Source
r/askscience•
CaptainCrunch

Why do planets, and stars in galaxies, only orbit along a single plane?

I asked this in a thread on r/science and was told I'd have some luck asking here.

This may be a dumb question, but why do planets orbit their stars on only one plane and in one direction? Satellites don't orbit the earth on only a single plane.

I guess it's got to do with centrifugal force and gravity, but I just fail to see how that works when gravity pulls in all directions from the star and not just from the equator.

It's got nothing to do with the spin of the star correct? If the star's spin were causing it, I'd completely understand why they ran along a single plane, but that doesn't seem to be the case.

I'm obviously not a physics major, but have always been curious about it. So I figured why not ask.

Posted Feb 26, 2011, 8:54 PM UTC

4117
shadydentistLasers•

If you're talking about why the planets in our solar system tend to lie in the same plane, it is because the dust cloud that our solar system came from started with some angular momentum. This causes the dust cloud to flatten out, similar to how spinning a ball of pizza dough causes it to flatten out. So now instead of a cloud of dust, you have now a spinning disk of dust. As the planets coalesce from the disk, they remain in the disk. This is also why all the planets are all orbiting the same direction.

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CaptainCrunchOP•

I guess I didn't think that it formed in that manner due to initial angular momentum. Thanks for the response, that helped clear it up.

So, hypothetically, if a new planet arrived in the solar system, and was trapped by the suns gravity into an orbit, it could be on a completely different plane, and perhaps even move in the opposite direction, correct?

13
Gieron•

This is exactly what has happened to Neptunes moon Triton.

12
siddboots•

This explains the angle and the direction. The next obvious question, "why do all of the planets have concentric orbits?", can be neatly explained by the fact that all of the objects whose orbits were not concentric have already collided into one another.

7
WorderOfWords•

Is the force that made it flatten out a centrifugal force?

2
mtheoryx•

Can we get this question added to an FAQ or something? It pops up more frequently than anything else I've seen in about 3 years.

8
GaryBusey-Esquire•

I don't really see most of reddit as falling into the FAQ reading crowd...

Now, if there was a 'best of AskScience' link on the side, maybe...

6
mtheoryx•

Whichever way is more beneficial is fine with me. It's just a trend that I happened to notice today.

1
tmannian•

Not all galaxies are in a plane, here's the wikipedia page on different shapes.

Spiral galaxies are thought to have formed when the center object has angular momentum about it. That momentum carries through and eventually the galaxy becomes "flat". The elliptical ones appear to just be a big blob of light because, well, they kinda are.

Edit: Here is more info on it

7
Jasper1984•

Previously asked a bunch, not that it is bad to ask. (Not really keeping that list much anymore.. But you might get terser/no answers from people having replied to it a bunch before.)

3
[deleted]•

shadydentist has it right with respect to the solar system, although because the dust cloud became roughly flat rather than perfectly flat, the planets don't quite all orbit in the same plane.

A list of the inclinations of the orbits of the planets relative to the ecliptic can be found here under "Orbit Inclination to Ecliptic".

Can anyone explain to me why the Earth orbits at 0.00005° to the Ecliptic according to that page? Should it be by definition, 0°?

2
shadydentistLasers•

Well, they probably define it by the total angular momentum of the solar system, not just the earth.

Not 100% sure, though.

2
[deleted]•

The "Invariable plane of the Solar System"? According to wiki, the Earth's orbit is inclined at 1.57° to that plane, so I don't think it's that. Good thought though.

1
UltraVioletCatastroAstroparticle Physics | Gamma-Ray Bursts | Neutrinos•

What happened was that the ecliptic was defined with respect to a catalog of stars. But, then when very long baseline interferometry started being used, the position of the catalog stars got much better, but slightly different. So, rather than change the definition of the ecliptic with respect to the cataloged stars they just accepted the fact that the Earth was slightly off axis from the defined ecliptic.

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[deleted]•

Interesting, thanks!

1
kouhoutek•

A planet's angle of inclination varies over time. So if this list is based on the earth's orbital plane at on one particular date, the plane could have proceeded that small amount over time.

2
[deleted]•

That sounds about right, thanks.

1
[deleted]•

[deleted]

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