In 2006, Pluto stopped being a planet.
“Pluto is dead,” said Mike Brown, a researcher from the California Institute of Technology, whose discovery a year earlier of a bigger world orbiting beyond Pluto led some astronomers to rethink what defines a planet—and ultimately decide that Pluto doesn’t count. “There are finally, officially, eight planets in the solar system.”
Fast forward a decade, and Brown is saying the opposite.
Brown, along with Caltech’s Konstantin Batygin, announced Wednesday that they have evidence that suggests a massive planet is orbiting in the edge of the solar system, far beyond Pluto, that would qualify as its ninth planet. The authors describe the planet, which they’re calling “Planet Nine,” in a paper published in The Astronomical Journal.
Planet Nine is big—really big. It is 10 times the mass of Earth, and 5,000 times the mass of Pluto. It dominates a region larger than any of the other known planets, which Brown says makes it “the most planet-y of the planets in the whole solar system,” according to a press release from Caltech. That’s the test Pluto failed to pass a decade ago—having enough mass to clear its orbit of other bodies with similar size.
Brown and Batygin have not directly observed Planet Nine, but have inferred its existence through mathematical models and computer simulations based on the movements of small, distant objects. From here, the planet is not even a speck of light in the vast darkness of space, and could only be seen—if it’s found—by powerful telescopes.