rriverstone wrote:I was just looking at an animation of Saturn; it reminded me of an old question I've had since I was a little girl, 45 yrs ago. Why are so many orbits flat discs? Why isn't Saturn within a bubble of snowballs, rather than rings? Why do planets orbit Sol in a relatively disc shaped order?
There are really two questions here: (1) why does each individual orbit fit into a plane? (2) why do all of the orbital planes in the solar system lie in roughly the same direction?
The first one is easy to answer. Consider the Sun and a planet. At some given moment in time we can imagine two lines in space: one that connects the centers of the sun and the planet, and another one that starts at the planets and indicates the direction the planet it moving in
right now, relative to the sun. These two lines are not parallel, and since they cross at the planet, there will be exactly one geometric plane that contains them both. Then,
as long as the gravitational interaction with the sun is the main influence on the planet, gravity will just move the planet's velocity vector around
within that orbital plane, which means that the planet itself will stay on that plane. Every orbit in a two-body system has to be plane, due to this argument.
The second question is harder. I think a good explanation will need to involve the angular momentum of the interstellar gas cloud that collapsed to form the solar system, and probably also some statistical dynamics of the collapsing cloud. But I cannot right off the cuff produce an explanation that would convince myself ...
However, note that the nice shared-orbital-plane property we see in the solar system is not universal. Globular clusters have no preferred orbital plane. Neither do elliptical galaxies. Closer to home, the Oort cloud is not thought to be particularly aligned with the Ecliptic.
[quote="rriverstone"]I was just looking at an animation of Saturn; it reminded me of an old question I've had since I was a little girl, 45 yrs ago. Why are so many orbits flat discs? Why isn't Saturn within a bubble of snowballs, rather than rings? Why do planets orbit Sol in a relatively disc shaped order?[/quote]
There are really two questions here: (1) why does each individual orbit fit into a plane? (2) why do all of the orbital planes in the solar system lie in roughly the same direction?
The first one is easy to answer. Consider the Sun and a planet. At some given moment in time we can imagine two lines in space: one that connects the centers of the sun and the planet, and another one that starts at the planets and indicates the direction the planet it moving in [i]right now[/i], relative to the sun. These two lines are not parallel, and since they cross at the planet, there will be exactly one geometric plane that contains them both. Then, [i]as long as the gravitational interaction with the sun is the main influence on the planet[/i], gravity will just move the planet's velocity vector around [i]within[/i] that orbital plane, which means that the planet itself will stay on that plane. Every orbit in a two-body system has to be plane, due to this argument.
The second question is harder. I think a good explanation will need to involve the angular momentum of the interstellar gas cloud that collapsed to form the solar system, and probably also some statistical dynamics of the collapsing cloud. But I cannot right off the cuff produce an explanation that would convince myself ...
[i]However[/i], note that the nice shared-orbital-plane property we see in the solar system is not universal. Globular clusters have no preferred orbital plane. Neither do elliptical galaxies. Closer to home, the Oort cloud is not thought to be particularly aligned with the Ecliptic.