AS science often does, it began with a "huh?" Some distant objects far beyond Pluto were behaving very oddly. The orbits of a handful of space rocks had aligned for no apparent reason. Though stumped at first, astronomers now have an explanation: a huge ninth planet at the edge of the solar system. If the researchers have their sums right, the mysterious new world is 10 times more massive than Earth and up to four times the size. Nicknamed Planet Nine, it moves on an extremely elongated orbit, and takes a staggering 10,000 to 20,000 years to swing once around the sun. The icy world, if it exists, has evaded detection because it is so far away. Scientists at the California Institute of Technology (Caltech) calculate that the closest it comes to the sun is 15 times the distance to Pluto. It then heads into uncharted territory, 75 times further out than Pluto, or about 93 billion miles from the sun. A ray of light would take a week to get there. "We saw a strange signal in the data that meant something odd was going on in the outer solar system," Mike Brown, an astronomer at Caltech, said. "All of these distant objects were lined up in a weird way and that shouldn't happen. We worked through the mundane explanations, but none of them worked out." The report was published in the Astronomical Journal. Researchers Konstantin Batygin and Mike Brown say have not yet observed the object directly. Rather, they found it through mathematical modeling and computer simulations. The presumed planet has about 5,000 times the mass of Pluto, and scientists believe its gravity has affected the motion of dwarf planets in the outer solar system, essentially perturbing celestial bodies in the field of icy objects and debris beyond Neptune known as the Kuiper Belt. "Like a parent maintaining the arc of a child on a swing with periodic pushes, Planet Nine nudges the orbits of distant Kuiper Belt objects such that their configuration with relation to the planet is preserved," explained CalTech in a statement. Brown, one of the co-authors on the paper, was a leading force in the downgrade of Pluto from planet to dwarf planet in 2006. He and colleagues had found a dwarf planet called Eris that was more massive than Pluto, and a potential candidate for a 10th planet. But when the International Astronomical Union decided in 2006, to issue a new definition of "planet," neither Eris nor Pluto made the cut. "OK, OK, I am now willing to admit," said Brown, who goes by @plutokiller on Twitter. "I do believe that the solar system has nine planets." But how could astronomers go so long without realizing another planet was out there? Brown and colleagues say Planet Nine could have been cast off during the early formation of the solar system, when four major cores grabbed up the gas around them and formed Jupiter, Saturn, Uranus and Neptune. Perhaps Planet Nine represented a fifth core, that may have gotten too close to Jupiter or Saturn and been ejected into its current, distant orbit, said Brown. A host of powerful telescopes are currently hunting for Planet Nine, including the twin 10-meter telescopes at the W. M. Keck Observatory and the Subaru Telescope on Maunakea in Hawaii. "Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we've become increasingly convinced that it is out there," said Batygin, an assistant professor of planetary science.
