The peculiar clustering of the six faraway objects that Brown and Batygin’s initial hypothesis highlighted is extremely unlikely to happen just by chance. According to the duo’s mathematical arguments, it would be naturally explained, though, by a planet about 10 times as big as Earth in the region known as the Kuiper Belt. Batygin has come up with more recent evidence, too: The orbits of other distant solar-system objects yoked to Neptune have gotten “detached” as well, and other objects’ orbits have gotten tilted to the side or reversed, so the solar system as a whole no longer resembles a thin record or CD with the sun at the center. Planet Nine, if it exists, could explain all of those phenomena.

“When all of these things come together, it’s becoming evident that without Planet Nine, the solar system has these weird puzzles and features that stand out,” Batygin says. His approach echoes that of astrophysicists when they inferred the existence of dark matter—which still can’t be seen—based on the rapid motions of stars in the outer realms of galaxies, and then clinched the idea with a more diverse array of evidence.

But other experts remain skeptical. Even the most basic facts are in dispute. Scientists with the Outer Solar System Origins Survey, or OSSOS, argue that Brown and Batygin’s data are actually biased by factors like bad weather and their telescope’s location, which could influence what’s seen and what’s missed, thereby introducing a spurious trend. If this is true, then there’s nothing weird about the little balls of rock and ice in the first place. The OSSOS researchers say these objects mostly seem randomly oriented and not aligned by some unseen force. If there’s no clustering, then Planet Nine’s linchpin disappears.

“While my research is skeptical of this planet, that’s not at all to say there isn’t a planet out there,” says Cory Shankman, an astronomer at the University of Victoria who led the research. He advocates for continuing the search for these hard-to-detect objects and understanding the biases while doing it. It’s slow, painstaking work.

Shankman’s survey only covered one-twentieth of the sky, however. Other astronomers, such as those affiliated with the Dark Energy Survey, question their findings, just like Shankman questions Brown and Batygin’s. “The more objects you can find that bear on the story, the easier it is to talk about them as a population rather than a small handful of curiosities” says David Gerdes, a University of Michigan astrophysicist. One way or another, he believes, the answer will be clear within the next year or two.

A theory’s more powerful if it doesn’t just explain what’s already known, but also makes successful predictions about things that haven’t been seen yet. If scientists find more objects throughout the Kuiper Belt and these objects are hardly clustered at all, it will deal a blow against Planet Nine. If the objects are similarly clustered as Brown and Batygin expect, it will strengthen their case.