Ever since Pluto was demoted from planet-status, it's felt like there's something missing in our solar system.
Originally considered to be the ninth planet orbiting around the sun, it was declassified in 2006 after scientists determined it to be too small.
Since then, Pluto has now been known as a dwarf planet and we’ve learnt to come to terms with the fact that there are now just eight neighboring planets to us… or so we thought.
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Astronomers have been suspicious for some time that there could be another planet out there.
It’s possible that it’s been difficult to spot because it’s far away from the sun so it would be dimly lit.
Researchers first began to wonder if there was something else we couldn’t see when they found a potential dwarf planet beyond the Kuiper Belt.
The Kuiper Belt is past Neptune and is made up of asteroids, comets, dwarf planets and more - it’s also where Pluto is situated.
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The dwarf planet, now called Sedna, had a strange orbit which suggested that another large mass was pulling it.
Astronomers have theorized that this could be a giant planet, with five to ten times the mass of Earth and an elongated orbit 400 to 800 times farther from the sun than our planet is.
Some suggest that this mysterious planet could have been ejected from its original orbit by Jupiter during the early stages of the solar system, or that it was once a rogue planet that formed a distant orbit.
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Speaking to Live Science, Mike Brown, who is an astronomer at the California Institute of Technology, said: “It's really difficult to explain the solar system without Planet Nine but there's no way to be 100% sure [it exists] until you see it.
“At the beginning, we didn't say there was a planet because we thought that was a ridiculous thing for there to be.
“But we tried a lot of different things to explain what we were seeing, and nothing else worked.
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“Our best estimates are that it's about seven times more massive than Earth.”
Current telescopes don’t seem to be able to find it and it would be like searching for a needle in a haystack if the planet is not well lit by the sun.
But all hope is not lost - a brand new telescope is in the works and it could be looking at the stars as soon as next year.
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The new telescope is based at the Vera C. Rubin Observatory in Chile and might be the answer scientists are looking for to finally spot what they're calling Planet Nine.
While other astronomers are quite as convinced that another planet exists, we might soon know for sure once and for all.