It was about 15 billion years ago, when the Universe began to form. The Earth was formed much later, about 4.5 billion years ago. We have come to know of this by studying rocks in meteorites, which landed on Earth from outer space. Meteorites are lumps of rocks which were formed at the same time as the Earth. The Earth's age has also been calculated from the rate at which elements of the radioactive metal uranium decay into lead.
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