The solar system appears to have a new ninth planet. Today, two scientists announced evidence that a body nearly the size of Neptune—but as yet unseen—orbits the sun every 15,000 years. During the solar system’s infancy 4.5 billion years ago, they say, the giant planet was knocked out of the planet-forming region near the sun. Slowed down by gas, the planet settled into a distant elliptical orbit, where it still lurks today.
Killing Pluto was fun, but this is head and shoulders above everything else.
Batygin and Brown published the result today in The Astronomical Journal. Alessandro Morbidelli, a planetary dynamicist at the Nice Observatory in France, performed the peer review for the paper. In a statement, he says Batygin and Brown made a “very solid argument” and that he is “quite convinced by the existence of a distant planet.”
Championing a new ninth planet is an ironic role for Brown; he is better known as a planet slayer. His 2005 discovery of Eris, a remote icy world nearly the same size as Pluto, revealed that what was seen as the outermost planet was just one of many worlds in the Kuiper belt. Astronomers promptly reclassified Pluto as a dwarf planet—a saga Brown recounted in his book How I Killed Pluto.
Now, he has joined the centuries-old search for new planets. His method—inferring the existence of Planet X from its ghostly gravitational effects—has a respectable track record. In 1846, for example, the French mathematician Urbain Le Verrier predicted the existence of a giant planet from irregularities in the orbit of Uranus. Astronomers at the Berlin Observatory found the new planet, Neptune, where it was supposed to be, sparking a media sensation.
Remaining hiccups in Uranus’s orbit led scientists to think that there might yet be one more planet, and in 1906 Percival Lowell, a wealthy tycoon, began the search for what he called “Planet X” at his new observatory in Flagstaff, Arizona. In 1930, Pluto turned up—but it was far too small to tug meaningfully on Uranus. More than half a century later, new calculations based on measurements by the Voyager spacecraft revealed that the orbits of Uranus and Neptune were just fine on their own: No Planet X was needed.
Yet the allure of Planet X persisted. In the 1980s, for example, researchers proposed that an unseen brown dwarf star could cause periodic extinctions on Earth by triggering fusillades of comets. In the 1990s, scientists invoked a Jupiter-sized planet at the solar system’s edge to explain the origin of certain oddball comets. Just last month, researchers claimed to have detected the faint microwave glow of an outsized rocky planet some 300 AU away, using an array of telescope dishes in Chile called the Atacama Large Millimeter Array (ALMA). (Brown was one of many skeptics, noting that ALMA’s narrow field of view made the chances of finding such an object vanishingly slim.)
Brown got his first inkling of his current quarry in 2003, when he led a team that found Sedna, an object a bit smaller than both Eris and Pluto. Sedna’s odd, far-flung orbit made it the most distant known object in the solar system at the time. Its perihelion, or closest point to the sun, lay at 76 AU, beyond the Kuiper belt and far outside the influence of Neptune’s gravity. The implication was clear: Something massive, well beyond Neptune, must have pulled Sedna into its distant orbit.