A huge planet that has never been seen by astronomers could be lurking, almost invisibly, at the dark edges of our solar system, some scientists believe.
By 1846, astronomers had pinpointed all the eight main planets – and have since found several other 'dwarf planets' including Pluto.
But the search goes on for the mysterious 'ninth planet' which is thought to sit far beyond Neptune in our solar system, said space expert Sara Webb.
There is a lot of evidence for the planet – thought to be up to 20 times further out from the Sun than Neptune – but it may be impossible to see with current technology.
The giant, hidden planet is thought to be 10 times larger than Earth and on an orbit that takes 10,000 or 20,000 years to go round the sun.
Writing in The Conversation, Webb, of Swinburne University in Australia, said: "There's a good reason astronomers spend many hundreds of hours trying to locate a ninth planet, aka Planet Nine or Planet X. And that's because the solar system as we know it doesn't really make sense without it.
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“It's through our understanding of gravitational pull that we get our biggest clue for a possible Planet Nine.
"When we look at really distant objects, such as dwarf planets beyond Pluto, we find their orbits are a little unexpected. They move on very large elliptical (oval-shaped) orbits, are grouped together, and exist on an incline compared to the rest of the solar system.
"When astronomers use a computer to model what gravitational forces are needed for these objects to move like this, they find that a planet at least 10 times the mass of Earth would have been required to cause this."
Previously, a scientist from the California Institute of Technology (Caltech) suggested that not only does Planet Nine exist, it could be five times the size of Earth.
Professor Konstantin Batygin said: "At five Earth masses, Planet Nine is likely to be very reminiscent of a typical extrasolar super-Earth."
Super-Earths are rocky bodies, like our planet. but much, much bigger.
On the planet, it is likely to be extremely dim, as it is so far from the Sun, so it could be up to 1,000 years before the planet is spotted.
Other researchers believe that there might be a huge disc of icy objects out there instead.
Webb is optimistic that Planet Nine could be found sooner rather than later, due to a new generation of space telescopes.
She said that there are unique challenges to spotting the huge object.
"We only have small windows of nights where the conditions must be just right. Specifically, we have to wait for a night with no moon, and on which the location we're observing from is facing the right part of the sky.
"In the next decade, new telescopes will be built and new surveys of the sky will begin. They might just give us the opportunity to prove or disprove whether Planet Nine exists."