Radiometric dating is a method of dating based on the rate of decay of radioactive isotopes present in all organic materials.

what is radioactive dating and what isotopes are used-61

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 age of the planet, though, was important to Charles Darwin and other evolutionary theorists: The biological evidence they were collecting showed that nature needed vastly more time than previously thought to sculpt the world.

A breakthrough came with the discovery of radioactivity at the beginning of the 1900s.

When we age date a planet, we are actually just dating the age of the surface, not the whole planet.

We can get absolute ages only if we have rocks from that surface.

From Wikipedia, radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation.

This decay, or loss of energy, results in an atom (element) of one type, called the parent nuclide transforming to an atom of a different type (another element or another isotope of the same element), named the daughter nuclide.

For the others, one can only use relative age dating (such as counting craters) in order to estimate the age of the surface and the history of the surface.