# Is this why solar systems are flat?

• Njorl
In summary, Njorl argues that the solar system formed from a cloud of material that collapsed under its own gravity (and perhaps from some initial cause such as a passing star that disrupted the cloud). As it came together asymmetrically, it had some net resulting spin. Also, due to gravity, friction, and conservation of angular momentum, this cloud came together as a rotating disk. The material in this disk was mostly in the middle (which collapsed to form the sun). The material in the outer portion of the disk was what collided and stuck together to form the planets. So, all the planets are essentially in the same plane (i.e., they go around the sun instead of "over" or "diagonally" around

#### Njorl

Most of the matter in our solar system lies roughly in a plane. I always wondered why. I imagine that there is a well understood reason for this, but I never came across it. I came up with this:

Assume we have a large star nearing nova. It spins rapidly, and has a large magnetic field. When it explodes, virtually all the matter it throws off is decomposed into charged particles. Though the matter is originally cast off in a nearly isotropic manner, as it interacts with the magnetic field, it is focussed into a planar disc around the remaining star. This disc becomes the solar system.

The problem I'm having is that the magnetic field of a star doesn't quite seem to me to be the right shape to do this.

My other thought (which just came to me now) was that charged matter that was cast off in the solar plane could be deflected by the magnetic field so that it would take up an eliptical orbit. Matter that was significantly out of the plane would either reach escape velocity, or return to the star.

Anybody know this?

Njorl

Check your models, solar systems are flat for conservation of momentum.

Rotational velocity/centrifugal force. Since the equator of the Earth is moving linearly faster than the poles, the Earth is flattened. And so it is with galexies and solar systems. But since gravitational attraction is far lower and the velocities far higher, you end up with something that is so stretched out it looks roughly planar.

In a nutshell...

The solar system formed from a cloud of material. This cloud collapsed under its own gravity (and perhaps from some initial cause such as a passing star that disrupted the cloud). As it came together asymmetrically, it had some net resulting spin. Also, due to gravity, friction, and conservation of angular momentum, this cloud came together as a rotating disk*. The material in this disk was mostly in the middle (which collapsed to form the sun). The material in the outer portion of the disk was what collided and stuck together to form the planets. So, all the planets are essentially in the same plane (i.e., they go around the sun instead of "over" or "diagonally" around it).

* Here's the key to your question. As the material came together, it began to spin faster due to conservation of angular momentum just like how a figure skaters spin faster when they pull in their arms close to their body. With a faster spin, the bits of material orbiting the center of the mass were flung out into more distant orbits...preferentially in the plane perpendicular to the axis of rotation of the cloud of matter. Gravity and friction (colliding bits of matter) tended to pull everything toward this central plane...the more matter that accumulated in this disk, the stronger the gravity became there to pull more matter into it.

p.s. A 'nova' (brightening star) is very different than a supernova (exploding star). Also, a supernova does not leave a stellar remnant like our sun...more like a neutron star or black hole.

A very plausible explanation, Phobos. But there remains a very intrigueing question. Everything seems to be turning in the same direction which is excellent for conservation of momentum, except for Venus, counter rotating stubbornly. How can we explain that in this model?

Much of this sounds familiar. I think I may have heard it before learning any physics, like age 8 or so. Without a framework to hang it on, it got lost in the back of the closet of my mind.

Njorl

Njorl-

The planetary disk while cooling off would reduce internal charge separation more rapidly than magnetic fields could influence its shape. See pictures of some high-energy nebulae for the magnetic effect you mention.

Don't click on the link... another one of the silly popularity spam contest thingies, it seems.

Originally posted by FZ+
Don't click on the link... another one of the silly popularity spam contest thingies, it seems.
Git 'im, mods!