It is thought Planet Nine could take 20,000 years to orbit the sun
Scientists have discovered evidence of a hidden planet 10 times bigger than the Earth on the outer reaches of the solar system.
Nicknamed 'Planet Nine' and thought to be similar to the gas giants of Uranus and Neptune, it has not yet been seen directly.
Its presence was indicated from observations by researchers using computer modelling.
The world is around 20 times further from the sun than Neptune, which lies at a distance of around 2.8 billion miles.
Given its far-flung location, experts think Planet Nine could take up to 20,000 years to make a single journey around the sun, which sits at the centre of the solar system.
There had previously been thought to be nine planets in the solar system, but then the most distant, Pluto, had its status downgraded to a "dwarf planet".
Professor Mike Brown, from the California Institute of Technology, who was one of the astronomers who announced the discovery in the Astronomical Journal, said: "This would be a real ninth planet".
"There have only been two true planets discovered since ancient times, and this would be a third".
"It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting".
Its existence was suggested by the influence its gravity had on a number of rocky bodies beyond Neptune known as the as the Kuiper Belt.
These objects had highly unusual orbits and computer simulations confirmed a giant previously unknown planet was the most likely explanation.
Prof Brown said: "We plotted up the positions of those objects and their orbits, and they matched the simulations exactly".
"When we found that, my jaw sort of hit the floor".
His Caltech colleague Dr Konstantin Batygin said: "Although we were initially quite sceptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we became increasingly convinced that it is out there".
"For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete".
Space and science commentator Leo Enright told Newstalk Breakfast there is an Irish link to all this.
Caltech's Konstantin Batygin and Mike Brown discuss the new research: