Did Planet Nine Give The Solar System Its Tilt?

Planet Nine (orange) could disturb not only the objects in the outer Solar System (purple) but also the eight planets' tilt. Caltech

A long-standing mystery in the Solar System might have been solved thanks to the help of an unlikely (and unconfirmed) object: Planet Nine.

Two different teams of astronomers, one from the US and one from Brazil and France, have independently discovered that the presence of a mysterious ninth planet beyond Neptune might have caused the orbit of the other eight planets to tilt with respect to the Sun.

The orbits of the eight planets we all know and love are not on the same plane. They have a small inclination compared to each other and they are tilted with respect to the Sun. But based on the interaction between the planets and the Sun, there shouldn’t be such an effect.

The discovery of other planetary systems has allowed us to understand our own Solar System better, and several explanations have been put forward to explain the tilt. A temporary interaction with a passing star might have given the planets the right tug, or maybe the slant was there right from the start as the magnetic field of the young Sun moved the protoplanetary disk that formed the planets.  

Those hypotheses are appealing but they lack a smoking gun, an undeniable proof to be the likely cause of the tilt. For this reason, the two teams were looking at alternative culprits, and the existence of a massive potential ninth planet might fit the bill.

Planet Nine has not been discovered yet, but it was proposed by Caltech’s Michael Brown and Konstantin Batygin to explain the complex orbits of objects in the outer Solar System. Now, with Caltech's Elizabeth Bailey, who led the study (available on arXiv), they have estimated the impact of Planet Nine on the orbit of objects in the inner Solar System.

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

Planet Nine is thought to be between five and 20 times the mass of Earth, and the large size of the icy planet could have created the tilt in the early Solar System. In their paper, the researchers show that the induced tilt from the simulation is consistent with the value that has been measured.

Independently, researchers from Brazil and France have come to the same conclusion, although the driving mechanism is not Planet Nine’s mass but the tilt of its orbit.

Planet Nine can help explain a lot of curious phenomena in the Solar System, but until somebody sees it with a telescope, it remains just a great hypothesis.

[H/T: New Scientist]

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