The mythical "Planet X" may actually be real, and scientists are calling it "Planet Nine."
Astronomers have found evidence for a planet 10 times more massive than Earth in the far outer solar system, orbiting about 20 times farther from the sun than distant Neptune does.
"This would be a real ninth planet," one of the researchers, Mike Brown of the California Institute of Technology (Caltech) in Pasadena, said in a statement. "There have only been two true planets discovered since ancient times, and this would be a third. It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting." [Evidence Mounts for Existence of 'Planet X' (Video)]
This potential "Planet Nine" has not yet been observed. But Brown and his colleague, Konstantin Batygin, also of Caltech, are inferring its likely existence based on modeling work and the weird orbits of a number of small objects in the faraway Kuiper Belt, which lies beyond Neptune.
Specifically, six Kuiper Belt objects (KBOs) cruise around the sun on elliptical paths that all point in the same direction, even though the bodies are moving at different speeds. In addition, the six KBOs' orbits all share the same tilt — roughly 30 degrees downward, relative to the plane of the eight officially recognized planets. (Pluto, which was the ninth planet until its 2006 reclassification by the International Astronomical Union, zips around the sun in a different plane.)
The odds of this latter phenomenon occurring by chance alone are about 0.007 percent, researchers said.
"Basically, it shouldn't happen randomly," Brown said. "So we thought something else must be shaping these orbits."