The astronomer who helped kick Pluto out of the planet club believes a much larger body may be lurking in the outskirts of the solar system.
If it exists, the solar system's ninth planet is estimated to be a gas world about 10 times bigger than Earth, California Institute of Technology (Caltech) astronomer Mike Brown wrote in this week's Astronomical Journal.
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Brown and colleague Konstantin Batygin, also at Caltech, used mathematical models and computer simulations to deduce the planet's existence, but they also have some observational evidence to support their claim. Several small icy bodies in the Kuiper Belt region beyond Neptune have quirks in their orbits that may be explained by the gravitational influence of a larger, more distant planetary cousin.
Scientists then realized that six of those bodies follow elliptical paths pointing toward the same direction in space.
"It's almost like having six hands on a clock all moving at different rates, and when you happen to look up, they're all in exactly the same place," Brown said in a press release, adding that the odds of that happening are about one-in-100.
The orbits also are tilted in the same direction, roughly 30 degrees downward relative to the orbital plane of the solar system's other eight planets.
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"We thought something else must be shaping these orbits," Brown said.
After checking if a batch of other Kuiper Belt objects might be responsible, the scientists started doing computer simulations that included a distant outer planet in various orbits.
They found an unusual match: a massive planet in an anti-aligned orbit, which is an orbit in which the planet's closest approach to the sun is 180 degrees across from the closest approach of the objects and known planets in the solar system.
"I was very skeptical," Batygin said in the release. "I had never seen anything like this in celestial mechanics."
Besides accounting for peculiarities in some Kuiper Belt objects' orbits, the predicted rogue planet, located at least 200 times farther away from the sun than Earth, also would pin other Kuiper Belt bodies into orbits perpendicular to the plane of the rest of the planets.
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"I realized there are objects like that," Brown said. "We plotted up the positions of those objects and their orbits, and they matched the simulations exactly."
"When we found that, my jaw sort of hit the floor," he added.
Planet Nine, as the mystery object is known, has not yet been observed directly, but it may have been captured during previous surveys.
"We hope that other people are going to get inspired and start searching," Brown said.