Planets were formed some time after the big bang. As the universe grew, it became cooler. This allowed gas and particles to start to collect together. Once a large amount of gas collects, the cloud starts to swirl around and a star is born in the centre. The rest of the cloud that now circles the new star eventually form planets that circle the star. The rounded shape is created because of gravity. Gravity compresses the particles in the planet, forcing them into a sphere.
Sodiumpolyacrylate – you have some good questions 🙂
The big bang was several billion years before the solar system was created, so it’s not really connected to the formation of the planets. Before the planets formed, the dust and material from the big bang has to collect into galaxies, and within those into solar systems, and within those into planets. It’s all driven by gravity – the force between matter that attracts matter to matter. It’s a weak force (you can jump, despite the gravity of the whole earth pulling on you), but it acts gradually and very slowly. It’s the same force that wears down mountains by pulling water and rain down them, even though it takes hundreds and thousands of years, and its the same force that moves glaciers of ice.
Big planets are smooth,because their gravity slowly pulls them into shape when their material is hot and can flow (the earth’s core is soft and molten – it’s a liquid). Even when the core seems solid to us, over millions of years it can still flow under the gradual pull of gravity. It’s a bit like “silly putty” – you can make a solid ball of it, but leave it on the shelf and it will flow into a pool because of the Earth’s gravity.
Interestingly asteroids, which are smaller pieces of material left over from the formation of the solar system, are bumpy. They are too small too have enough gravity to smooth them over time.
Someone once explained the process of dust and things pulling towards a central point like candy floss being collected on a wooden stick at a funfair or at the seaside. A little bit of candy floss sticks first, then attracts more to it as it becomes stickier, and before you know it, this swirling mass of sugar has formed into a big huge ball of the stuff, all around a wooden stick.
My astro friend laughed at my answer to how the Earth was formed, so I’d better not let him see my candy floss story! It does make sense though!