How old is the Earth?

About 4.5 billion years based on radio active dating using uranium and thorium isotopes. It could be a little bit older if it formed as a liquid mass, since this state resets nuclear isotope clocks which only date from the onset of a solid phase. The oldest meteorites are about 4.8 billion years old so that is probably the upper limit for the Earth too.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY Education and Public Outreach program.