Curious tilt of the sun traced to planet nine.

• Position: Solar System

Planet Nine--the undiscovered planet at the edge of the solar system that was predicted by the work of Konstantin Batygin and Mike Brown of the California Institute of Technology, Pasadena--appears to be responsible for the unusual tilt of the sun.

The large and distant planet may be adding a wobble to the solar system, giving the appearance that the sun is tilted slightly. "Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment," says graduate student Elizabeth Bailey, lead author of a study announcing the discovery.

All of the planets orbit in a flat plane with respect to the sun, roughly within a couple degrees of each other. That plane, however, rotates at a six-degree tilt with respect to the sun--giving the appearance that the sun itself is cocked off at an angle. Until now, no one had found a compelling explanation to produce such an effect. "Ifs such a deep-rooted mystery and so difficult to explain that people just don't talk about it," notes Brown, professor of planetary astronomy.

Brown and Batygin's discovery of evidence that the sun is orbited by an as-yet-unseen planet--that is about 10 times the size of Earth with an orbit that is about 20 times farther from the sun, on average, than Neptune's--changes the physics. Planet Nine, based on their calculations, appears to orbit at about 30[degrees] off from the other planets' orbital plane--in the process influencing the orbit of a large population of objects in the Kuiper Belt, which is how Brown and Batygin came to suspect a planet existed there in the first place.

"It continues to amaze us; every time we look...