[Dusan]

Dusan asked the Naked Scientists:
   
What is the factor that propels the Earth and other planets to orbit around the sun and what propels moons orbit around their planets? And why is the speed of the orbiting of planets and moons always the same?

What do you think?

[Mr. Scientist]

Gravity.

Study Newtons equations for a
classical approach, which is also approximately correct. If you don't know any, i can give you a few examples.

[LeeE]

Our solar system and everything in it was created from the debris of an earlier supernovae explosion.  We know this from the presence, in the solar system and on Earth, of heavy metals, such as Gold, Lead and Uranium, which can only be created inside stars.  These heavy metals can only be 'liberated' to subsequently turn up in a planet when the star explodes in a supernovae explosion and blasts itself to pieces.

When this primeval supernovae occurred the debris from the explosion, including the heavy metals, was blasted off in to space, mostly in the form of dust and gas.  Over a very long time though, as the debris cloud drifted through space, it started to collapse upon itself due to its gravity.

Initially, all the dust and gas would have been moving in random directions as they started to orbit their mutual center of gravity, but over time the randomness gets balanced out as molecules of gas and particles of dust collide with each other.  However, all the random directions will never exactly cancel, because being random they're not exactly balanced, and you'll end up with an overall direction of rotation.  Once everything has finally settled down, with the Sun at the center of the solar system, and containing most of its mass, the remaining dust and gas that condenses into the planets and moons etc. will be orbiting in the same direction.

When a relatively small body orbits a relatively massive body, the speed that the small body orbits at just depends upon its distance from the massive body, so for example, Mercury, which is closest to the Sun, takes only 88 days to go around the Sun, whilst the Earth, which is further away takes 365.25 days.