Researchers Konstantin Batygin and Mike Brown, two CalTech scientists, say the new planet is about 10 times the mass of Earth and has an atmosphere of hydrogen and helium. Science magazine reports that the mysterious "Planet X" moves in a distant orbit beyond Neptune.
The researchers haven't observed Planet X itself, but believe it exists because of the unique configuration of six objects when they come closest to the sun, according to Science.
The scientists say that there's a 0.007 percent probability that the configuration is due to chance, and instead are confident it's a ninth planet. They believe they will observe the planet with a telescope within five years.