The object "has a mass about 10 times that of Earth" and follows a "bizarre, highly elongated orbit in the distant solar system," said a statement by researchers at the California Institute of Technology (Caltech).
"In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the Sun."
'Hello, is this planet Earth?' Astronaut dials wrong number on Christmas call from space
The report was published in the Astronomical Journal.
Researchers Konstantin Batygin and Mike Brown say they found the planet through mathematical modeling and computer simulations, and have not yet observed the object directly.
The celestial body has about 5,000 times the mass of Pluto.
"This would be a real ninth planet," said Brown, a professor of planetary astronomy.
"It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting."
LONDON: Tim Peake, the first British astronaut on the International Space Station, dialled a wrong number after trying to phone home for Christmas, asking a woman “Is this planet Earth?”
“I’d like to apologise to the lady I just called by mistake saying ‘Hello, is this planet Earth?’ – not a prank call…just a wrong number!” he tweeted late on Thursday.
UK astronaut to take part in London marathon from space
I'd like to apologise to the lady I just called by mistake saying 'Hello, is this planet Earth?' – not a prank call…just a wrong number!
— Tim Peake (@astro_timpeake) December 24, 2015
Peake, 43, became the first British astronaut to travel to the space station after blasting off from the Moscow-operated Baikonur cosmodrome in Kazakhstan earlier this month for a six-month mission.
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