# Thread: Planetary Systems off axis

1. Newbie
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Nov 2009
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## Planetary Systems off axis

a question:

As evidenced in planets by Uranus and Venus, the axis of rotation can be flipped or twisted so that things don't exactly rotate as-is with the general pattern of counter-clockwise rotation. Is it possible to have a complete system be flipped off-axis from the glactic plane so that it rotates either at a perpendicular (ala Uranus) or even in retrograde as relative to the galactic center, and if so, how?

thanks for the potential answers!

Chris Eisele
(that's pronounced Isz-lee--gotta love Americanized German)

2. Yes.

The galactic plane has little effect on a planetary system that is forming. Local effects are what drive it.

Take a look at this.

http://antwrp.gsfc.nasa.gov/apod/ap091001.html

and you can see how it doesn't align with any plane.

3. Order of Kilopi
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Dec 2004
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11,220
Hello, Chris!

The plane of the orbits of the planets in a solar system has no relationship
to the plane of the galactic disk. The axes of rotation of the planets in a
solar system probably does have a relationship to the plane of the
planet orbits. The direction of rotation of a planet is determined mainly by
the direction of motion of the matter that hits it while it is forming.

Virtually all the matter in a protoplanetary disk is moving in nearly circular
orbits around the forming central star. Matter that collides with a forming
planet would be in orbits that are just slightly elliptical, so it would tend to
hit the night side of the planet while near periastron and hit the day side
of the planet while near apastron. That gives the planet a rotation in the
same direction as its revolution around the star.

Later on in planet formation, planets of various sizes that are in orbits close
to one another will collide, as happened to the Earth. Such giant impacts
can alter the direction of rotation of the planet.

-- Jeff, in Minneapolis
Last edited by Jeff Root; 2009-Nov-09 at 02:05 AM. Reason: fixed grammatical error

4. As it has been said a solar systems orientation has little to do with the galactic plane. While visually similar to a spiral galaxy the effect of a new solar system the gravitational effect of the galaxy is much less then a solar system. The disk around a new star is only partly formed by gravitational effects while the majority of the reason the disk forms is from actual collisions of the material.

In galaxies you might even get stars that rotate around the galactic centre opposite to the galactic rotation but this would be due to galactic mergers and over time those systems would probably be slowed down by gravitational pull and drop into a closer orbit around the galactic centre before orbiting in the same direction.

5. Newbie
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Nov 2009
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thanks for the fast answers, Aurora, Jeff and Wayne!

So if the formation is based upon local effects and is irrespective of the galatic plane, does that mean that there are any examples of a perpendicular system corkscrewing along through the Milky Way?

I know that most space is largely empty, and a parent star's gravity will hold tightly onto the planets that revolve around it, but is it possible (as Wayne suggested) that a galactic merger or other high-mass event (like passing near enough to a rediculously large black hole, etc.) could result in a "flipping" of an entire planetary system without just flinging the planets off into space?

Also, are there any examples of stars whose axial tilt is drammaticly off alignment as related to the galatic spin?

6. Originally Posted by Mr_Grey

Also, are there any examples of stars whose axial tilt is drammaticly off alignment as related to the galatic spin?
I think I'm kind of repeating what has been written before, but stars are not generally aligned with the galactic spin. Ours isn't. It's about perpendicular to the plane of the galaxy. The thing with Uranus is that its own rotation is at odds with the plane of its orbit around the sun. But so is the earth's, about 23 degrees in our case.

7. Established Member
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Our own Solar System is at 60 degree angle to Milky Way.

Few other planetary systems are known, and those that are often have unknown plane or are subject to selection effects. But there are plenty of known multiple stars.

Are the orbital planes of multiple stars strictly random and isotropic?

8. Newbie
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Wow! thanks again for the replies. Jens, I had no idea we were at a tilt relative to the galatic plane. Interesting!

Thinking more on the question:

Chornedsnorkack makes a valid opint. Due to how we detect the presence of planets--the gravitic "wobble" of a parent star while observing the star itself for transits--it would seem quite difficult to say "Oh yes, this star has X planets and they are all rotating in this direction." It's like staring at a flashlight at a half-mile distance and then having people wave their hands in front of it. You can't really tell exactly where the obstruction came from, only that a hand passed in front of the light. Is that a correct analogy?

So then, even though everything is generally spinning in one direction when viewed from a big enough frame of reference, it's when you get into the specifics that the whole thing becomes largely a seemingly random array of options. I.e. The solar system has planets rotating counter-clockwise, BUT, in specific some of their axes of rotation operate quite differently. The same holds with the galaxy--it spins collectively in one direction, but the individual components don't neccessarily all spin along in the same way locally.

interesting.

9. Originally Posted by Mr_Grey
Chornedsnorkack makes a valid opint. Due to how we detect the presence of planets--the gravitic "wobble" of a parent star while observing the star itself for transits--it would seem quite difficult to say "Oh yes, this star has X planets and they are all rotating in this direction." It's like staring at a flashlight at a half-mile distance and then having people wave their hands in front of it. You can't really tell exactly where the obstruction came from, only that a hand passed in front of the light. Is that a correct analogy?
I don't know all that much about it, but I think I can say this much. When the planet transits in front of the star, you look at how the star itself moves. If the star moves from right to left (slightly!) as the planet passes in front of it, then you know that the planet is moving from left to right. Similarly, if the star bobs upward, you know the planet is going downward. I'm just a layperson myself, and share your awe that this could be done at such a distance. But somehow they manage it.

Originally Posted by Mr_Grey
So then, even though everything is generally spinning in one direction when viewed from a big enough frame of reference, it's when you get into the specifics that the whole thing becomes largely a seemingly random array of options. I.e. The solar system has planets rotating counter-clockwise, BUT, in specific some of their axes of rotation operate quite differently. The same holds with the galaxy--it spins collectively in one direction, but the individual components don't neccessarily all spin along in the same way locally.
I think you almost have it right, but a solar system is different from a galaxy. Solar systems are generally like planes, with all the planets more or less on the same plane. But a galaxy is more complex, looking sort of like flying saucers, right? So the plane is not so clearly defined, and that may be why there is a difference. As a contrast, think of a globular cluster. The stars are all scattered around, so I imagine that their ecliptic planes would be all over the place as well.

Also, I'm not sure if you're always using rotation and orbits in the right way. All the planets in our solar system go around the sun in the same direction. It's just that for some the rotation (two or three, depending on the definition of planet ) of the planet itself is off.

10. Originally Posted by Mr_Grey
Wow! thanks again for the replies. Jens, I had no idea we were at a tilt relative to the galatic plane. Interesting!

Thinking more on the question:

Chornedsnorkack makes a valid opint. Due to how we detect the presence of planets--the gravitic "wobble" of a parent star while observing the star itself for transits--it would seem quite difficult to say "Oh yes, this star has X planets and they are all rotating in this direction." It's like staring at a flashlight at a half-mile distance and then having people wave their hands in front of it. You can't really tell exactly where the obstruction came from, only that a hand passed in front of the light. Is that a correct analogy?

So then, even though everything is generally spinning in one direction when viewed from a big enough frame of reference, it's when you get into the specifics that the whole thing becomes largely a seemingly random array of options. I.e. The solar system has planets rotating counter-clockwise, BUT, in specific some of their axes of rotation operate quite differently. The same holds with the galaxy--it spins collectively in one direction, but the individual components don't neccessarily all spin along in the same way locally.

interesting.
Actually we can detect the movement quite well. The problem is the orientation of the planet's orbit in relation to us causes the investigation to be a bit more difficult. There is a difference between the eclipse method (measuring the changes in brightness) and measuring the wable.

Using spetrum analysis we can say very accurately how far the star is wabbling. Down to a speed that you can keep up with on a brisk walk.

The resulting data over a long time results in a nice SIN curve that tells us a lot about the planetary system.

If a solar system was tangent to us then detecting the planets would be near impossible with current methods as there would be
• no eclipsing
• no red/blue shifting of the star's spectrum
• no real chance of detecting the motion via parallax

Future methods that block out the light from the star and look for the shine of the planets will be needed to detect these systems.

11. Established Member
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Originally Posted by WayneFrancis
If a solar system was tangent to us then detecting the planets would be near impossible with current methods as there would be
• no eclipsing
• no red/blue shifting of the star's spectrum
• no real chance of detecting the motion via parallax

Have any planets been seen through astrometry?

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