If you have carefully observed the solar system model, you may have noticed that the sun, planets, satellites and asteroids are all approximately on the same plane. But why is this so? To answer this question, let us first go back to about 4.5 billion years ago, when the solar system was first formed. At that time, the solar system was just a huge, constantly rotating cloud of dust and gas, with a diameter of about 12,000 astronomical units (one astronomical unit is the average distance between the earth and the sun, which is about 150 million kilometers). Because the cloud is so large, even though it contains only dust…