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Whenever I have learned about the solar system I always see the orbits displayed as a virtually flat plane. 2D Solar System

Are all of the orbits in the solar system really like this? If so, why? It seems like a rather large coincidence for all of our planets + the asteroid belt to be on virtually the same plane.

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  • $\begingroup$ Potentially related/dup: astronomy.stackexchange.com/questions/130/… $\endgroup$
    – user96
    Apr 21, 2014 at 21:50
  • $\begingroup$ See this video. $\endgroup$
    – Yashbhatt
    Apr 22, 2014 at 7:56
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    $\begingroup$ Because it is roughly a plane. But no scientifically accurate depiction of the Solar System will show it as a 2D plane. I mean it is obvious that the picture you show is not to scale or anything, and I don't think it is meant to be. $\endgroup$
    – harogaston
    May 3, 2014 at 3:26

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This is not a coincidence at all, but a direct consequence of the way the solar system was formed.

The generally accepted model is that solar systems (including our own) form out of a Protoplanetary disc. Gravitation causes mass to collapse around a protostar, which always has some angular momentum (as does everything). Wikipedia explains it better than I can:

Protostars typically form from molecular clouds consisting primarily of molecular hydrogen. When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As this collapsing cloud, called a solar nebula, becomes denser, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. Conservation of angular momentum causes the rotation to increase as the nebula radius decreases. This rotation causes the cloud to flatten out—much like forming a flat pizza out of dough—and take the form of a disk.

And then, from this protoplanetary disk, planets form. Consequently, they're all in the same plane.

As such, the inclinations of each planet's orbit are pretty close to that of Earth's:

Planet  Orbital Inclination
Mercury 7°
Venus   3.39°
Earth   0°
Mars    1.85°
Jupiter 1.3°
Saturn  2.49°
Uranus  0.77°
Neptune 1.77°
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  • $\begingroup$ Are they all positive inclinations? $\endgroup$
    – Dgrin91
    Apr 21, 2014 at 15:05
  • $\begingroup$ It appears so, as for why, that's a different question. $\endgroup$
    – gerrit
    Apr 21, 2014 at 15:06
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    $\begingroup$ @Stu I believe what was meant are retrograde orbits with an inclination over 90° (below 90° are prograde and around 90° polar orbits), e.g. Saturn's Phoebe that's suggested might be a captured Kuiper belt object and in 173° orbital inclination to the ecliptic. Those orbits are sometimes called "negative" or even their inclination marked as negative degrees to the polar orbit. E.g. Sun-synchronous "frozen" polar orbits are often said to be at around -1° (or better said 360°/365.25 days in a year, clockwise, so negative to prograde anti-clockwise). $\endgroup$
    – TildalWave
    Apr 21, 2014 at 15:16
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    $\begingroup$ Inclination isn't always positive. It can be zero, after all. Inclination is always non-negative, between 0 and pi radians (0 and 180 degrees), inclusive. $\endgroup$
    – David Hammen
    Apr 21, 2014 at 15:23
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    $\begingroup$ The reason that inclination is always non-negative is by the way it's defined, $\arccos \frac{\vec h \cdot \hat z}{||\vec h||}$ . The range of the inverse cosine is $[0,\pi]$ (or 0 and 180 degrees if you insist on using degrees). $\endgroup$
    – David Hammen
    Apr 21, 2014 at 15:30

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