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  1. Most images you see of the solar system are 2D and all planets orbit in the same plane. In a 3D view, are really all planets orbiting in similar planes? Is there a reason for this? I'd expect that the orbits distributed all around the sun, in 3D.
  2. Has an object made by man (a probe) ever left the Solar System?
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welcome to physics.SE! Please try to ask only one question per ... question ;-) That way it's easier to determine the correct/best answer –  Tobias Kienzler Apr 12 '11 at 13:24
Incidentally, this is part of why Pluto was "demoted" from planethood--it orbits with an appreciable angle with respect to the ecliptic, unlike the eight planets. –  Jerry Schirmer Apr 12 '11 at 13:41
this freeware program lets you zoom around a 3D model:google.co.uk/… –  Nic Apr 12 '11 at 13:55
short form of @Nic's link: shatters.net/celestia (no need to tell google everything. Did I mention I hate this automatic clipboard replacement behaviour?) –  Tobias Kienzler Apr 12 '11 at 14:21
This question belongs on Astronomy. Wait... It is not created yet (is in the commitment phase), but nearly everybody can help create it! –  Peter Mortensen Apr 12 '11 at 19:05
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3 Answers

up vote 9 down vote accepted

Nic and Approximist's answers hit the main points, but it's worth adding an additional word on the reason the orbits lie roughly in the same plane: Conservation of angular momentum.

The Solar System began as a large cloud of stuff, many times larger than its current size. It had some very slight initial angular momentum -- that is, it was, on average, rotating about a certain axis. (Why? Maybe just randomly! All of the constituents were flying around, and if you add up those random motions, there'll generically be some nonzero angular momentum.) Because angular momentum is conserved, as the cloud collapsed the rotation rate sped up (the usual example being the figure skater who pulls in her arms as she spins, and speeds up accordingly).

Further collapse in the direction perpendicular to the plane of rotation doesn't change the angular momentum, but collapse in the other directions would change it. So the collapse turns the initial cloud, whatever its shape, into a pancake. The planets formed out of that pancake.

By the way, you can see the signs of that initial angular momentum in other things too: not only are all of the planets orbiting in roughly the same plane, but so are most of their moons, and most of the planets' rotations about their axes as well.

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nice answer, Uranus rotates in a very funny direction and this suggests something significant has happened to disturb its rotation. –  Nic Apr 12 '11 at 14:34
Right! I have a lousy memory for which planet(s) have anomalous rotations, but I knew there were some. –  Ted Bunn Apr 12 '11 at 14:51
Uranus orbits on the same plane and in the same direction as the other planets. It's rotation about it's axis is odd as it's practically laying on it's side and rolling along like a ball rolled on the ground, as opposed to like a spinning top. Venus, however, is the real oddball, as it rotates about it's axis in the opposite direction of the rest of the planets (retrograde rotation), like it's completely upside down or rotating backwards. –  Phoenix Apr 12 '11 at 19:12
Just one thing I didn't get clearly: Why the rotations of the planets about their own axes are also in similar planes? –  becko Apr 12 '11 at 23:20
@becko -- you're right that that wasn't clear from my explanation. The collapse to a pancake happens before the planets form. Velocities in the perpendicular direction are much smaller than velocities in the plane. The motion of the cloud is a more-or-less uniform "swirling" around the rotation axis. –  Ted Bunn Apr 13 '11 at 13:24
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  1. More or less yes. The planets mostly orbit in the same plane but with small deviations compared to the size of the system. The largest relative tilt is around 4 to 6 degrees. This 'flatness' is due to orbital mechanics, where the solar system started spherical it now has 'decayed' into a flat disc. This disc is essentially a stable low energy configuration and therefore the system has become more and more disc like over time.

  2. The Voyager 1 probe is 'leaving' the solarsystem as we speak but there is no real firm boundary and so it's hard to give a definitive answer.

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2) depending on the definition of the boundary, Voyager 2 and Pioneer 10 and 11, too –  Tobias Kienzler Apr 12 '11 at 13:35
largest relative tilt is mercury 7 degrees –  Approximist Apr 12 '11 at 14:36
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  1. The orbital planes of different planets has small inclinations to the ecliptic plane. The corresponding wikipedia diagram should give a better view than the 2D images you've seen.

  2. Both Voyager 1 and 2 are beyond the Solar System

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If you include Voyager 2, you might as well include Pioneer 10&11, see en.wikipedia.org/wiki/File:Outersolarsystem-probes-4407b.svg –  Tobias Kienzler Apr 12 '11 at 13:36
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