Big bully? Caltech/R. Hurt (IPAC)

A jealous Planet Nine may have shoved its siblings for attention. If a massive ninth planet exists in our solar system, it might explain why the planets are out of line with the sun.

The eight major planets still circle the sun in the original plane of their birth. The sun rotates on its own axis, but surprisingly, that spin is tilted: the axis lies at an angle of 6 degrees relative to a line perpendicular to the plane of the planets.

There are a few theories to explain this jaunty slant, including the temporary tug of a passing star aeons ago, or interactions between the magnetic fields of the sun and the primordial dusty disc that formed the solar system. But it is hard to account for why the sun’s spin is aligned the way it is relative to the planets.


Two teams of astronomers have just announced a new explanation: a hypothetical massive planet in the outer solar system could be interfering with all the other planets’ orbits.

Earlier this year, Michael Brown and Konstantin Batygin at the California Institute of Technology in Pasadena argued that this Planet Nine could be responsible for some of the erratic movements of icy worlds in the outer solar system. With that planet plugged in to our models, the machinations of the heavens begin to make more sense.

Now the idea can be extended to the orbit of all the planets, says Elizabeth Bailey, also at Caltech, who did the work together with Brown and Batygin.

“Because we think Planet Nine has a significant inclination, if it exists, then that means it would tilt things,” Bailey says, and by the just right amount. “It’s one puzzle piece that seems to fit together, and it really seems to be in support of the Planet Nine hypothesis.”

The planet would have between 5 and 20 times Earth’s mass and be in a wildly eccentric orbit, reaching 250 times the sun-Earth distance at its farthest point. That elongated trajectory has led some to suggest that it was once an exoplanet and was kidnapped by the sun.

If that happened early enough, then its gravitational influence since the solar system was born would be enough to pull the planets’ orbital plane out of alignment with the sun, Bailey says. Jupiter, Saturn, Uranus and Neptune would move as one, so Planet Nine would not be able to shift them individually like pinballs. Instead, the entire solar system would tilt as a whole.

Planet Nine’s tilt, not its mass, is key, says Alessandro Morbidelli at Côte d’Azur Observatory in Nice, France, who has independently come to a similar conclusion. If it were a question of mass, Jupiter would be the prime suspect.

“What is important is that the perturbing planet is off-plane. Jupiter cannot cause its own tilt,” he says.

The sun’s tilt doesn’t prove that Planet Nine exists, however. That would require seeing it with a telescope.

Journal reference: Bailey, Brown and Batygin: arxiv.org/abs/1607.03963v1; Morbidelli and colleagues: arxiv.org/abs/1607.05111