A plane of dust and gas around the Sun keeps everything orbiting the same way with its momentum.
But the bizarre planet dubbed Niku, Chinese for rebellious, is moving the opposite way, meaning it was knocked off course.
Now scientists have admitted they don’t know what – or who – is responsible, but say there’s something going on we don’t know about.
“There’s more going on in the outer solar system than we’re aware of”
The planet, which could be less than 200km in diameter, orbits in the outer solar system beyond the distant world of Neptune.
Michele Bannister, an astronomer at Queens University, Belfast, said the discovery was “confusing” for astronomers.
She told New Scientist: “Angular momentum forces everything to have that one spin direction all the same way.
“It’s the same thing with a spinning top, every particle is spinning the same direction.”
She added: “I’m looking forward to seeing what the theoretical analysts do once they get their hands on this one.”
Matthew Holman of the Harvard-Smithsonian Center for Astrophysics said something must be happening humans don’t know about.
“It suggests that there’s more going on in the outer solar system than we’re fully aware of,” he told the magazine.
It was initially thought that the gravity of a giant planet we can’t see in the outer solar system was to blame.
But that world, called Planet Nine, is believed to be too far away and has been ruled out.
Instead scientists looked for a dwarf planet like Pluto that could provide the answer.
Yet that line of enquiry has proved fruitless too, leaving us none the wiser.
“We don’t know the answer,” says Holman.