G'day Team,
Well out of my comfort zone on this one, but after responding to post which had compared the orbit of electrons around a nucleus with those of planets around the solar system I started thinking. Why do all the planets orbit on the same axis? So I did a little bit of research and I'd love to share what I've learned with everyone!
Today I'll be welcoming as much feedback and as many corrections as people would like to throw at me. I'm more than happy to admit this is well outside my field of expertise and I'd love to hear about any misunderstandings I have!
The Problem (In My Mind)
So let's start thinking this through logically. It's about four billion years ago, the sun is burning away and a lot of rocks and other debris are floating around. One rock bumps into another rock they stick together. This happens a few billion times and eventually, we get a planet... "because gravity".
But dust is everywhere right? Why would all the dust sit in the same horizontal (or what's represented as horizontal) axis? Or did the planets form in all different positions and then migrate to a single orbit? Does this have something to do with the spinning of the sun? (Does the sun spin?) Or does it have to do with each planet's gravity pulling them together? Where do I start? Why am I here? What is the meaning of all this?
A Stupid Question
The first thing I discovered was that, technically, my question wasn't even correct. The planets orbit on almost the same plane, but not quite. There is a line called the 'invariable plane' (no idea why, or what determines this plane) but here's how far out the planets are from this imaginary line!
- Mercury = 6.3
- Venus = 2.2 degrees
- Earth = 1.7 degrees
- Mars = 1.6 degrees
- Jupiter = 1.0 degrees
- Saturn = 0.9 degrees
- Uranus = 0.7 degrees
- Neptune = 0.3 degrees
But still... this is pretty close to a single plane, so my questions stands and I want my answer!
The Start of a Solution
The first thing I learned, is that I am not the first person to have asked this question (in my experience you never are) and the people who have answered it have some pretty cool terms. When all the planets sit on a single plane they are called "coplanar".
The next thing I learned is that planets formed from a ring of dust around our sun (like the one around Saturn) and this ring is the origin of the "coplanar" arrangement of planets.
So we've got the start of a solution. But what gives dust the tendency to form on a plane?
Getting Complicated
Why does the dust form a disc? Well because of a thing called angular momentum.
This angular momentum essentially describes the spin of the cloud of dust around our sun. It's like normal momentum, but in a circle! This spin, in turn, origionates from the cloud of dust that our sun and solar system were created from, which (on a massive scale) had some form of rotation.
As this rotation collapses down under the force of gravity, it not only conserves this spin, but also it's momentum. To understand this we think of a spinning ice-skater. As they spread themselves out they have a slow spin across a large radius, as they pull their arms and legs in their spin speeds up to conserve angular momentum over a smaller radius.
Think of this happening, but on the scale of our solar system.
As the cloud that created our sun and planets collased it maintained the same amount of angular momentum, but in order to do so the rate of spin increased dramatically. Now the best way to spin fast is to spin close to the rotational axis, and as such all matter tends to fall into this single plane.
And now we've got a single ring of dust! Out of which a single ring of planets can form!
There's More
Now I didn't originally think of this but there are answers to two more good questions hidden in angular momentum.
Not only do the planets all orbit on the same plane because of angular momentum, but it's also the reason that every planet except Uranus and Venus spin on the same axis too. Both uranus and venus appear to have been struck in the past, resulting in changes to their orbits.
- Uranus spins on a 90 degree angle
- Venus spins the wrong way
Beyond this, the same angular momentem explains why all the planets orbit the sun in the same direction.
Disclaimer
Now I'm well aware I'm out of my depth here, so any feedback or corrections are more than welcome!
Thanks
I hope everyone had as much fun learning about this as I did!
Thanks
-tfc
Resources
Without which I would have had no chance!
Ask an Astronomer
Ask Ethan #82
Van Physics
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