Scientists have discovered "solid evidence" of a mysterious gaseous planet in the outer reaches of the solar system - which if true would make it the ninth official planet.
Here is (the currently theoretical) Planet Nine explained in nine points.
A 'real' planet
Planet Nine could be the real ninth planet in our solar system, after Pluto was demoted to the status of "dwarf planet".
It's big. Really big
Believed to be 10 times the mass of Earth and 5,000 times the mass of Pluto.
It's made of gas
Thought to be gaseous, like Neptune and Uranus.
Long time in orbit
The planet could take between 10,000 to 20,000 years to complete one journey around the sun.
Ninth rock from the sun
Orbits 20 times further from the sun than Neptune which orbits at a distance of 2.8 billion miles.
Long way from Earth
Scientists think it would never get closer to the sun than 200 times the distance between the sun and our own planet.
Meddles with orbits
Planet Nine's existence was inferred from the gravitational influence it was having on objects with unusual orbits in the Kuiper Belt surrounding Neptune.
Solar system headcount in doubt
Experts claim it's the first solid evidence more than 150 years that we don't have a full count of the solar system's planets.
Could be anywhere
The scientists who discovered it are urging astronomers to scour the skies to find the elusive Planet Nine.
ITV News' Science Correspondent Alok Jha reports: