Asked by Dan Hampton
The Sun and the planets are staying approximately the same distance apart and have been in roughly the same places for the last several billion years. Planetary orbits are a fine balancing act between momentum and gravity. As the planets fly through space they want to move in a straight line but the gravity from the Sun attempts to pull the planets towards it, which curves their paths. The planets currently lie in a perfect balance that results in each planet moving fast enough to not be pulled closer to the Sun, but not too fast that it moves away from the Sun and out of the Solar System.
Answered by Josh Barker from the National Space Centre