As far as we know, there is no reason why the solar system is arranged exactly as it is. It might have been arranged differently, just as there are other solar systems in the universe arranged differently.
This has to do with the way it originated. But man has discovered certain laws of nature that seem to keep the solar system in its present pattern. Earth, like the other planets, follows its path, or orbit, around the sun.
The period of time that the earth takes to go around the sun is called a year. The other planets have orbits larger or smaller than the earth. How this solar system came to be and how the planets came to have the size, location, and orbits they have, astronomers cannot fully explain.
But they have two main types of theories. One type of theory suggests that the formation of the planets was a part of the gradual change of the sun from a whirling mass of hot gas to its present size and brilliance.
The planets formed as small whirling masses in the giant gas and dust cloud as it turned. Another group of theories is based on the idea that at some time there was a near-collision between the sun and another star passing nearby.
Large pieces of the sun were pulled away and began to revolve around the sun at different distances. These are now planets. No matter which theory is right, the solar system came to be as it now is more or less by chance.
Why does it stay this way? Kepler’s Laws of Planetary Motion state that all planets travel about the sun in an elliptical (oval) path, that a planet moves faster in its orbit as it nears the sun, and that there is a relation between its distance from the sun and the time it takes to make an orbit.
Newton’s Law of Gravitation, of which Kepler’s three laws were an indispensable part, explained how two objects attract each other. So the solar system remains as it is because certain laws of nature maintain the relationship of the sun and the planets.
Read: How Big Is the Universe?