Many great thinkers throughout history have tried to figure out Earth’s age.
For example, back in 1862, Lord Kelvin calculated how long Earth might have taken to cool from its original molten state.
After the second half-life has elapsed, yet another 50% of the remaining parent isotope will decay into daughter isotopes, and so on.
For all practical purposes, the original isotope is considered extinct after 6 half-life intervals. A small portion of a meteorite is vaporized in the device forming ions.
Thus, although "extinct", these nuclides are present in meteorites, but produced by a more recent process.
"The idea that Rb-Sr is the most used chronometer for meteorites is largely based on work done 10-30 years ago.
Also, because Earth formed as part of our sun’s family of planets – our solar system – scientists use radiometric dating to determine the ages of extraterrestrial objects, such as meteorites.
These are space rocks that once orbited our sun, but later entered Earth’s atmosphere and struck our world’s surface.
Taken together, these methods give results that suggest an age for our Earth, meteorites, the moon – and by inference our entire solar system – of 4.5 to 4.6 billion years old.
But Earth’s layers of rock did not give up the secret of Earth’s age easily. However, from working with layer upon layer of rock laid down on Earth over long time spans, early 20th century scientists came to believe Earth not of atoms of one chemical element into another.
They led to the discovery that certain very heavy elements could decay into lighter elements – such as uranium decaying into lead.
The use of 14C in meteorite dating is solely based on its production by cosmic rays (and for terrestrial samples, with its production in the atmosphere).
26Al and some other nuclides not mentioned are also used in this way.