The existence of Planet Nine - a hypothetical planet on the edge of the solar system - which is twice the size of Earth, came one step closer in January.
It's thought to be orbiting our sun ten times further away from Pluto, and there were even some spurious reports that it could destroy life on Earth.
Finally, scientists have some ideas about how it exists so far away.
The planet is thought to circle our sun from approximately 40 to 140bn miles away, by far the most distant planet of them all.
In a newly published paper by the Harvard Smithsonian Centre for Astrophysics (cfA) astronomers Gongjie Li and Fred Adams used millions of computer simulations to come up with three theories on how the planet came to being:
1. A passing star dragged the planet away from the sun with its gravitational pull
This type of interaction would explain not only the distance but also the reason for Planet Nine’s strange elliptical orbit.
The only weakness in this theory is if this happened, there’s a high chance that the star would have pulled the planet completely out of our solar system.Picture: Stock/NASA/Getty, Spiral Galaxy
2. Jupiter “kicked” it away
Astronomer Scott Kenyon argues the “planet” is actually a huge gas giant - like Saturn, Neptune and Jupiter - which formed nearer the sun. The latter planet, which has a habit of either eating up material or expelling it, may have given Planet Nine a series of 'gravitational kicks' which, over time, pushed the planet far out.
Think of it like pushing a kid on a swing. If you give them a shove at the right time, over and over, they'll go higher and higher.
Then the challenge becomes not shoving the planet so much that you eject it from the solar system.
3. The planet came from another solar system
Dipping her toes firmly into the pool of strange, Li argues that perhaps it's an exoplanet, having formed in another solar system next to our own.
The edges of our solar system may have once shared material - maybe even a planet or two - with other solar systems.
While it remains largely speculation, the third theory would mean that Planet Nine, having been born outside of our solar system, is an alien.