Forget invaders from Andromeda galaxy -- an entire planet might have gatecrashed our solar system.
Astronomers are finding growing evidence of a ninth planet well beyond Neptune, and it's not Pluto.
Planet 9, as it's known for obvious reasons, is believed to be about 10 times bigger than Earth and 75 times further away than Pluto. It would take between 10,000 and 20,000 years for it to orbit the sun.
Astronomers are basing its estimated size and distance on the effects it's having on objects in the Kuiper Belt, a sphere of rocky bodies that orbit the sun at the edge of the solar system -- dwarf planet Pluto is a relatively close-in member of the belt.
Scientists at the California Institute of Technology announced in January they had found a strange alignment of rocks in the belt with only a 0.007 percentage of it occurring by chance, rather than from the gravitational pull of a real-life mystery planet.
But it raises a difficult question -- assuming it exists, how did such a large planet end up so far away?
"The rest of the Solar System appears to be so compact, and then you have this enormous gap where there's really not very much, and then a very large planet, nearly 10 times the mass of Earth," astronomer Alexander Mustill said in a video released by Lund University in Sweden.
"It seems a little implausible to us that you would just 'create' this thing out at such a distance."
After running a number of computer simulations of how Planet 9 might have got there, they came to the conclusion it's 300 times more likely our sun stole it off another solar system.
"Planet 9 may very well have been 'shoved' by other planets, and when it ended up in an orbit that was too wide around its own star, our sun may have taken the opportunity to steal and capture Planet 9 from its original star."
"When the sun later departed from the stellar cluster in which it was born, Planet 9 was stuck in an orbit around the Sun."
An exoplanet right on Earth's doorstep would be a huge gain for science.
"It is almost ironic that while astronomers often find exoplanets hundreds of light years away in other solar systems, there's probably one hiding in our own backyard."
As for whether it even exists, the scientists at the California Institute of Technology who found the Kuiper Belt anomaly say there's only a 0.007 percent chance it was caused by chance, rather than a real-life mystery planet.