Is a mysterious Planet Nine creating a slight tilt in the solar system?

Astronomers have known about a mysterious tilt in our solar system for decades, but that tilt has never been satisfactorily explained.

The solar system formed about 4.5 billion years ago, when a spinning cloud of material collapsed into a disk, and forming a star and the planets orbiting around it in a plane, like a spinning record. The disk is remarkably flat, with only slight wobbles in the orbit of each planet relative to the line of the disk around the sun.

But there's one problem. According to this dominant theory of the solar system's creation, the disk should be in line with the sun's equator. While some degree of variation from the flatness of the disk is to be expected, scientists realized that the solar system, on average, has around a 6 degree tilt relative to the solar equator. But now, new findings from researchers at Caltech suggest that the tilt may be the result of the mysterious and hitherto unobserved Planet Nine, possibly solving a longstanding astronomical puzzle as well as providing more compelling evidence of the planet's existence.

The existence of Planet Nine was proposed in January 2016 by astrophysicists Michael Brown and Konstantin Batygin of the California Institute of Technology, as a way to explain anomalous gravitational behavior at the edge of the solar system. They found that some of the movements of a number of objects in the outer regions of the system could be explained by the gravitational influence of an unobserved planet about 2 to 15 times more massive than Earth and much farther out than any other known planetary body. Since then, astronomers have been scouring the sky for further evidence of Planet Nine's existence, hoping to one day spot the planet directly.

Until then, evidence for the planet will be grounded in indirect gravitational measurements. This latest study, which will be released in the next issue of The Astrophysical Journal, calculates that Planet Nine has an unusual orbit, about 30 degrees off of the sun's equatorial plane. Over billions of years, that kind of oddity would have had a significant effect on the motion of planetary bodies.

"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 Elizabeth Bailey, a graduate student at Caltech and lead author of a study, in a statement from Caltech.

All orbiting planets have angular momentum. As it revolves around its star, it contributes force via angular momentum to the spinning disk of the system as a whole. Because Planet Nine has such a large mass and is so far from the sun, its unusual 30-degree wobble orbit is enough to exert considerable force on the rest of the solar system, creating a 6-degree deviation in what would have otherwise been a flat plane around the solar equator.

"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 Dr. Batygin in the statement.

Of course, this solution provides the researchers with a new mystery as well. If Planet Nine's 30-degree deviation from the center of the plane created the 6-degree wobble, how did Planet Nine's orbit come about?

It is impossible to say for sure at this point, especially since Planet Nine has not been directly observed. Batygin suggests that its unusual orbit might be explained if it had been ejected from a closer orbit to the sun by Jupiter, or perhaps it might have been affected by the gravitational pull of other objects in the early days of the solar system that are no longer there.

Bailey pointed out that while the study seems to support Planet Nine's influence, it is possible that the tilt could be explained by other phenomena, especially with alternative hypotheses about the formation of the system.

"However, all these other ways to explain why the solar system is tilted are really hard to test — they all invoke processes that were possibly present really early in the solar system," Bailey told Space.com. "Planet Nine is the first thing that has been proposed to tilt the solar system that doesn't depend on early conditions, so if we find Planet Nine, we will be able to see if it's the only thing responsible for the tilt, or if anything else may have played a role."