Scientists say Planet Nine could have been cast off during the early formation of the solar system.
A previously unknown giant planet may have been discovered lurking in the outer reaches of our solar system, US scientists announced on Wednesday.
Nicknamed Planet Nine, 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."
The report was published in the Astronomical Journal.
Researchers Konstantin Batygin and Mike Brown say have not yet observed the object directly.
Rather, they found it through mathematical modeling and computer simulations.
The presumed planet has about 5,000 times the mass of Pluto, and scientists believe its gravity has affected the motion of dwarf planets in the outer solar system, essentially perturbing celestial bodies in the field of icy objects and debris beyond Neptune known as the Kuiper Belt.
"Like a parent maintaining the arc of a child on a swing with periodic pushes, Planet Nine nudges the orbits of distant Kuiper Belt objects such that their configuration with relation to the planet is preserved," explained CalTech in a statement.
Brown, one of the co-authors on the paper, was a leading force in the downgrade of Pluto from planet to dwarf planet in 2006.
He and colleagues had found a dwarf planet called Eris that was more massive than Pluto, and a potential candidate for a 10th planet.
But when the International Astronomical Union decided in 2006, to issue a new definition of "planet," neither Eris nor Pluto made the cut.
"OK, OK, I am now willing to admit," said Brown, who goes by @plutokiller on Twitter.
"I DO believe that the solar system has nine planets."
But how could astronomers go so long without realizing another planet was out there?
Brown and colleagues say Planet Nine could have been cast off during the early formation of the solar system, when four major cores grabbed up the gas around them and formed Jupiter, Saturn, Uranus and Neptune.
Perhaps Planet Nine represented a fifth core, that may have gotten too close to Jupiter or Saturn and been ejected into its current, distant orbit, said Brown.
A host of powerful telescopes are currently hunting for Planet Nine, including the twin 10-meter telescopes at the W. M. Keck Observatory and the Subaru Telescope on Maunakea in Hawaii.
"Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we've become increasingly convinced that it is out there," said Batygin, an assistant professor of planetary science.
"For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete."
Other planets have been discovered by mathematical modeling, including Neptune in 1846.
But not every prediction has led to an actual planet, said Robert Massey, deputy executive director of Royal Astronomical Society in London.
"There have been instances in the past where planets have been predicted... but weren't found," he told AFP.
But, he added, the researchers who have published their paper are respected in the science community and their hypothesis is definitely worth following up.
"It would be a really exciting thing to find. At the moment it's simply a prediction."