This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable.
We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars.
This can be interpreted in two ways: why it is important to know the age of a planet or how is age dating important in determining the age of a planet?
Based on our study of meteorites and rocks from the Moon, as well as modeling the formation of planets, it is believed (pretty much well-established) that all of the objects in the Solar System formed very quickly about 4.56 billion years ago.
The third day you eat half again of what's left (leaving 1/8th), and so on until you finish the pie (or get sick of it).
If your mom walked in and saw 1/16th of a pie (and knew that you had been eating half of what was left every day), she could quickly determine that the pie was four days old. Some elements (uranium, for example) are unstable, and decay to a different element (uranium to lead, for example).