h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I'm sure I could look up the answer to this, but I'd rather ask it here. Why are the planets of our solar system arranged in a plane? Why doesn't stuff just randomly go round the sun at any old angle?

The solar system started out as a big cloud of gas and dust. The gravity of the cloud caused each molecule of gas or piece of dust to orbit around the centre.

For those going in one direction, there were nearly always an equal number going in the opposite direction, but it would be a huge coincidence if all the motions exactly cancelled each other out. So by the laws of chance, the whole cloud had an overall average spin around one axis.

As time went on, the gravity pulled the cloud together, but the rotation around the "average axis" caused it to only collapse in a direction parallel to the axis. It stayed spread out in the plane perpendicular to the rotation axis. So it collapsed into a spinning disc of dust rather than a smaller sphere.

This disc then gradually separated out into hundreds of "protoplanets", with a big lump at the centre. These protoplanets collided with each other, and gradually coalesced into the planets we know and love. But because they started out as a disc, they ended up all orbiting in the same plane.

Thanks Gnomon. That makes sense. I guess the same effect on a much larger scale is responsible for disc shaped galaxies.

Exactly.

Incidentally, this is one of the reasons why Pluto got demoted - it does not orbit in the same plane as the eight planets. Its orbit is at a significant tilt to the plane of the rest of the orbits, which is one of many reasons to consider it as something other than a planet.

And the fact that the people who wanted Pluto demoted did a sneaky rear-guard action at the end of the conference where the vote happened and most people had gone home...

Sounds like there's a Peer Review Thread here that people need to comment on...

F48874?thread=8292168

Did anyone else pick up the experiment that's going on trying to see if the UNIVERSE is actually 2D, and our "reality" is just a hologram?

Saw mention of it in Focus the other week....