The age of solar system (including that of earth) has been estimated to be about 5000 million years by the radiometric dating technique which uses the phenomenon of radioactivity. This is described below.
The Age of Earth :
The earth contains various types of rocks. Some of these rocks contain a radioactive substance called uranium. This uranium is unstable and gradually decomposes to form a stable element lead. The rate at which the atoms of radioactive uranium decay to form non-radioactive lead is a constant which has been determined by experiments.
For example, it has been found that about one-third of the radioactive uranium atoms present in a sample of uranium rock decay to form lead in every 4.5 billion years (4.5 x 109 years). Now, by using a mass spectrograph, it is possible to find out the ratio of the amount of uranium to that of lead in a given earth rock. And knowing the overall rate of disintegration of uranium to lead, it is possible to calculate the age of rock (that is, it is possible to calculate the time for which the radioactive disintegration has been going on). This gives an approximate idea of the age of the earth. The assumption made in these calculations is that the rock being examined is as old as the earth. However, as there is reason to believe that the uranium rock may have been created a little later than the earth, so the age of earth obtained by this method is somewhat on the lower side.
The oldest known rock on the earth is in Greenland whose age has been estimated to be 3700 million years by the radiometric technique. This estimate of the earth’s age is said to be on lower side because many changes have taken place in earth since its formation which may have destroyed the original, more older rocks. Now, since the earth was formed at the same time as other planets, asteroids and meteorites, so a better estimate of the age of earth and hence that of solar system has been obtained by the radiometric dating of meteorites falling on the earth from outer space.