The planets in our solar system started out some 4.5 billion years ago as dust and gas in a protoplanetary disk surrounding the infant Sun. Radioactive dating of meteorites and other rocks indicates that Earth grew to its present size over a period of some 100 million years. (See the article by Bernard Wood, Physics Today, December 2011, page 40.)
Ironically, the much larger solid cores of the gas-giant planets had to have formed much more quickly. Looking around the Milky Way at young star-forming regions, astronomers have found that gas disks surrounding young stars dissipate within the first 1 million to 10 million years. The cores of gas giants had to grow large enough—to 10 Earth masses—to be able to sweep up that nebular gas in time.
In the traditional picture of planet formation, dust in the protoplanetary disk clumps up to form pebbles that in turn...
