Has the Mystery of Uranus' Tilt Been Solved?

Evolution of Uranus’ spin-axis caused by a large moon (Gwenael Boue/Jacques Laskar)

One of the most enduring mysteries of the Solar System may be a step closer to being solved.

It is well known that Uranus is an oddball, orbiting around the sun on its side, but little is known about how the huge planet came to be this way.

Although the general consensus is that Uranus was involved in some kind of cosmic hit-and-run, two researchers from Paris think the gas giant may have gradually wobbled over millions of years, eventually tipping due to the presence of a large moon.

Usually the planets orbit the sun upright, with the axis of rotation perpendicular to the solar system’s plane (i.e. in relation to Earth, pointing “north”). That is, apart from Venus and Uranus. Venus, however, is a more extreme case where the entire planet was turned upside down causing it to rotate in an opposite fashion to Earth.

Uranus is tilted 97 degrees to the vertical. The Earth’s tilt is a little over 23 degrees, and it is this tilt that gives our planet seasons. Needless to say, the seasons on Uranus are a little more extreme than ours; each Uranian hemisphere experiences 42 years of continuous sunlight (a year on Uranus is 84 Earth years).

This is all very interesting, but how did Uranus come to be this way? After all, the planet is really big (14.5 times the mass of Earth), it would take some kind of cataclysmic event to knock it on its side (it is impossible for the planet to be “born” this way, it should have an upright axis like all the other planets).

Generally it is assumed that another planet must have collided with Uranus, pushing it off-kilter, but new computer models suggest a scenario that is far more elegant.

Gwenaël Boué and Jacques Laskar from the Paris Observatory in France started out with the idea that Uranus may have once had a very large moon, approximately one percent of the gas giant’s mass. Through the gravitational “tugging” by the large moon’s mass, over the course of 2 million years Uranus may have wobbled to such an extent that it was pulled onto its side.

However, the researchers admit that such a large moon may not be plausible as current satellite formation models don’t allow moons of this size. As indicated in their unpublished paper’s conclusions, a smaller satellite of only 0.1 percent the mass of Uranus may be sufficient to pull the planet on its side over a longer period.

But what happened to this moon? It is not uncommon that planets disrupt (or even steal) other planet’s moons, so the gravitational influences of the other massive gas giants may be a factor. For example, Neptune’s large moon Triton is thought to have been “kidnapped” from the Kuiper Belt as it has a retrograde orbit (i.e. it orbits the “wrong way” when compared with the other Neptunian moons) and it has a similar composition to the dwarf planet Pluto.

Perhaps a massive moon orbited Uranus long enough to wobble the planet onto its side, only for the satellite itself to be kicked out of orbit by a passing planet.

Although the jury is likely to be out for the foreseeable future as to the mechanism that knocked Uranus onto its side, this collisionless scenario is a fresh look at an old mystery that will continue to fox astronomers for some time yet.

Publication: “A Collisionless Scenario for Uranus Tilting,” Gwenael Boue and Jacques Laskar, 2009, arXiv:0912.0181v1 [astro-ph.EP]

Source: New Scientist.

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