How to Move an Entire Gas Giant Planet
One of the advantages of working in academia is that there are often lectures on interesting topics. (Those of my friends who went drinking last night before heading out to the VNV Nation concert last night may not share my assessment, though.) Yesterday, I went to a talk by Doug Hamilton about the axial tilt of the planets, and in particular about Saturn’s 20-some degree tilt.
Mercury and Venus have spin axes pretty much perpendicular to their orbital plane, because that’s a side effect of their proximity to the sun, which robs them of both spin and axial tilt through tidal forces. Jupiter also spins pretty much straight up, which is what you’d expect from a planet that coalesced from the gas cloud that formed our solar system.
Earth most likely got tilted when it was smacked by a big-ass rock in the event that formed our moon. Pluto too, perhaps. You can even imagine the same thing happening to Uranus, if you could find a big enough rock. And Mars is tugged on by all sorts of bodies, including the sun and Jupiter, so that its axial tilt varies chaotically.
But Saturn is tilted about 27°, and it’s a gas giant, which means that it’s frickin’ massive, and if the “got smacked by a big-ass rock” was implausible for Uranus, then all the more so for Saturn.
What Hamilton found, if I understood correctly, was that the Saturn’s spin precesses and makes a full circle every 1.8 million years or so. And as it turns out, this is almost exactly the same as the rate at which Neptune’s orbit precesses. In other words, the solar system happened to form in such a way that Neptune was well positioned to keep tugging on Saturn in a consistent way, so that billions of years’ worth of tugs added up to the tilt we see today.
Okay, so far I’ll admit it sounds a bit like numerology: finding similar numbers in the data, and cooking up an explanation after the fact. But he says he’s run simulations and found that this happens for a fairly wide range of starting conditions. So while I’m not 100% convinced, I don’t think the guy’s a kook.
During and after his talk, we talked about the origin of Saturn’s rings. He says it’s not clear where they came from: they could have formed at the same time as the solar system &emdash; i.e., they’ve always been there &emdash; or perhaps a moon got pulled in and reduced to dust by tidal forces. In fact, this may have happened several times; the rings we see now might be less than 100 million years old.
(Dunno about Jupiter, though I hear Saturn keeps teasing Jupiter about them.)