Planetary orbits

Ok this has been bugging me fer a couple days now and if the almighty Cecil has written about this already please honor me with a link. At any rate I was thinking about the planets the other day, don’t ask why, and it occured to me that whenever I see the solar system diagramed all of the planets are orbiting on pretty much the same plane. Why is that? Why aren’t any of them orbiting on a plane perpendicular to the others? Also all of the planets’ moons and or rings seem to orbit on pretty much the same plane except Uranus which IIRC is different because it’s undergoing some sort of flip which the apocolyptic types think is gonna happen here but thats another thread. Help me out if y’all can, thanks.

Well, I don’t know nearly as much physics as I’d like to, but here’s the basic idea as I understand it; I’m sure someone will be along to correct any inaccuracies I have.

At its “birth”, the solar system was a “spinning ball of mass”, for lack of a better term. Imagine a line through the center of the ball that the ball spins around (the axis of rotation). Now imagine a plane perpendicular to that line at the center of the ball. The ball tends to flatten out along that plane due to inertia (“centrifugal force”), just as there is a slight bulge along the earth’s equator, for example. That plane became the orbital plane for the planets.

Just to add a bit to Cabbage’s answer (essentially correct): Pluto is an oddball, its orbit being tilted at approximately 30[sup]o[/sup] from the ecliptic (the orbital plane of the planets). Except for Terra/Luna, the major satellites and rings tend to be in the plane of their primary’s equator, not the ecliptic, which is typically tilted somewhat-- Luna is within 5[sup]o[/sup] of the ecliptic. Minor satellites, comets, and Kuiper belt objects can go any which way. In fact, the latter is the reason why Pluto is an oddball, as it’s actually a Kuiper belt object itself.

The ball of mass and the centrifugal force are certainly a part of the answer, and the rest of the answer is a whole lot more complicated than it looks at first glance. I recently got into this whole issue on the macrocosmic scale when I began doubting that the forces described were sufficient to flatten the entire galaxy during the forty or so revolutions which represents the age of the universe. For the solar system, of course, the number of revolutions is far larger, even if the system is only five or six billion years old.

The larger answer includes the fact that the structures of the universe seem to have developed from huge “sheets” of matter, rather than balls of matter. That fact arises from the geometric consequences of the “inflationary epoch” which created the non-uniform distribution of matter. While that sheeting is less a part of the creation of the solar system than of the galaxy, it does have repercussions, such as the fact that the solar ecliptic is not greatly skewed from the galactic ecliptic. (The direction of the center of the galaxy lies in the constellation Sagittarius, which is extends only a few degrees from the solar ecliptic.) The great accumulation of angular momentum of the galaxy is reflected in the general local angular momentum and the planets as well.

There are also complex interactions among freely orbiting particles in a spherical region, which tend to favor the exchange of momentum along the ecliptic at the expense of momentum away from it. This statistical trend flattens the system as it coalesces toward the center, and results in the planar orientation of the bodies that condense out of the cloud. Since the mathematics of this phenomenon are at the fringes of my understanding I shall refrain from trying to describe them. Any rocket scientists in the audience are invited to try.


Chronos says:

Pluto is an oddball, but not that odd. It’s orbital tilt is about 17[sup]o[/sup].

Triskadecamus says:

Sorry, the galactic plane is no where near the solar ecliptic. My astronomy books are all packed away, but I believe that these two are tilted at about 60[sup]o[/sup].

But the part about the galactic center being near the ecliptic is correct. But you must remember, all of these kinds of circles look to us like they intersect at two points. The galactic center being near one of these points is pure coincidence.

Hmmmmmm. You seem to be correct. That would imply that I was wrong. I hate it when that happens. I shall review the information available and see what it was I misunderstood. (do you recall if the solar system is at least prograde with respect to the rotation of the galaxy? That seem to be included in the same block of memory which has me confusing the two planes of orientation.)

I’m fairly sure they are prograde. At least I can’t recall anywhere that it says they are retrograde and that kind of factoid is one that’s likely to be mentioned.