An unknown object, possibly 10 times the mass of Earth, is hovering on the edge of the solar system. Its gravity is warping the orbits of the dwarf planet Sedna and 18 similar space bodies and causing them all to tilt on their axes at precisely the same angle. The odds of that happening coincidentally are 0.007%.

“So we thought: something else must be shaping these orbits,” Mike Brown, Ph.D., a professor of planetary astronomy at the California Institute of Technology (Caltech), said in a statement.

But scientists aren’t sure what, exactly, that something else is. No one has been able to catch sight of the object. Its existence remains theoretical, with only mathematical calculations to substantiate its presence.

“The reason it’s such a tough search is because most astronomical surveys are not looking for a single thing,” Konstantin Batygin, Ph.D., also a Caltech planetary astronomer, told the BBC. Most searches are for a certain kind of object. Even if it’s rare, the sky is wide — search enough of it, and you’ll stumble upon it. But with the unknown object, “there’s only one tiny portion of the sky that has it.” Plus, there’s only one telescope in the world currently powerful enough to search for an object that hangs around in the dark, shadowy edge of the solar system, too far to reflect light from the sun. Researchers chasing the mysterious object get maybe three nights a year with the Subaru Telescope in Hawaii.

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The Vera Rubin telescope — new, mega-powerful lens under construction in Chile — will change that when it is completed in the news few years. But until then, scientists are left guessing as to what the object is. One leading theory, proposed by Brown and Batygin in a recent paper, is the long-sought ninth planet, a role briefly filled by Pluto before its demotion to dwarf planet in 2006. If this is so, scientists posit it is a cold, icy, rock-core place, much like Uranus or Neptune. Mathematical models suggest an elongated orbit; at its closest point to the sun, it’s still 300 times the distance between the Sun and Earth; at the farthest point in its lopsided orbit, it’s 600 times the Sun-Earth distance.

But with only gravitational proof, and no visual, another possibility is just as likely: that it’s not a planet at all, but a primordial black hole. Primordial black holes are theoretical, too — never seen, but thought to have coalesced in the initial moments of the Big Bang, when matter and energy were so chaotic a mass equivalent to a large planet might have been squeezed into the tiniest of spaces so dense no light can escape. But if the mystery object is this kind of black hole, though its mass would be up to 10 times that of Earth’s, it would be roughly the size of an orange. It would also appear only as a dark space between stars — making it even more unlikely to spot from Earth than a ninth planet.

Whether the unknown object is a planet or a primordial black hole will remain a mystery for some time. But its theoretical existence is not only very real to the scientists studying it, but also very real to the space objects it attracts with its gravity — and thus, very real to the solar system we call home.