The bits that make up Earth apparently took their time pulling themselves together. New research hints that our home didn't form as a fully-fledged planet until 70 million years after its currently accepted birth date, making the planet younger than scientists believed. The evidence appears in Nature and looks at the Earth's "accretion"--the swirling together of gas and dust that formed our planet. Researchers previously believed that the Earth's accretion was a fairly steady process, happening in about 30 million years, but this study suggests that Earth took a lot longer to form.
“The whole issue hinges on working out how long it took for the core of the Earth to form, which is one of the big unknowns in this area of science,” said Dr. John Rudge, one of the authors at the University of Cambridge. “One of the problems has been that scientists usually presume Earth's accretion happened at an exponentially decreasing rate. We believe that the process may not have been that simple and that it could well have been a much more staggered, stop-start affair.” [The Telegraph]
Specifically, the scientists compared isotopes in our planet's mantle with those found in meteorites, which are as old as the solar system. The researchers used meteorites as samples of our embryonic planet's materials, and by comparing the isotopes in these building materials to the final product--the earth's mantle--they could make several computer models to determine how the planet formed. After looking at models using different isotopes, the researchers believe that the planet had one great growth spurt (sticking together about two-thirds of the Earth's current mass) followed by a period of long slow growth. They say the formation could have ended with a walloping by a planet-sized chunk of materials that gave us the last of our mass and also broke off a chunk to form the Moon.
"If correct, [this model] would mean the Earth was about 100 million years in the making altogether," Dr. Rudge said. "We estimate that makes it about 4.467 billion years old--a mere youngster compared with the 4.537 billion-year-old planet we had previously imagined." [BBC]
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