A note on rocky planet formation

Bernard Wood presents a nice overview in his article, “The formation and differentiation of Earth” (PHYSICS TODAY, December 2011, page 40 ). For many years the prevailing model has been that rocky planets formed in our solar system as a consequence of a succession of impacts of large objects during the first 100 million years or so following the Sun’s formation. That picture is the outcome of theoretical considerations combined with the study of asteroids, meteorites, and the rocky planets—Mercury, Venus, Earth, and Mars. Only recently has it become possible to confirm or deny by observation whether the rocky-planet formation time scale of 100 million years generally applies for Sun-like stars.

A collision of large, rocky-planet embryos that orbit young stars would typically eject a mélange of rocky debris, as illustrated in Wood’s figure1 . Abundant dusty debris has now been observed in orbit around a handful of young, nearby stars with ages between 30 million and 100 million years. ¹ The dust temperature, generally somewhat greater than the temperature of Earth, suggests that the colliding objects typically orbit at a distance from their stars similar to the distance from our sun to Venus. By contrast, there are no known examples of stars between, say, 100 million and 1000 million years old that have large quantities of orbiting warm dust particles. The most straightforward interpretation of those observations is that rocky planet formation around solar-mass stars is pretty much complete by the time the stars are 100 million years old, which agrees with what theory would have predicted.

The same study indicates that rocky planet formation, in a zone analogous to the region of the rocky planets in the solar system, is common and perhaps nearly ubiquitous around Sun-like stars. ¹