How Old Is Planet Earth?

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Abstract
Using a precise hafnium-tungsten "clock," three groups have recently revised the time table of the early events that led to Earth's formation. In his Perspective, Jacobsen explains how this isotopic clock works and what it has revealed about early Earth. Just 10 million years after the first grains formed in the solar system, Earth had already accreted about 64% of its mass. About 20 million years later, a Mars-sized impactor smashed into Earth, leading to the formation of the Moon. The data also shed light on the origin of the materials from which the solar system was made.