While in astronomy class my teacher mentioned the solar system was a plane. Ofcourse I knew this, that the planets are all on the same field (exept for pluto), and I began to wonder why this was. Shuoldn’t planets also be orbitting vertically, and diagonally, and all screwball, instead of a neat little plane? Explain.
maybe the sun’s magnetic field kind of balances all the planets between the north and south poles…
this is assuming the sun has north and south poles… i wanna know this too! bump.
When the initial ball of gas that formed the solar system began to condense, things did move around in all sorts of screwball orbits. They had to because the cloud was spherical, right ? The trouble is, all those differently angled orbits intersected each other, or came close enough for gravitational interactions to throw the orbiting bodies into different paths. Those new orbits were also subject to large perturbations, and the process continued. The exception to this continual perturbation and change of trajectory is for objects moving in a single disk in roughly circular orbits around the central star. The objects in such a disk don’t have close encounters with each other. Any object passing through this plane, such as a body orbiting at 45° to the ecliptic, would periodically suffer changes to its orbit caused by interaction with the mass in the protoplanetary disk.
The short answer is that the sun, planets, moons, asteroids, etc all formed from the same blob of intersteller stuff. This stuff had some rotation, so as it collapsed due to gravity, it formed a pancake shape. So when the planets started actually forming, they were already in a fairly neat little plane.
The last I heard, Pluto and Charon were moons that got flung out of orbit around Neptune (and out of the ecliptic plane) by some passing asteroid/comet/UFO.
Here’s a pretty good explanation.
And it’s basically the same reason that rings around planets are in a plane.
Its due to angular momentum, which must be conserved. Hence as time goes by, the rotation of the planets forces the entire solar system into the pancake shape.
its too big to fit in a truck
Think about spinning a ball of dough in the air, and how it flattens out into a disk. Gravitational attraction pulls the objects together, and angular momentum pulls them outward. The end result is a flat disk.
I think I’ve got it!!!
THE PIZZA PARLOR!. Big blob of mass, spin it, end up with nice flat plane.
Makes sense to me.
Wait a minute, people…let’s not be sloppy about this! The truth is the eight planets (sans Pluto) lay in ABOUT the same plane. But, this is a generality! The truth is, each planet’s orbit has a slight tilt (some more than others) to the plane containing the imaginary line between sun-earth centers. This plane IS the ecliptic. Even the moon’s orbit has a slight tilt to the plane of the ecliptic.
Also, it is NOT true that the rings of the ringed planets lay in the same plane as the ecliptic. In fact, Saturn’s rings (for one) have quite a pronounced tilt to Saturn’s orbit.
I will look up the actual angles, if you wish.
- Jinx, amateur astronomer
I don’t think that conservation of momentum alone is sufficient to account for the formation of an ecliptic plane. You also need to take into account the topological constraints on sheaves of randomly oriented circular orbits. Something akin to the Hairy Ball theorem is likely operating here.
Squink got it right in his first post. Conservation of momentum and centrifugal force (pizza dough analogy) do not explain why the cloud flattens out. If it weren’t for collisions, particles are perfectly happy to stay in randomly oriented orbits.
In case of Saturn’s rings, there is an additional contribution from the non-spherical shape of the planet. Since Saturn bulges out at the equator, you can’t treat it as a point mass.
What is the additional contribution? Saturn’s rings would still be in a plane if Saturn were not spinning at all. Wouldn’t it?
And Jinx, I didn’t mean to imply that the rings around planets are in the same plane that the planets orbit the sun. I meant that rings are in a plane for the same reason that the planets’ orbits are in a plane - you start out with a bunch of random orientations, and because of the collisions with each other, it ends up as a disk.
I agree with scr4 that the pizza dough analogy doesn’t work. Pizza dough is pulled into a disk because it’s sticky.
Do all of Jupiters moons (what are there, 14 or something?) and rings orbit in roughly the same plane? If not, why not, given the discussion above?
I’m not perfectly clear myself, but my understanding that it may have formed a plane at a different angle (i.e. not perpendicular to the planet’s axis of rotation) if it weren’t for the non-spherical shape of the planet.
Planetary rings lie in in the planet’s equatorial plane, as do the moons of most planets. In the case of moons, this is due to tidal friction. In the case of the rings, it is indeed due to the oblateness oblateness of the planet.
Planets are oblate due to their rotation–they bulge out at the equator and are flattened at the poles. (Think of the pizza example again.) Any ring particle not in an equatorial orbit will have its orbit changed slightly by the gravity of the equatorial bulge. A change in its orbit which will tend to cause it to crash into another ring particle on short order. The end result of all these collisions leads to a flat ring in the equatorial plane, where the particles are no longer perturbed by the oblateness of the planet.
For a perfectly spherical planet, collisions will just create a ring that has whatever orientation is dictated by the initial angular momentum of the ring particles. However, such a beast don’t exist in nature.
For moons it’s a little more complicated. The difference in the strength of the planet’s gravity between the near and far side of the moon distorts the shape of the moon. The moon wants to rotate so that it always keeps one face toward the planet (the way the Earth’s moon does). So far, so good. However, if the moon is on an orbit that is inclined (tilted relative to the planet’s equator) then the gravity of the equatorial bulge of the planet will constantly be trying to change its orientation, trying to pull the moon’s face southward when it’s north of the bluge, or northward when it’s south of the bulge. Friction within the moon will dissipate energy, decreasing the inclination of the moon’s orbit, until it lies in the equatorial plane. (That’s a little bit vague, I know, but I’m not sure if I should delve into the exact mechanism of tidal dissipation before I’ve had any caffeine.)
Oh, hell, I screwed up. You don’t need an equatorial bulge to get rid of the inclination of the moon. The distorted shape of the moon is enough–the planet’s gravity will always try to pull the moon so that its long axis is lined up with the center of the planet. No oblateness necessary.
This is what I get for PBT (Posting Before Tea). And no, I’m not going to try to tackle how friction within the moon decreases the inclination until after I’ve had a hearty, nutrituous lunch. If you can’t wait until then: blah blah blah rotational angular momentum, blah orbital angular momentum, blah blah.
Am I clear …?
So, the Sun also bulges according to its own spin. It spins at a certain rate, drawing a certain amount of mass outward due to centrifugal force, giving it a “middle”.
This middle will have more gravity that the rest of the Sun, so throughout the solar system, a wave of gravity on that particular eliptic will be strongest, pulling all orbital bodies closer to that eliptic disk.
Bodies rotating on a perpendicular eliptic are possible, but would (actually have in the past) collide with the greater number of bodies at the strong eliptic.
Therefore, after all these billions of years, bodies running on the perpendicular eliptics were destroyed and have assumed positions along the strong eliptic - home of the greatest gavetic plane.
Am I right?
You’re about halfway right, Gorgon.
The oblateness of the Sun has nothing to do with it, so just forget that.
A collapsing, spinning cloud, due to collisions and conservation of angular momentum, will settle into a disk.
Think of it this way: most (though not all) of the particles are rotating in a vaguely counterclockwise direction. Some are orbiting perpendicular to the plane, some are orbiting in the plane, and some are inbetween. If two particles crash into each other, it’s likely that one of them will be in the up-going part of its orbit, and the other will be in the down-going part, so their up-and-down motion cancels out–but they are both going counterclockwise, so in the end, both particles end up in a less inclined motion, but still going counterclockwise.
Obviously this isn’t the case in every collision, but when you look at the average of bajillions of collisions between bajillions of particles, on average, that’s how it all works out, so after a while (a few hundred thousand years, IIRC) most particles are no longer on inclined orbits–they’re all orbiting instead in a disk in the plane of the Solar System.
As it turns out, this disk is roughly in the equatorial plane of the Sun, but that’s mostly because the Sun and the rest of the Solar System formed from the same spinning cloud, not because the Sun’s orientation dictated the way the whole disk formed.
The following is about rings, so if you just care about the Solar System as a whole, you can stop reading here. If you find rings endlessly fascinating–as I do–by all means read on:
The whole oblateness thing just explains why a ring aroud a planet–which did not form from the same spinning could of gas as the planet–ends up in the planet’s equatorial plane. And it’s not because the stuff is extra-attracted toward the equatorial bulge–you might think it’s the case but, as a rule, orbital mechanics defies all common sense.
What happens is that, for an object on an inclined orbit, the equatorial bulge perturbs the orbit in some subtle ways. So if you somehow made a perfectly flat ring, what would happen is that the particles in the ring would get perturbed, leading to a not-perfectly-flat ring, leading to lots of collisions between ring particles. Any ring particle that ends up in a nice equatorial orbit, though, does not recieve any perturbations from the equatorial bulge, so you can start accumulating ring particles in nice stable orbits there. Non-equatorial particles keep getting perturbed, and therefore have collisions–oftentimes with the equatorial particles, actually, because an inclined particle has to pass through the equatorial plane twice per orbit!
While an individual collision might end up knocking some equatorial particles into inclined orbits, again we have to think about the whole average picture, where up-goers tend to collide with down-goers, and cancel their up-and-down motion out. Pretty quickly (just thousands of years, actually, no matter what crazy distribution of ring particles you start out with) all the particles end up in the equatorial plane.
I hope that clarifies things. (And again, I apologize for my oblateness & tidal dissipation screw-up. I hope that didn’t confuse anyone.)
Hm. Thank you, Podkayne. I’m in a rish at the moment and the terminology isn’t all that clear, it does make sense. I’ll be sure to sit down later and give this the pondering it deserves.