Question: how was the world made?

James Verdon answered on 23 Jun 2012:

Hi Auntiepolly,
Gosh, this is quite a complicated one, but it’s a few hours until the next Euros game, so I’ll do my best.

The key factor is gravity. The law of gravity means that all objects with mass are attracted to each other, and the more mass an object has, the larger the attraction. So, for example, because the earth has lots of mass, we’re strongly attracted to it.

Now, our solar system would have started out as a nebula of space-dust. Mainly hydrogen atoms, but also small amounts of all the other elements we know. We can see examples of such nebulae still in space – the Orion nebula is probably the most well known.

Just by random chance, a certain part of that nebula would have had slightly more particles in it, and so slightly more mass. Because it had more mass, all the rest of the particles, all the hydrogen and other atoms, would have been attracted towards it. The majority of the atoms therefore collapse towards the center. As they collapse, the particles gain energy (they are essentially falling towards the center) until there’s enough energy for fusion to begin between the hydrogen atoms, and that’s basically what a star is. That’s how our sun formed.

However, not all the particles collapse to the center. If they have enough speed, particles would not have collapsed, but would have orbited the sun instead. As an example, space rockets work because they have enough speed so that, rather than falling back to earth, they orbit instead. So around the new star, which contains most of the mass, there’s an orbiting disc of material.

Gravity comes into play again here. The bits of the disc that have the most mass begin to pull in the surrounding material, because they have more mass and so attract surrounding matter. Again, material collapses toward the center of each, this time forming the 8 planets of our solar system.

Initially, the earth would have been a big molten ball containing all the elements. Most of the hydrogen and helium would have burned away at this point. The heavier elements, like iron, would sink to the middle, forming the core, and around it the mantle. On the outside, which is the coolest part, the material becomes completely solid and brittle, which is the crust, where we live. The crust is only about 30km thick, which is a tiny part, when you think the earth has a radius of about 6000km. Below the crust is the mantle, which is sort of semi-solid, not molten, but not completely solid either.

In summary, the formation of the earth is all about the power of gravity, pulling together an inter-stellar dust cloud to form the sun, and the planets orbiting around. I hope that answers your question. Please comment if you would like more information.