Humans have explored only a tiny, tiny fraction of the universe, so we can’t rule out the possibility objects exist out there that we haven’t seen yet — even giant planets right in our backyard.

You can divvy up the Solar System into four parts: the inner rocky planets, the Asteroid Belt, the gas giants, and a huge cluster of small icy bodies called the Kuiper Belt. Some of those icy chunks in the Kuiper Belt are big enough to count as dwarf planets (like Pluto and Eris).

Now scientists think something bigger — perhaps four times bigger than Earth — is lurking out there, just beyond the Kuiper Belt.

“I think there are definitely things out there bigger than Pluto that are yet to be discovered,” Scott Sheppard, an astronomer at the Carnegie Institution, told the Washington Post.

Sheppard worked on a paper published in the journal Nature in 2014 that describes the discovery a mystery object on the fringe of the Kuiper Belt. The astronomers nicknamed it “Biden.” It’s no bigger than a dwarf planet, and they estimate that it’s three times the distance that Pluto is from Earth, or roughly 9 billion miles away.

In fact, there’s evidence that quite a few of these mystery objects, none of them larger than a dwarf planet, drifting at the fringes of the Kuiper Belt, according to the paper. And they aren’t just randomly floating around out there: They all appear to follow a similar orbital pattern, albeit a really strange one.

This means it’s possible some especially large, really distant planet — called a “perturber” in Sheppard’s study — is massive enough that its gravity can tug Biden and the other mystery objects into a similar path through space. Unlike Biden, the “perturber” could be four times the size of Earth.

If this giant “Planet X” exists, we probably haven’t spotted it because it’s so far away. At such a great distance it would appear 200 times fainter than the Sun appears from Earth. Or there might be some entirely different explanation for why those mystery objects appear to have the same orbital pattern. The bottom line is that the researchers don’t know enough yet.

The researchers based their observations on only 12 of these outer Kuiper Belt objects, and the orbital pattern they appear to follow might vanish with future observations. In other words, it might just be a coincidence they all seem to follow a similar path.

Since Sheppard first announced distant objects in 2014, however, he’s found more — and they, too, follow the same pattern. So that’s more evidence that “Planet X” does exist. Of course we won’t know for sure until someone spots it, but right now it seems like the most plausible explanation.

“There is a not insignificant chance that there are Earth-mass or larger objects yet to be discovered in the outer solar system,” Mike Brown, the astronomer who discovered dwarf planet Eris, told the Washington Post.

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