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.

Professor Mike Brown speaks in front of a computer simulation of the probable orbit of Planet Nine (yellow) Credit: REUTERS/Mario Anzuoni

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.

The orbits of Planet Nine and the six most distant objects in the solar system Credit: Caltech/R. Hurt

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: