Scientists have fiercely debated the existence of ‘Planet 9’ for a decade. Some say evidence is piling up

Our solar system used to have nine planets. Astronomer Mike Brown, also known as “the man who killed Pluto,” said he got hate mail from kids and obscene calls at 3 a.m. for years after his most famous finding helped change that.

Brown, a professor of planetary astronomy at Caltech, discovered another small world called Eris in the Kuiper Belt — a vast ring of icy objects beyond Neptune’s orbit that also happens to be the former ninth planet’s neighborhood. The 2005 revelation set off a chain of events that led to Pluto’s still-controversial demotion from planet status the following year.

But now, just as the Kuiper Belt effectively took a ninth planet away, Brown and other scientists believe it could give one back.

The belt, which astronomers believe is made of leftovers from the solar system’s formation, extends 50 times farther from the sun than Earth, with a secondary region that reaches beyond it for nearly 20 times that distance. Pluto, now classified as a dwarf planet along with Eris, is just one of the largest among the scores of icy bodies that exist there — and doesn’t dominate its own orbit and clear the orbit of other objects. That’s why it can’t have the same standing as the remaining eight planets, according to guidelines laid out by the International Astronomical Union.

Because objects in the Kuiper Belt are so far away from the sun, however, they are difficult to spot. For more than a decade, astronomers have been searching that area for a hidden planet that has never been observed, but its presence is inferred by the behavior of other nearby objects. It’s often called Planet X or Planet Nine.

“If we find another planet, that is a really big deal,” said Malena Rice, an assistant professor of astronomy at Yale University. “It could completely reshape our understanding of the solar system and of other planetary systems, and how we fit into that context. It’s really exciting — there is a lot of potential to learn a tremendous amount about the universe.”

The excitement comes with some controversy — different camps have competing theories about the planet itself while some researchers believe it doesn’t exist at all.

“There are definitely full-blown skeptics about Planet Nine — it’s kind of a contentious topic,” Rice said. “Some people feel very passionately that it exists. Some people feel very passionately that it doesn’t. There’s a lot of debate in trying to pin down what it is, and whether it’s there. But that’s the hallmark of a really interesting topic, because otherwise people wouldn’t have heated opinions about it.”

Soon, the debate could be settled, once a new telescope capable of surveying the entire available sky every few nights comes online in late 2025. Until then, a team of researchers believes it has found the most compelling evidence yet that the hidden planet is real.