Planet Nine and the Obliquity of the Solar System

Recently, a mysterious planet Nine was hypothesized to linger about in the outskirts of our solar system. Even with our most sophisticated equipment, we had largely been oblivious to its presence. But, deep theoretical evidence based upon the gravitational perturbations caused in the outer regions of the solar system that confirm predictions and account for the unexplained eccentric movements of the Kuiper Belt Objects have accorded much traction to this particularly thought-altering idea, the likes of which have not always held water in the history of astronomy. It is a world that, albeit larger than the Earth and indeed much larger than Pluto, remains quite dark, drowned into inconspicuity by the much brighter lights surrounding it. Its closest approach to the sun is 200 Astronomical Units, whereupon it moves quite fast in its perihelion, making its detection quite difficult indeed.

Simulations showed that Planet Nine has an orbit that is anti-aligned to the six most distant Kuiper Belt Objects. Indeed, the odd orbits of these objects tend to lie in the same plane, all tilted 20 degrees downwards relative to the other planets of the solar system. The Kuiper Belt Objects are consequently locked in mean motion resonances with Planet Nine, wherewith they avoid close encounters with themselves and Planet Nine and by which they are also ultimately accorded more elliptic orbits.

Hypothesized Planet Nine, conferring orbital eccentricities to Kuiper Belt Objects and distant objects in the solar system. Credit: Caltech

Such a model is also able to account for the eccentric behaviour of a particularly intriguing member of our solar system, Sedna. The very eccentric orbit of Sedna does not even position it anywhere near as close to Neptune, suggesting an external influence. Previous hypotheses have posited that Sedna’s unusual orbit is the result of gravitational perturbations due to a companion star to the Sun. But, this characteristic orbit seems to also be exhibited by other objects too, in particular a trans-Neptunian object by the name of 2012 VP113. Both Sedna and 2012 VP113 have a perihelion greater than 50 Astronomical Units and a semi-major axis of 150 Astronomical Units. Further still, very recently, a dwarf planet by the name of V774104 was also discovered, lying a hundred times farther from the sun than Earth and which could very well join this camp when data about its orbit arises. Planet Nine thus offers a good model to explain such convergent behaviours.

Objects (blue) orbit perpendicular to the plane of the solar system, a predicted consequence of Planet Nine. Credit: Caltech

More recently still, Planet Nine has been implicated as possible reason for the unexplained wobble of the solar system. A very remarkable feature of the sun is that it its spin axis is tilted 7 degrees relative to the angular momentum vector of the solar system. Indeed, the likelihood that two vectors chosen at random would be tilted likewise is about 0.003 and thus this is very small to be attributed to chance alone. It is very likely that such a tilt is consequent of Planet Nine’s perturbing the plane of planetary orbits, such that the sun appears to be slightly tilted.

The sun is tilted 7 degrees to the ecliptic, a possible consequence of Planet Nine. Credit: Caltech

It seems that our own cosmic backyard itself is richer than we ever thought. But, such an elusive planet could not escape the power of our theoretical predictions which have almost certainly pinned it down as a possibility. Direct evidence of such a planet would shed some light upon the many mysteries which we have so far utterly failed in all of our endeavours to answer.

Featured image: Caltech. Artist’s impression of Planet Nine.


Bailey, E., Batygin, K., and Brown, M. E. 2016. Solar Obliquity Induced by Planet Nine. arXiv: 1607.03963

Batygin, K. and Brown, M. E. 2016. Evidence for a Distant Giant Planet in the Solar System. The Astronomical Journal. 151 (2): 22.



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