Astronomy / Space Planet Rotation direction

Javert

Volunteer Moderator
Javert
Can I ask a silly question.


As far as we know, do planets in solar systems always tend to rotate the same way so that sunrise / sunset would always be from "east to west" based on our system of seeing things, or is it random and planets could rotate in either direction (or from south to north or whatever).

Also while I'm at it, why do the planets in a solar system in ED always lie roughly on the same plane and is this realistic?

cheers.
 

Aquamonkey

Aquamonkey
The reason for both is conservation of angular momentum.

Occasionally you can have things not orbiting in the ecliptic plane. This is due to interactions with other gravitational bodies flying through the system or collisions with such bodies. You can also have planets with extremely tilted axes, this is usually due to collisions.
 
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Javert

Volunteer Moderator
Javert
Aquamonkey said:
The answer to both is conservation of angular momentum.

Occasionally you can have things not orbiting in the ecliptic plane. This is due to interactions with other gravitational bodies flying through the system or collisions with such bodies. You can also have planets with tilted axes, this is usually due to collisions.
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I'm not sure that actually answers my first question. The question is whether all planets without a tilted axis would naturally tend rotate in in the same direction, or would it be random which direction they rotate in?

In other words, could there be an earth like planet out there where the sun rises in the west and sets in the east?
 

Javert

Volunteer Moderator
Javert
Zieman said:
About 1.1-1.8 million years ago Earth itself was such a planet. :)
You see, Earth's magnetic field reverses itself every now and then (geological timescales).
https://en.wikipedia.org/wiki/Geomagnetic_reversal
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Understood, but again I don't think this is exactly what I meant - magnetic pole reversal would show that the sun is rising in the west from magnetic heading point of view, but this would still be the East from a "True" bearing perspective. I'm actually refeeering the direction the planet is physically rotating in rather than the magnetic pole orientation.
 

Zieman

Zieman
Javert said:
Understood, but again I don't think this is exactly what I meant - magnetic pole reversal would show that the sun is rising in the west from magnetic heading point of view, but this would still be the East from a "True" bearing perspective. I'm actually refeeering the direction the planet is physically rotating in rather than the magnetic pole orientation.
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Oh yeah, "true bearing", like when viewed from "above" of the Solar System (Sun's north pole pointing "up").

In that case, conservation of angular momentum is the answer, 99% of bodies do rotate counterclockwise when viewed from "above", and they also orbit the Sun same way.

http://scienceblogs.com/startswithabang/2010/10/07/counterclockwise-but-there-are/
 
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Javert

Volunteer Moderator
Javert
Zieman said:
Oh yeah, "true bearing", like when viewed from "above" of the Solar System (Sun's north pole pointing "up").

In that case, conservation of angular momentum is the answer, 99% of bodies do rotate counterclockwise when viewed from "above", and they also orbit the Sun same way.

http://scienceblogs.com/startswithabang/2010/10/07/counterclockwise-but-there-are/
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OK so I read that and a few other articles on google (oh dear!).

So this explains why the planets in this solar system where the Earth is located mainly rotate anticlockwise (even though actually Venus rotates clockwise), but as far as I can see it doesn't say that all other solar systems would be the same.

Isn't it entirely possible that in other solar systems the starting direction of the formation of planetary bodies would be the opposite, so in those solar systems most planets would rotate clockwise instead?
 

Surfinjo

Surfinjo

Komemiute

Komemiute
[Tongue firmly in cheek]
Venus is the planet that represents femininity.

It spins opposite to the others.

Many things are clearer now.

[/Tongue firmly in cheek]
 

Aquamonkey

Aquamonkey
Javert said:
OK so I read that and a few other articles on google (oh dear!).

So this explains why the planets in this solar system where the Earth is located mainly rotate anticlockwise (even though actually Venus rotates clockwise), but as far as I can see it doesn't say that all other solar systems would be the same.

Isn't it entirely possible that in other solar systems the starting direction of the formation of planetary bodies would be the opposite, so in those solar systems most planets would rotate clockwise instead?
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As Surfinjo says, all matter possesses spin. When there's a clump of gas and dust, all the pieces have movement which averages out to a total spin for the whole clump. As that clump collapses in to form a new solar system, all the little random movements cancel out except for the overall average spin. This forces the clump to flatten out into a disk (kinda like when a pizza maker spins pizza dough over his/her head). As bodies coalesce within this accretion disk into stars and planets, they all maintain that initial spin direction (both in their rotation and revolution) left over from when it was a big clump of gas. This is the conservation of angular momentum. I was OMW to work and didn't have time to explain it thoroughly.

The orientation of this spin is arbitrary depending on whatever was going on in the original clump and whatever interstellar winds or supernovas, etc. acted on it. We have assigned the right hand rule as a convention for what is "up" when looking at spinning systems, hence why everything goes CCW when "looking from above."

[video=youtube;tmNXKqeUtJM]https://www.youtube.com/watch?v=tmNXKqeUtJM[/video]
 
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Surfinjo

Surfinjo
That video is quite good, but why the reference to a 4 dimensional universe?

It's didn't make things any clearer, the suggestion that a 3d universe is necessary for flat galaxies to exist is illogical and frankly irrelevant.
 

racer1

racer1
Putting it simply the galaxy is spinning, so when a solar system forms any spin is taken from the spin of the galaxy unless it has had a close encounter with a large body locally. As a solar system forms from dust and gas any bodies take their spin from that and thus tend to all lie in the same plane, the ecliptic.

So without strong local interactions everything in a solar system will spin in the same direction and all solar systems within a galaxy are likely to be spinning the same way. Same for solar systems within a galaxy and I expect galaxies themselves and so on.
 

Javert

Volunteer Moderator
Javert
So the bottom line is, it's perfectly possible to find an ELW that spins in the opposite direction but just much less likely.
 

Mephane

Mephane
Javert said:
So the bottom line is, it's perfectly possible to find an ELW that spins in the opposite direction but just much less likely.
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Well, in space there is no up and down, so north and south are also arbitrary. There is no "opposite" spin direction. One could define "east" as simply "spinward" (towards sunrise), and suddenly no planet could rotate "opposite" of that, by definition. Then magnetic fields could be "opposite orientation" instead.

I'd rather say magnetic field and rotation can be coaligned or not. In fact, if you take the terms literal, Earth is anti-coaligned. The magnetic south pole is near the geographic north pole... :D
 
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Aquamonkey

Aquamonkey
racer1 said:
Putting it simply the galaxy is spinning, so when a solar system forms any spin is taken from the spin of the galaxy unless it has had a close encounter with a large body locally. As a solar system forms from dust and gas any bodies take their spin from that and thus tend to all lie in the same plane, the ecliptic.

So without strong local interactions everything in a solar system will spin in the same direction and all solar systems within a galaxy are likely to be spinning the same way. Same for solar systems within a galaxy and I expect galaxies themselves and so on.
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It doesn't really have much to do with the spin direction of the galaxy. Ours and most solar systems are not oriented with the disk of the galaxy.

The basic conception of how the galaxy rotates is that it is a flat disk and everything goes around it like a record player. However, this is not actually how it works. Even the disk is several thousand light-years thick. Each individual body within the galaxy goes its own path around the center of mass for the galaxy. This means that everything is bobbing up and down vertically through the disk as it orbits around the center of the galaxy. The extent of this depends on the inclination of their orbits. Things in the galactic bulge can have an orbital inclination near 90 degrees.

EDIT: Like something above the middle of the disk now, won't just stay above the disk and go around the center like it were stuck there. Because it has to go around the CoG (which at that point would be lower in elevation), its orbit will eventually take it down below disk
 
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