Planet Nine may be behind curious tilt of Sun: study

This is because the massive planet has a tilted orbit and solar system has no choice but to slowly twist out of alignment

Planet Nine - the undiscovered planet at the edge of the solar system - may be responsible for the unusual tilt of the Sun, according to a new study.

The large and distant planet that was predicted to exist in January this year may be adding a wobble to the solar system, giving the appearance that the Sun is tilted slightly, researchers said.

"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," said Elizabeth Bailey, a graduate student at California Institute of Technology (Caltech) in the US.

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.

"It's such a deep-rooted mystery and so difficult to explain that people just do not talk about it," said Caltech's Mike Brown.

The 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 our understanding of 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 Konstantin Batygin, also from Caltech, came to suspect a planet existed there in the first place.

"It continues to amaze us; every time we look carefully we continue to find that Planet Nine explains something about the solar system that had long been a mystery," said Batygin.

The tilt of the solar system's orbital plane has long befuddled astronomers because of the way the planets formed: as a spinning cloud slowly collapsing first into a disk and then into objects orbiting a central star.

Planet Nine's angular momentum is having an out-sized impact on the solar system based on its location and size.

A planet's angular momentum equals the mass of an object multiplied by its distance from the Sun, and corresponds with the force that the planet exerts on the overall system's spin.

Since the other planets in the solar system all exist along a flat plane, their angular momentum works to keep the whole disk spinning smoothly.

Planet Nine's unusual orbit, however, adds a multi-billion-year wobble to that system. Mathematically, given the hypothesised size and distance of Planet Nine, a six-degree tilt fits perfectly, Brown said.

The finding was published in the Astrophysical Journal.