This is a question to which we may never have the exact answer. Man has wondered about the age of the earth since ancient times, and there were all kinds of myths and legends that seemed to have them. But he couldn’t begin to think about the question scientifically until about 400 years ago.
When it was proven at that time that the earth revolved around the sun (in other words that the earth was part of our solar system), then scientists knew where to begin. To find the age of the earth, it was necessary to explain how the solar system was born. How did the sun and all the planets come into being?
One theory was called the nebular hypothesis. According to this theory, there was once a great mass of white-hot gas whirling about in space and getting smaller and hotter all the time. As the gas cloud grew smaller, it threw off rings of gas. Each of these rings condensed to form a planet, and the rest of the mass shrank into the center to become the sun.
Another explanation is called the planetesimal theory. According to this, millions and millions of years ago, there was a huge mass made up of small, solid bodies called planetesimals, with the sun at the center. A great star came along and pulled on the sun so that parts of it broke away. These parts picked up the tiny planetesimals the way a rolling snowball picks up snow, and they became planets.
Whichever theory is right, astronomers have figured out that it all probably happened about 5,500,000,000 years ago! But other scientists besides astronomers have tracked this question. They tried to find the answer by studying how long it took for the earth to become the way we know it. They studied the length of time it takes to wear down the oldest mountain, or the time needed for the oceans to collect the salt they now contain.
After all their studies, these scientists agree with the astronomers:
The earth is about 5,500,000,000 years old!