The major planets of our Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) all orbit within a single plain, sometimes called the Plain of the Solar System, with none of the planets varying from this by more than a degree. This is consistent with the theory that the Sun and Planets formed from an initial disk of swirling matter within a larger nebulae. However the Sun itself is angled to this plain, rotating on a plane of axis six degrees out from that of the Plane of the Solar System. This is insignificant compared to some of the highly tilted orbits seen in exoplanets, with some known planetary systems hosting planets with orbits that vary by tens of degrees, but does require an explanation if our model of how the Solar System (and other stellar systems) formed is correct.
Several theories have previously been proposed to explain this obliquity. The young Sun could have been tilted from the plane by the activity of the stellar magnetosphere and its interactions with the protoplanetary disk, or the Sun could have started out with an asymetric distribution of mass in its core, leading to it acquiring an axial tilt. Alternatively the disc could have been tilted by a close encounter with another star or even a dense molecular cloud early in its life, or the Sun could have started out with a companion star that has subsequently been lost. However while all of these theories work on paper, there is no real way of either proving or disproving any of them at this time, which leaves us no closer to finding a solution to the mystery.
On 20 January 2016 Konstantin Batygin and Michael Brown of the Division of Geological and Planetary Sciences at the California Institute of Technology published a paper in The Astronomical Journal which suggested the observed clustering of objects in the Kuiper Belt might be attributable to the shepherding actions of a previously unknown planet with a mass 5-20 times that of the Earth and a highly eccentric orbit with an average distance from the Sun of 250 AU (i.e. 250 times the average distance between the Earth and the Sun).
Top view of the Keplerian trajectories of all bodies with a > 250AU as well as dynamically stable objects with a > 150AU. The two thin purple orbits correspond to stable bodies within the 150 < a < 250AU range. For each object, the directions of the angular momentum and Runge-Lenz (eccentricity) vectors are additionally shown. Batygin & Brown (2016).
In a paper published on the online arXiv database at Cornell University Library on 14 July 2016, Elizabeth Bailey of the Division of Geological and Planetary Sciences at the California Institute of Technology, along with Konstantin Batygin and Michael Brown, discus the posibility that such a hypothetical planet, refered to as Planet Nine, might be responsible for the obliquity of the Solar System.
It theory a large enough body in the Kuiper belt could exert sufficiant gravitational pull on the planets to alter their orbits, yet the planets would remain in the same plane of orbit as they gravitational pull they exert upon oneanother is greater than that exerted upon them by Planet 9. Importantly, unlike the other theories currently available, the existance of Planet nine, plus its size and orbit are potentially testable (since if such a planet exists it must be possible to directly observe it, even if this is not possible with current technology).
To this end Bailey et al. constructed a model of the Solar System, imputting the known parameters of the discovered planets, and assuming that they started orbitting in the same plane as the Solar Equator, plus a variety of different Planet 9s, in different orbits and having masses equivalent to 10, 15 ans 20 times that of the Earth.
Bailey et al. found that a planet of any of the sizes modelled could potentially have caused the current obliquity of the Solar System over the 4.5 billion years of its existance, but (importantly) in order to do this it would need to be in an orbit much closer to that of the Plane of the Solar System then is predicted for the current orbit of Planet 9, based upon its influence on bodies in the Kuiper Belt. However this does not preclude the possibility that such a planer could be responsible for the obliquity, as it's orbit would also be effected by the gravitational interplay with the other planets of the Solar System (which are expected to have a much greater mass), potentially pushing it into a new orbit.
See also...
Hubble Space Telescope discovers a moon orbiting the Dwarf Planet Makemake. Makemake is the largest known body in the Kuiper Belt and the
second-brightest Trans-Neptunian Object (after Pluto). It has a very
bright albedo (it reflects a lot of light), with a surface apparently
covered by...
Seasonal exospheres detected on Dione and Rhea. Exospheres are gas envelopes around planets and moons too thin to be
considered true atmospheres. Unlike atmospheres, the gasses in
exospheres are not thought to interact with one another, rather
individual molecules are thought to be lost from the body's surface,
spend some time in the exosphere, then either be lost into space or
resettle onto the surface...
Sputnik Planum: An apparently young feature on the surface of Pluto. Pluto was the first known
and is one of the largest bodies in the Kuiper Belt, a field of Dwarf
Planets and smaller bodies beyond Neptune which are thought to have
been beyond the zone of true-planet formation, and therefore to
reflect the nature of the early Solar System. In July 2015 the New Horizons spacecraft flew past Pluto, making a detailed survey of the
surface...
It theory a large enough body in the Kuiper belt could exert sufficiant gravitational pull on the planets to alter their orbits, yet the planets would remain in the same plane of orbit as they gravitational pull they exert upon oneanother is greater than that exerted upon them by Planet 9. Importantly, unlike the other theories currently available, the existance of Planet nine, plus its size and orbit are potentially testable (since if such a planet exists it must be possible to directly observe it, even if this is not possible with current technology).
To this end Bailey et al. constructed a model of the Solar System, imputting the known parameters of the discovered planets, and assuming that they started orbitting in the same plane as the Solar Equator, plus a variety of different Planet 9s, in different orbits and having masses equivalent to 10, 15 ans 20 times that of the Earth.
Parameters of Planet Nine required to excite a spin-orbit misalignment of i = 6 deg over the lifetime of the solar system, from an initially aligned state. Contours in a9-e9 space denote i9, required to match the present-day solar obliquity. Contour labels are quoted in degrees. The left, middle, and right panels correspond to m9 = 10; 15, and 20mEarth respectively. Due to independent constraints stemming from the dynamical state of the distant Kuiper belt, only orbits that fall in the 150 < q9 < 350AU range are considered. The portion of parameter space where a solar obliquity of i = 6 deg cannot be attained are obscured with a light-brown shade. Bailey et al. (2016).
Bailey et al. found that a planet of any of the sizes modelled could potentially have caused the current obliquity of the Solar System over the 4.5 billion years of its existance, but (importantly) in order to do this it would need to be in an orbit much closer to that of the Plane of the Solar System then is predicted for the current orbit of Planet 9, based upon its influence on bodies in the Kuiper Belt. However this does not preclude the possibility that such a planer could be responsible for the obliquity, as it's orbit would also be effected by the gravitational interplay with the other planets of the Solar System (which are expected to have a much greater mass), potentially pushing it into a new orbit.
See also...
Hubble Space Telescope discovers a moon orbiting the Dwarf Planet Makemake. Makemake is the largest known body in the Kuiper Belt and the
second-brightest Trans-Neptunian Object (after Pluto). It has a very
bright albedo (it reflects a lot of light), with a surface apparently
covered by...Seasonal exospheres detected on Dione and Rhea. Exospheres are gas envelopes around planets and moons too thin to be
considered true atmospheres. Unlike atmospheres, the gasses in
exospheres are not thought to interact with one another, rather
individual molecules are thought to be lost from the body's surface,
spend some time in the exosphere, then either be lost into space or
resettle onto the surface...Sputnik Planum: An apparently young feature on the surface of Pluto. Pluto was the first known
and is one of the largest bodies in the Kuiper Belt, a field of Dwarf
Planets and smaller bodies beyond Neptune which are thought to have
been beyond the zone of true-planet formation, and therefore to
reflect the nature of the early Solar System. In July 2015 the New Horizons spacecraft flew past Pluto, making a detailed survey of the
surface...
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