The Origin of The Solar System

Alex (telephone caller): What makes the Earth rotate on its axis?

Mike - The solar system formed about 5 billion years ago. When a star forms, material collapses into the centre to form a star. Around it, it forms a planetary disc. That disc fragments due to gravity. Bits coalesce which have more gravity, and you get planets and planetessimals forming. When the bits collapse to form the Earth, the Earth rotates because the disc was rotating already. If you have ever seen an ice skater with arms out, she spins quite slowly. When her arms are drawn in, she spin much faster. This is called conservation of angular momentum. Even though this disc was rotating, it began to spin in a similar way to when an ice skater draws her arms in.

Chris - Why do they all rotate in the same direction?

Mike - Because the disc was spinning around the sun in a particular direction, the rotation of the planets follow the direction of the disc. Uranus is different because it's on its side. It's thought that some interactions after the formation of Uranus moved it on its side - perhaps a big collision.

Kat - The solar system is surprisingly flat. Why?

Mike - A good picture of the solar system as its being form is like a fried egg with the yolk being the sun. The flat disc rotating around the sun fragmented and formed our planets.

Kat - Is the Milky Way - our galaxy - rotating as well?

Mike - Rotation is ubiquitous in astrophysics, and the reason is conservation of angular momentum. You have large clouds of gas that collapse under gravity to form either galaxies on very large scales or, within galaxies, stars or even planets. As the material falls in, there's no reason for its angular momentum to change because there is nothing to change its direction (called 'torque').