The amount of time it takes for an unstable isotope to decay is determined statistically by looking at how long it takes for a large number of the same radioactive isotopes to decay to half its original amount.
We can get absolute ages only if we have rocks from that surface.
For others, all we are doing is getting a relative age, using things like the formation of craters and other features on a surface.
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.
When we age date a planet, we are actually just dating the age of the surface, not the whole planet.
The biggest assumption is that, to first order, the number of asteroids and comets hitting the Earth and the Moon was the same as for Mercury, Venus, and Mars. The bottom line is that the more craters one sees, the older the surface is.
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?
These differing atoms are called isotopes and they are represented by the sum of protons and neutrons in the nucleus. Carbon has 6 protons in its nucleus, but the number of neutrons its nucleus can host range from 6 to 8.
We thus have three different isotopes of carbon: Carbon-12 with 6 protons and 6 neutrons in the nucleus, Carbon-13 with 6 protons and 7 neutrons in the nucleus, Carbon-14 with 6 protons and 8 neutrons in the nucleus.
Earth) and what could happen to Earth in an extreme case, etc.
From Wikipedia, radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation.
On the other hand, the number of neutrons that can be contained in the nucleus can vary.