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.
Brown and Batygin say Planet Nine helps explain a peculiar feature of the Kuiper Belt, a field of icy and rocky objects beyond Neptune: a mysterious clustering of six small objects that, by the laws of the Kuiper Belt, shouldn’t cluster. In 2014, Scott Sheppard of the Carnegie Institution of Science in Washington and Chad Trujillo of the Gemini Observatory in Hawaii suggested the configuration resulted from the presence of a planet large enough to warp the orbits of the nearby objects, lassoing them together. Brown and Batygin sought to disprove that theory. But they realized that, actually, something must be there—“a massive perturber”—because they found that, among other things, the orbits of the objects, even though they traveled at different rates, all tilted in the same way. The probability of that happening on its own, without some external force, is about 0.007 percent.
So, Planet Nine. An icy, rocky world. The researchers posit that the planet could have been flung out to deep space when it got too close to the gravitational forces of Jupiter or Saturn. It orbits 20 times farther from the sun than does Neptune, which on average is about 2.8 billion miles from our star. It takes Planet Nine between 10,000 and 20,000 years to make one full orbit around the sun. Had it stuck closer, the planet would have been the core of a gas giant, like the four in the solar system—Jupiter, Saturn, Uranus, and Neptune.
Planet Nine’s potential existence actually makes the solar system less of an anomaly in the grand scheme of the universe. It’s not unusual to find extremely far-flung planets orbiting stars, and the mass of most common planets circling other stars can be up to 10 times that of Earth.
The next challenge for astronomers will be spotting Planet Nine. It will be evaluated by the same standards that kicked Pluto out of science textbooks, 76 years after it was discovered, just over a third of the way into its 248-year-orbit around the sun, and nearly a decade before New Horizons revealed it to be a complex world with an atmosphere and icy mountains.
Brown, who tweets under the handle @plutokiller, said Wednesday that the new research should be welcome news for those still smarting over an eight-planet solar system.
“All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found,” he said. "Now we can go and find this planet and make the solar system have nine planets once again.”