Solar Obliquity Induced by Planet Nine

TL;DR

This paper proposes that the hypothesized Planet Nine could explain the Sun's 6-degree obliquity and the solar system's spin-orbit misalignment through secular gravitational interactions, providing a testable hypothesis.

Contribution

It introduces an analytic model showing Planet Nine's potential to generate the Sun's obliquity and spin axis orientation from an initially aligned state.

Findings

Planet Nine's parameters can produce the observed solar obliquity.

The model explains the Sun's spin axis pole position.

Results support Planet Nine as a cause of solar system misalignment.

Abstract

The six-degree obliquity of the sun suggests that either an asymmetry was present in the solar system's formation environment, or an external torque has misaligned the angular momentum vectors of the sun and the planets. However, the exact origin of this obliquity remains an open question. Batygin & Brown (2016) have recently shown that the physical alignment of distant Kuiper Belt orbits can be explained by a 5-20 Earth-mass planet on a distant, eccentric, and inclined orbit, with an approximate perihelion distance of ~250 AU. Using an analytic model for secular interactions between Planet Nine and the remaining giant planets, here we show that a planet with similar parameters can naturally generate the observed obliquity as well as the specific pole position of the sun's spin axis, from a nearly aligned initial state. Thus, Planet Nine offers a testable explanation for the otherwise mysterious spin-orbit misalignment of the solar system.