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 - 20m+ planet on a distant, eccentric, and inclined orbit, with an approximate perihelion distance of ~ 250AU. 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 speciffic 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.
The axis of rotation of the sun is o set by six degrees from the invariable plane of the solar system (Souami & Souchay 2012). In contrast, planetary orbits have an RMS inclination slightly smaller than one degree1, rendering the solar obliquity a considerable outlier. The origin of this misalignment between the sun's rotation axis and the angular momentum vector of the solar system has been recognized as a longstanding question (Kuiper 1951; Tremaine 1991; Heller 1993), and remains elusive to this day.
With the advent of extensive exoplanetary observations, it has become apparent that signifficant spin-orbit misalignments are common, at least among transiting systems for which the stellar obliquity can be determined using the Rossiter-McLaughlin effect (Rossiter 1924; McLaughlin 1924). Numerous such observations of planetary systems hosting hot Jupiters have revealed spinorbit misalignments spanning tens of degrees (Hebrard et al. 2008; Winn et al. 2010; Albrecht et al. 2012), even including observations of retrograde planets (Narita et al. 2009; Winn et al. 2009; Bayliss et al. 2010; Winn et al. 2011). Thus, when viewed in the extrasolar context, the solar system seems hardly misaligned. However, within the framework of the nebular hypothesis, the expectation for the offset between the angular momentum vectors of the planets and sun is to be negligible, unless a speciffic physical mechanism induces a misalignment. Furthermore, the significance of the solar obliquity is supported by the contrasting relative coplanarity of the planets.
Because there is no directly observed stellar companion to the sun (or any other known gravitational influence capable of providing an external torque on the solar system sufficient to produce a six-degree misalignment over its multi-billion-year lifetime Heller 1993), virtually all explanations for the solar obliquity thus far have invoked mechanisms inherent to the nebular stage of evolution. (...)
Yves Herbo, Sciences et Fictions et Histoires, http://herboyves.blogspot.com/, 21-07-2016, 25-07-2016