Scientists think that the Earth is 4.54 billion years old.
Coincidentally, this is the same age as the rest of the planets in the Solar System, as well as the Sun.
How do scientists know Earth is 4.54 billion years old?
It’s actually difficult to tell from the surface of the planet alone, since plate tectonics constantly reshape its surface.
Since they decay in a very known rate, these isotopes can be measured to determine how long the elements have existed.
Of course, it’s not a coincidence; the Sun and the planets all formed together from a diffuse cloud of hydrogen billions of years ago.
In the early Solar System, all of the planets formed in the solar nebula; the remnants left over from the formation of the Sun.
Scientists assume that all the material in the Solar System formed at the same time.
Various chemicals, and specifically radioactive isotopes were formed together.