Scientists estimate that the Earth is about 4.5 billion years old, based on radioisotope dating techniques. To understand how this process works, you need to know a little bit about atoms and isotopes.
Often, any one atom has several different forms, called isotopes. Atoms are made up of electrons, protons, and neutrons, and the number of electrons and protons determines the type of atom. Hydrogen, for example, has one electron and one proton. Sometimes, it also has a neutron, in which case it is called deuterium. Heavy water refers to water in which each hydrogen atom has a neutron.
Some isotopes, like deuterium, are stable; they're perfectly happy with the number of electrons, protons, and neutrons they have. Other isotopes are unstable because the different number of neutrons interacts with the other atomic components in such a way that, over a period of time, the isotope changes into some other atom. When these unstable isotopes change to a different atom, they emit radioactivity. For that reason, they're called radioisotopes.
An important property of radioactive isotopes is the half-life — the time it takes for half of the atoms to undergo the transition from one atom to the other. In the first half-life, half the atoms make the transition. In the second half-life, half of the remaining atoms transition, leaving one quarter of the original parent material. In the third half-life, half again transition, and so on.
To determine the age of material, researchers compare the ratio of the parent and daughter products that were initially in the sample with the ratio of these products at the current time. By doing so, they can calculate how much time has passed.
Numerous radioactive isotopes exist. One system that has been very successful in dating the ages of fossils is potassium-argon dating. Potassium is an extremely common element. Although most potassium isotopes aren't radioactive, one of them is, and one of its decay products is the gas argon.
Although potassium is a solid, argon is a gas. When rock is melted (think lava), all the argon in the rock escapes, and when the rock solidifies again, only potassium is left. The melting of the rock and releasing of any argon set the potassium-argon clock to zero. As time passes, argon accumulates in the rock as a result of radioactive potassium decay. When scientists analyze these rocks and compute the ratio of argon to potassium, they can determine how long it's been since the lava cooled. When scientists date rocks from our planet this way, the oldest dates they find are 4.5 billion years.
By dating the lava flows above and below a fossil find, scientists can put exact boundaries on the maximum and minimum age of that fossil.
With radioactive dating, scientists can now get within a few percentage points of the actual date. They know this because they have been able to accurately date lava flows that happened recently enough for their dates to be known historically, such as the eruption of Mount Vesuvius at Pompeii.
