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I always see pictures of the solar system where our sun is in the middle and the planets surround the sun. All these planets move on orbits on the same layer. Why?

Qmechanic
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Stephan Schielke
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    Possible duplicate: http://physics.stackexchange.com/q/8502/2451 Related: http://physics.stackexchange.com/q/12140/2451 and links therein. – Qmechanic Sep 06 '13 at 08:21

1 Answers1

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We haven't ironed out all the details about how planets form, but they almost certainly form from a disk of material around a young star. Because the disk lies in a single plane, the planets are broadly in that plane too.

But I'm just deferring the question. Why should a disk form around a young star? While the star is forming, there's a lot of gas and dust falling onto it. This material has angular momentum, so it swirls around the central object (i.e. the star) and the flow collides with itself. The collisions cancel out the angular momentum in what becomes the vertical direction and smear the material out in the horizontal direction, leading to a disk. Eventually, this disk fragments and forms planets. Like I said, the details aren't well understood, but we're pretty sure about the disk part, and that's why the planets are co-planar.

Warrick
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    Very interesting, I've been wondering about this everysince I learned what a solar system was. +1 – OmnipresentAbsence Mar 16 '13 at 14:20
  • I find this clearer: With a heavy enough cloud left around the just collapsed cloud (=sun), 2 reasons often evade further collapse into the sun. 1. Solar wind 2. The angular momentum. Subtracting this attraction towards the center leaves a kind of "net attraction" perpendicular to it (in each point of the cloud). So logically this eventually forms a disk with the net angular momentum. (There is a chance for some chunks to not follow the disk, but it becomes smaller with its deviation and with each additional one. Pluto?) Similar:http://en.wikipedia.org/wiki/Protoplanetary_disk#Formation – e-motiv Mar 04 '14 at 11:16
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    My question is why can there not exist, say, two orthogonal axis of rotations, for various planets to revolve around and not colliding? – Hans Jul 09 '14 at 18:04
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    @Hans Then the discs would intersect and their material would interact, dispersing and rearranging the material into a more stable configuration. What that configuration is depends on the initial conditions. – zibadawa timmy Oct 24 '14 at 06:46
  • @zibadawatimmy: Why do the planets belonging to two orthogonal axis of rotation have to be "discs" as you say and interact? The simplest counterexample is two concentric circles of different radii perpendicular to each other. – Hans Oct 24 '14 at 14:21
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    @Hans Sorry, I missed the planets part and was thinking dust. For planets the gravitational interactions will make the orbits chaotic over time, unless they are far apart. Get too far and you're unlikely to align on a plane. The Oort cloud is spherical rather than planar, for example. Closer to the star and the angular momentum, collisions, and interactions during the system's beginning stages will flatten things to nearly coplanar. – zibadawa timmy Oct 24 '14 at 15:23
  • @zibadawatimmy: Thank you for introducing me to the Oort cloud and supplying evidence of spherical distribution of orbits. Even with the dust and with collisions, why can dust not form, say, different annuli of different orientation at close distance to the sun? The rationale of chaos does not seem to be too convincing, since the planets in the solar system have been pretty stable with the current mutual distance. Does the orientation of the orbit so crucial even when we, say, make the mutual distance of those annuli to ten times that of the current planets? – Hans Oct 24 '14 at 16:26
  • @Hans I lack the expertise on what kind of distances you need for long term stability in that case. As for chaos, our solar system has not always been in this configuration, nor will it stay in it. The orbits in a many body problem are known to always be chaotic (meaning small changes become exponentially large on long time scales). It's one way we know there isn't a second earth always on the other side of the sun from us. – zibadawa timmy Oct 24 '14 at 16:35
  • @zibadawatimmy: I know many body gravitation problem is chaotic. Yet the solar system in the time scale of the earth life time until now is "relatively" stable. My question is what mathematical prohibition is there against a spherically distributed planet or dust orbital axis system with the same size orbit just like the solar system? Our solar planet system happens to be mostly planar. Is there a mathematical rationale stipulating the axis of rotation have all to be almost parallel and stable within the earth-life-time-until-now time scale? – Hans Oct 24 '14 at 18:01
  • @Hans I'd suggest making your own question, asking in the chat, and searching for existing answers. Not necessarily in that order. – zibadawa timmy Oct 24 '14 at 18:26
  • @Hans Any configuration has to be stable for a long time for us to be able to have time to exist and make telescopes, etc. Unstable elements will tend to either have fallen into the sun or other planets. A few may have reached escape velocity, or be out in the Oort cloud, etc. – Dronz Nov 03 '14 at 02:47
  • @Dronz: "Stable" here is not required to be for time infinite and is defined relative to a given time interval. However, how does your comment relate to my question at all? – Hans Nov 03 '14 at 23:38
  • @Hans Good point. I am thinking of the assumption that they were formed by accretion from an cloud that gradually condensed by gravity. So I imagine a gas cloud with irregularities whose concentrations form denser pieces and eventually usually one is largest which becomes the main star, and as it forms, more and more matter swirls into or around it. The matter swirling around it will initially be spread out and still condensing, so it will tend to condense towards concentrations... – Dronz Nov 04 '14 at 00:23
  • ... and as it started as a cloud that was more even than not, it's going to tend to have a continuous history from that point which will have some large-scale continuity unless small-scale concentrated variations lead to some asymmetry. So I think yes you could end up with the inner planets on a different plane, but it would be the exception rather than the usual case. (I would love to see some attempted solar system accretion simulation animations - I wonder if there are some.) – Dronz Nov 04 '14 at 00:30
  • @Dronz: The concentration should be local. My question is why there can not be, with high probability, disjoint concentric elliptical annuli, at the distance of the solar planets in scale or larger for dust cloud, that are stable at the current solar system time scale, that planets or dust can rotate around the central star about very distinct axises. – Hans Nov 04 '14 at 14:54
  • @Hans I may well not be saying anything you haven't heard several times before, but... Since the way solar systems actually form is still not completely known, we don't know, but assuming solar accretion, it seems to me the reason would be that the probability is not high because the large-scale distribution tends to have no major local concentrations before accretion, and a cloud has particle collisions that tend to cancel out differences, and concentrations attract more and more and cause more collisions, tending to lead to one plane unless something major happens to throw it off. – Dronz Nov 04 '14 at 18:13
  • @Dronz: Yes, I have heard these before, and again this line of argument appears to form in hindsight: because in most of the examples we have observed --- I would not think the number is high as most planets and dust clouds in other solar systems are dark and thus unobservable --- the orbits are almost planar, so we fill whatever SEEMINGlY plausible reasons to convince ourselves this is almost always the case, whatever the mathematical rigor. It is a bit akin to the joke about psychologist's theory. Now to be more specific, why does "the large-scale distribution tend to have no major ... – Hans Nov 05 '14 at 06:13
  • @Dronz: "local concentrations"? If the elliptical regions that different orbits reside in are all disjoint and sufficiently far, there would be no collisions and no cancellations, and concentrations. You, just like the others, seem to put cart in front of the horse and cherry pick the reasons just to be able to draw the prescribed conclusion. That seems to fit more the characteristic of logical fallacy than scientific method. – Hans Nov 05 '14 at 06:20
  • @Hans It's true that I don't know. Scientific method does include putting out hypotheses, and yes this one is informed mainly by our one main example of a solar system, and it could be the exception.. There are a lot of assumptions or even guesses. As I wrote, "we don't know." I'm talking about what I think I understand about theories. I'm trying to explain how this theory seems to make sense to me. The thinking of the guess I'm aware of, is that solar systems form from lots of dust assumed to usually be fairly homogeneous before it started accreting into larger objects. – Dronz Nov 05 '14 at 07:06
  • @Dronz: I know what you are saying. Even for spherical initial distribution, as assumed in your last sentence, with perhaps some perturbation, why should most the resulting semi-stable orbits lie on a single plane? – Hans Nov 05 '14 at 14:52
  • @Hans Ok, I will put what I think I get about this theory in an answer. – Dronz Nov 05 '14 at 19:13
  • I added an answer on the "possible duplicate" question, here: http://physics.stackexchange.com/questions/8502/questions-about-the-solar-system/145043#145043 . Even better than my answer is the site I found here: http://www.scholarpedia.org/article/Accretion_discs – Dronz Nov 05 '14 at 20:08