How do we know the age of the earth? What about the age of the solar system?
The Earth is 4,54 billion years old. This age has been determined with the radioactive dating technique. The precise decay rate of radioactive elements is used as a clock: the number of daughter products in one rock indicates its age.
The oldest meteorites ever dated in the Solar System are 4,56 billion years old, the oldest minerals on Earth are 4,4 billion years old, and the oldest rocks on Earth are 4 billion years old. These ages are very consistent because the meteorites had to form before the accretion of our planet, and the Earth had to cool down before the first minerals could crystallise.
The Solar System was formed around 4.6 billion years ago, out of the collapse of a dense cloud composed of dusts and gases. Meteorites, which are the very components of our planets (through the process of accretion), are the remnants of the Solar System’s origins. Dating meteorites thus allows us to give a lower age to the Solar System (4,56 billion years old).
Lead isotope isochron that Clair Patterson used to determine the age of the solar system and Earth (Patterson, C., 1956, Age of meteorites and the earth: Geochimica et Cosmochimica Acta 10: 230-237). The animation shows progressive growth over 4550 million years (Myr) of the lead isotope ratios for two stony meteorites (Nuevo Laredo and Forest City) from initial lead isotope ratios matching those of the Canyon Diablo iron meteorite. Courtesy of Wikipedia.