Most of us grew up saying our solar system was made up of a sun and nine planets — including our distant sibling Pluto. In 2005, however, Pluto lost its standing as a planet, owing its downfall in large part to discoveries of other planetary bodies like Eris, a dwarf planet of the same basic size as Pluto that also shares our celestial zip code. Soon more dwarf planets were discovered, making Pluto a bit less special than it was once considered. Ultimately, the decision was made to strip Pluto of its title, making it the king of the dwarf planets, rather than the runt of the group.

Since then, our solar system’s planetary count has officially been eight… but that may not be the case for long. There have long been legends of mysterious planets (and even dwarf stars) lurking in the distant expanse of our solar system, beyond where the sun’s light can reach with enough effect to illuminate it in our skies. Some have argued such a planet could be responsible for the cyclical extinction events that seem to occur on Earth every 31 million years or so. The theory suggests that a massive orbital path may take earth’s mysterious cousin into the interior of the solar system periodically, disrupting the state of the asteroid belt via gravitational forces and creating an environment ripe for asteroid strikes.

Legends of “Planet X” were more or less dismissed by the scientific community, however, until 2016 — when one of the very men responsible for Pluto’s demotion helped to publish a paper proposing that there may really be another planet way out in the nose-bleed section of our own solar system.

“We have found evidence that there’s a giant planet in the outer solar system,” Astronomer Mike Brown said at the time. “By ‘giant’ we mean the size of Neptune, and when we say ‘outer solar system’ we mean 10 to 20 times farther away than Pluto.”