I've always wondered....

[Hunter Parasite]

I was just wondering, you know how when planets orbit a star, why are they always horizantal? Why can't some planet orbit a star like Charon orbits pluto? Same thing with galaxies, why are they discs?

[selden]

Objects tend to group in planes because their gravity pulls on each other. Whenever one of the objects is above or below the plane, all the others that are still near the plane are pulling it back toward the plane.

A similar effect makes Saturn's rings so thin and flat. Saturn has a bulging equator that tends to pull its satellites toward Saturn's equatorial plane.

[WildMoon]

I've sometimes wondered about that too. Thanks.

[Hunter Parasite]

Thanks, because i was thinking, when i learn how to make solar systems, i was gonna make one of those funky ones, but that would be to unrealistic.

[bdm]

I think you can include planets with inclined orbits, but they would not be close to the parent star.

Consider the solar system. All the major planets are roughly in the plane of the ecliptic, because they are close to each other and will pull each other into the same plane. Farther out in the Kuiper belt, the Kuiper Belt objects are not so strongly confined to the ecliptic plane. 2003 UB313 has an inclination of 45 degrees!

If there was another large planet the size of Neptune in the depths of the Kuiper belt, say 150 AU from the sun, it would be likely to have an orbit with a relatively high inclination to the ecliptic plane when compared to Neptune itself.

[Matt McIrvin]

Objects tend to group in planes because their gravity pulls on each other. Whenever one of the objects is above or below the plane, all the others that are still near the plane are pulling it back toward the plane.

A similar effect makes Saturn's rings so thin and flat. Saturn has a bulging equator that tends to pull its satellites toward Saturn's equatorial plane.

I don't think that a gravitational restoring force pulling things back toward the plane is the crucial effect. Saturn's equatorial bulge is big but not that big. It's more likely that if something starts out in the plane, the gravitational forces that tend to change its orbit won't pull it far out of the plane.

And as for how all that got started in the first place, a rotating cloud that coalesces into a system of objects will tend to have them revolving in a plane from conservation of angular momentum.

Notice that the outer, probably captured moons of the four giant planets are in much looser clouds, not so concentrated in any plane whatsoever.

[Tanketai]

Saturn's equatorial bulge is big but not that big.

Keep in mind that it had billions of years to work. Give it some credit.