Lesson 22: The Age of the Solar System
In this lesson, we will learn about how radiometric dating works and
see how it has been used to study the age of our solar system.
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.
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.