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The Planetary Society Blog

By Emily Lakdawalla

How Uranus got its tilt

Apr. 28, 2006 | 16:41 PDT | 23:41 UTC

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Uranus in the infrared
Uranus as seen in 2004 through the Keck II Telescope. It is well known for being tipped on its side -- with its rings and moons.

Here's the other story I picked up from today's magazine reading. Argentinian scientist Adrián Brunini published a paper in Nature proposing a novel mechanism by which the planet Uranus could have acquired the nearly horizontal tilt of its rotation axis. For the inner planets, it is pretty well accepted that major impacts in which very large planetesimals struck glancing blows to the still-forming planets could have produced the currently observed axial tilts, including the nearly upside-down tilt of Venus. (You could also call it Venus' backwards rotation. Either way, something big -- or several big somethings -- had to happen to make Venus rotate the opposite way from the "prograde" direction of rotation of the bodies in the solar system.) This kind of impact scenario also conveniently explains the origin of Earth's moon. So people have generally assumed that the outer planets acquired their tilts in similar ways. Jupiter is nearly upright at 3 degrees, but Saturn, Uranus, and Neptune currently have axial tilts of 27, 98, and 28 degrees, respectively. The problem is that the outer planets are far more massive than the inner planets, so you need really big bodies -- Earth-sized -- to do the impacting and produce the observed tilts, especially for Uranus. But most solar system formation models just can't produce spare planetesimals that big in the outer solar system. What's worse, it's been hard to imagine how you can form a planet and a whole swarm of satellites and then have an impact to the planet tip over both the planet and its whole satellite system. It can be made to work, but a lot of stuff has to happen very quickly, and it has been hard to write down mathematical models that naturally produce systems that look like this.

That's where Brunini's paper comes in. He says you don't need impacts to make the giant planets tilt. "The present obliquities of the giant planets were probably achieved when Jupiter and Saturn crossed the 1:2 orbital resonance," he proposes. What that means is that as they were forming and evolving, Jupiter and Saturn's orbits were shifting around a bit, and they wound up in orbital positions where Jupiter went around the Sun exactly twice for every one time that Saturn did. This had direct effects on smaller Saturn but it also made Saturn's and Jupiter's gravity work together to make changes in the orbits of other bodies in the solar system, namely Uranus and Neptune. There is a graph in the paper that shows how the orbits evolve over time in one of the 30 simulations he ran. Jupiter and Saturn start out at roughly 5 and 8 astronomical units; Uranus and Neptune begin much closer to the Sun than their current positions, at about 13 and 14 AU. They stay pretty comfortably in those positions for about 100,000 years. Then, quite suddenly, that 1:2 resonance is reached. Saturn and Jupiter don't change a lot initially, but the orbits of Uranus and Neptune go nuts. They get much more eccentric, so that their orbits cross; at times Uranus even gets very close to Saturn. After about a million years, the eccentricity dies down, and Uranus and Neptune are on their way out to more distant positions in the solar system, at the same time that Saturn begins to acquire its present orbit eccentricity.

During all of this orbital dancing, Brunini says, the planets exchange a great deal of angular momentum. In particular, the very close approaches of Uranus to Saturn causes them to exchange momentum, which, over time, changes their axial tilts. Now, this process is relatively fast, happening over a few hundred thousand years, but it is much slower than a single humongous impact tipping over a planet. The slower pace of the process that Brunini proposes means that as the planets slowly tilt, their satellite systems can actually follow the change in tilt. (All of these planets are fatter at the equator than at the poles, a geometry that tends to make their satellites' orbits flatten out into their equatorial planes over time.)

Mathematical models can never be considered proof of how something happened in the distant past. However, it does make scientists feel better to be able to show that a proposed explanation could work. And good models can generate testable hypotheses that you can use to design new experiments or observations that can test whether the models hold true for data that postdates the development of the models. However, nature is always more complicated than your model, so you will always find some data that the oversimplified model just can't explain. At that point, you scratch your head, and begin looking for a new model that might explain the observations better. The tilt of Uranus' satellite system was a clue that the accepted model might not be the perfect one; Brunini has done that model one better. At some point, someone else will improve on Brunini, and the process will go on.

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