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Although the planet, dubbed Planet Nine, has not been directly observed, computer simulations of the orbits of several distant objects beyond Neptune indicate it exists, scientists from the California Institute of Technology 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 become increasingly convinced that it is out there," said Konstantin Batygin, an assistant professor of planetary science.
The new findings are reported in the Astronomical Journal.
Dwarf planets Pluto and Ceres were originally classified as planets at the time of their discovery — but were both later demoted.
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