Planet Nine is a huge alien world roughly four times the size of Earth that continues to evade astronomers – but it's not the only space mystery that has experts baffled.
From the search for intelligent alien life to the dark matter that binds our cosmos, our universe is hiding plenty of juicy secrets.
Read on to discover the biggest space conundrums that science has yet to crack.
Four times the size of Earth and around ten times its mass, this mysterious ninth planet could be lingering on the icy outskirts of our universe.
There's just one problem: scientists claim it is invisible to telescopes, making it a pain to track down.
But Nasa is optimistic because something out there is pushing a bunch of mini ice-worlds in its orbit.
Last year, the space agency said Planet Nine may be 20 times further from the Sun than Neptune, adding: "It is now harder to imagine our solar system without a Planet Nine than with one."
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