Does Planet Nine Solve the Riddle of the Sun’s Obliquity?

The sizeable tilt of the proposed Planet Nine body could explain other unexplained features of the solar system as well as the observed clustering of extended scattered disk object beyond the Kuiper Belt.  The Caltech astrophysics team who introduced the world to Planet Nine in January (1) think it may also explain the Sun’s six degree tilt with respect to the plane of the ecliptic (2).  In addition, the presence of a distant, sizeable Planet X object, whose closest approach to the Sun is argued to be 250 Astronomical Units away (3), could be affecting the tilt of the entire planetary system orbiting the Sun.

“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.” (3)

The key to this quite dramatic effect is the high inclination of Planet Nine’s projected orbital plane with respect to the rest of the solar system, rather than the object’s inherent mass.  This point was made by Alessandro Morbidelli (2) who, along with his colleagues, has also published similar work at the same time as the Caltech team.  Their work goes further, providing a series of scenarios for the orbit of Planet Nine, and thus offering further constraints upon its present position:

“Under some specific configurations for planet 9, this precession [of plane of the planets relative to the total angular momentum vector of the Solar System] can explain the current tilt of approximately 6 degrees between the invariant plane of the giant planets and the solar equator. …Some of the planet 9 configurations that allow explaining the current solar tilt are compatible with those proposed to explain the orbital confinement of the most distant Kuiper belt objects. Thus, this work on the one hand gives an elegant explanation for the current tilt between the invariant plane of the inner giant planets and the solar equator and, on the other hand, adds new constraints to the orbital elements of planet 9.” (4)

An Artist's impression of Planet Nine. Image credit: Caltech/R. Hurt (IPAC)

An Artist’s impression of Planet Nine. Image credit: Caltech/R. Hurt (IPAC)

Over very long time periods, the computer simulations independently deployed by both team shows that Planet Nine might generate the observed solar system anomalies naturally.  Significantly, this would imply that Planet Nine would have to have taken its place within the ranks of the planets quite early on in the lifetime of the solar system.  This, in turn, implies that its orbit is a stable feature of the solar system, rather than the result of a relatively recent capture of an exoplanet.

Of course, this is all conjecture based in turn upon hypothesis.  It should be pointed out that there are other possible explanations for the tilt of the rotational axis of the Sun, and the relative obliquity of the Sun’s equatorial plane compared to the invariant plane of the ecliptic (2).   But the existence of a Planet X object at a high inclination could readily explain these effects, as well as the anomalies of the extended scattered disk beyond the Edgeworth-Kuiper Belt.

Written by Andy Lloyd, 30th July 2016


1  K. Batygin & M. Brown “Evidence for a Distant Giant Planet in the Solar System” 20th January 2016, The Astronomical Journal, Volume 151, Number 2,

2)  Rebecca Boyle “Planet Nine may have tilted entire solar system except the sun” 19th July 2016 with thanks to Mark

3)  Elizabeth Bailey, Konstantin Batygin & Michael Brown “Solar Obliquity Induced by Planet Nine” 20th July 2016

4)  Rodney Gomes, Rogerio Deienno & Alessandro Morbidelli “The inclination of the planetary system relative to the solar equator may be explained by the presence of Planet 9” 18 Jul 2016

5)  Shannon Hall “Planet Nine might be an exoplanet stolen by the sun” 4th April 2016

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