How Come Scientists Have Been Able To Work Out Exactly How Old The Earth Is?

The age of the Earth can never be known 'exactly' but we can make a very good and accurate estimate based on the evidence available. Scientists today believe the Earth to be 4.5 billion years old and they use information from isotopes to make their calculations. Some elements exist in slightly different forms called isotopes and some of these are radioactive – they decay to form the non-radioactive form of the element.

This decay happens at the same rate all of the time and, if we know the decay rate of an isotope, it is possible to date the age of a specimen of it by working out the proportion of remaining radioactive isotopes to decay products. Isotopes of uranium and lead are particularly useful for this purpose.

By studying minerals from Earth and rocks brought back from the moon and meteorites, it is possible to work out the age not only of the Earth, but of the Solar system as well.