Kepler spacecraft, we now know that objects 10 times the mass of the Earth — like Planet X — are incredibly common in the Universe. "Other systems are full of things that are just the right size for this," says Macintosh. "It kind of makes our Solar System look a lot like other systems."

And now we may have a better chance of spotting it too. The Large Synoptic Survey Telescope will soon conduct a search of the entire Southern Hemisphere sky; if Planet X is really out there, the telescope should find a lot more Kuiper Belt objects manipulated by the planet's path, Macintosh says. The telescope may even be able to spot planets twice the size of Earth at distances up to several hundred times the distance from the Earth to the Sun. Brown and Batygin also plan to use Hawaii's Subaru telescope to look for Planet X, as well. "This is just a great example of science in progress," says Seager.

But until the planet is actually seen, its existence is no more than an exciting idea. And ideas aren't always permanent. "Theories and models come and go, but a good observation lasts forever," says Jewitt.