Nature surveys the growing body of evidence of *something* very big orbiting at the fringes of the solar system:
“If I read this paper out of the blue, my first reaction would be that it was crazy,” says Mike Brown, an astronomer at the California Institute of Technology in Pasadena who was part of the research team. “But if you look at the evidence and statistics, it’s very hard to come away with any other conclusion.”
Brown and his colleague Konstantin Batygin propose Planet Nine in a paper published on 20 January in the Astronomical Journal (K. Batygin and M. E. Brown Astronom. J. 151, 22; 2016).
Alessandro Morbidelli, an orbital-dynamics specialist at the University of the Côte d’Azur in Nice, France, who has reviewed the paper in detail, says he is “quite convinced” that the planet exists. Others are not so sure.
The story of Planet Nine began in 2014, when a pair of astronomers reported finding a KBO called 2012 VP113. Its stretched-out orbit never came closer than 80?au to the Sun (C. A. Trujillo and S. S. Sheppard Nature 507, 471–474; 2014). (Pluto, at its most distant, is 48?au from the Sun.) VP113 joined the dwarf planet Sedna as only the second known object with a very distant orbit. In their report, Chadwick Trujillo at the Gemini Observatory in Hilo, Hawaii, and Scott Sheppard of the Carnegie Institution for Science in Washington DC said that the orbits of these objects suggested that yet another object, a planet bigger than Earth, could exist at around 250?au….
Batygin and Brown picked up the challenge. “Our main goal at that point was to show that this idea is crazy,” says Brown.
But Trujillo and Sheppard had noted that Sedna, VP113, and several other KBOs all shared a peculiar property: their closest approach to the Sun lay in the plane of the Solar System, and they all moved from south to north when crossing that plane.
Batygin and Brown analysed the orbits further and discovered that their long axes were physically aligned, too, as if something had nudged them to occupy the same region of space around the Sun. The team concluded that a massive object must be shepherding the objects. “We have a gravitational signature of a giant planet in the outer Solar System,” Batygin says.
Planet Nine — informally known as Phattie — is probably smaller than Neptune and icy with a gassy outer layer.