- Radiometric dating
- Earth rocks, moon rocks, and meteorites


[SLIDE 1]

Studying the oldest rocks in our solar system indicates that the planets began to form through accretion just over four-and-a-half billion years ago.

How did we arrive at this number? A process called radiometric dating allows scientists to measure a rock's age by studying the proportions of various atoms and isotopes in the rock.

Specifically, radiometric dating looks for the presence of radioactive isotopes in a rock, which are isotopes that decay over time. Take, for example, the radioactive isotope Potassium-40, which is a natural ingredient of many minerals in rocks. Over time, Potassium-40 gradually decays and turns into Argon-40.

The amount of time it takes for half of the Potassium-40 to turn into Argon-40 is known as its "half-life". Each radioactive isotope has its own half-life, and this decay always occurs at the same, measurable rate. For Potassium-40, the half-life is 1.25 billion years, so if a scientist finds an equal amount of Argon-40 and Potassium-40 in a rock, then he can estimate that it is about 1.25 billion years old.


[SLIDE 2]

Radiometric dating allows scientists to determine how long ago a rock solidified, which is not necessarily the same as the age of the planet. For example, many rocks on Earth are relatively young while the oldest date to about four billion years ago. The age of the planet itself must be older than this.

Rocks that have been brought back from the Moon and tested have been dated to be about 4.4 billion years old, which must still be younger than the Moon itself.

To find the age of the solar system, scientists have tested meteorites, many of which appear to have remained unchanged since they accreted in the early solar system. The oldest meteorites appear to be about 4.56 billion years old. Scientists have therefore estimated that Earth and the other planets formed by about 4.5 billion years ago.