Most meteorites found on Earth are believed to be fragments of asteroids - ancient rocks that formed during the creation of the solar system about 4.56 billion years ago. Thousands of asteroids still orbit the Sun in the asteroid belt between Mars and Jupiter, about 140 million miles from Earth.

"Asteroids and meteorites are solids that never got incorporated into the planets. These objects have survived, unchanged, for 4.56 billion years," says physicist Anders Meibom, a postdoctoral fellow in the Stanford Department of Geological and Environmental Sciences who co-authored the Science study.

Chondrites and chondrules
Using electron microscopy and other laboratory techniques, Meibom and his colleagues conducted a detailed chemical analysis of two chondrites - primitive meteorites made up of thousands of tiny round particles called chondrules.

"Chondrules are among the oldest objects in the solar system, dating back to the birth of the Sun," says Meibom, "so when we look at chondrules, we're actually looking at the very first steps towards the creation of our solar system."

Meibom points out that most chondrules are made of silicates and metals that can only be produced at very high temperatures. Exactly how chondrules formed in the early solar system is a hotly debated topic among scientists.

"The conventional view," notes Meibom, "is that chondrules started out as dust balls in the asteroid belt region some 4.56 billion years ago. Today, the asteroid belt is ultra-cold, but at that time, the temperature was just below 700 degrees Fahrenheit. The dust balls melted after they were zapped by quick bursts of lightning or shock waves, which briefly raised temperatures to about 3000 degrees F."

According to this theory, as the melted particles cooled, they turned into millimeter-size chondrules, which eventually clumped together to form larger chondrites.

New theory
But in 1996, astronomer Frank Shu of the University of California proposed a different theory based in part on dramatic images from the Hubble Space Telescope, which - for the first time - allowed astronomers to witness the actual birth of new stars elsewhere in the Milky Way.

The Hubble revealed that most young stars are created from enormous disks of whirling gas and dust.

As the disk contracts, it rotates faster and faster, funneling tons of interstellar dust toward the center, where temperatures reach 3000 degrees F or more - hot enough to melt metal and vaporize most solids.

The rotating disk also produces enormous jets of gas capable of launching debris far into space at speeds of hundreds of miles per second.

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