by Robert Sanders
A new and detailed picture of how the Sun and planets formed from a swirling mass of gas and dust also explains the puzzling composition of the most common meteorites, the chondrites, which date from the formation of the solar system some 4.5 billion years ago.
Ever since the first meteorite was cracked open over a hundred years ago astronomers and meteoriticists have puzzled over the paradoxical mix of minerals-some heated to high temperatures and others formed at cold temperatures.
Based on a theoretical model of how clouds of gas and dust condense into stars and planets, astronomer Frank Shu and colleagues Typhoon Lee and Hsien Shang propose that stars like the Sun recycle some of the dust falling into the star, throwing it out from the center in a fiery spray that seeds the colder matter with small "chondrules" or beads of melted rock.
Chondrules eventually coalesced with the remaining cold matter in the planetary disk to form asteroids, which are thought to have aggregated into the planets.
The theory predicts that chondrules of only a certain size, ranging from a millimeter to about a centimeter, would fall back into the disk, in agreement with the size of chondrules found in the most common types of meteorites.
The theory has broader implications, since the formation of large asteroids and planets may not have been possible without these droplets of melted star dust in the early planetary nebula. Plus it explains why the Earth and many asteroids are deficient in certain elements.
"This is the first theory to explain all the disparate features of chondrites, such as the narrow range of sizes of chondrules and why their composition is different from the Sun," Shu says. "And because planets formed from chondritic asteroids, it also explains the composition of the planets."