Aluminum emerges as early timekeeper.

It isn't easy trying to determine when objects formed in the early solar system. What planetary scientists need is an accurate timekeeper, and a new study supports the notion that a short-lived isotope of aluminum fills the bill.

By analyzing some of the oldest known bits of rock, the scientists say, they have found strong evidence that aluminum-26 was widespread in the early solar system. The isotope's ubiquity, together with its half-life of only 730,000 years, gives researchers a means of clocking key events during the first few million years of the solar system's existence.

Because earlier objects would capture a higher proportion of this isotope, its relative abundance in asteroids and other rocky bodies provides a clue to when they were formed. Some of these bodies provided the building blocks for planets.

Scientists have known since the 1970s that significant amounts of aluminum-26 resided in some meteorites that have fallen to Earth. However, these ancient rocks, called carbonaceous chondrites, are rare, so astronomers had no guarantee that this aluminum isotope was widely distributed and bore witness to events throughout the newborn solar system.

In the new study, Glenn J. MacPherson of the National Museum of Natural History in Washington, D.C., and his colleagues measured the amount of magnesium-26-the isotope produced when aluminum-26 decays-in a far more common class of meteorites, the ordinary chondrites. They deduced that, early in the solar system's existence, the ratio of aluminum-26 to its stable sister isotope, aluminum-27, had been the same in ordinary chondrites as in the much rarer carbonaceous chondrites.

Their results indicate that aluminum-26 was common throughout the inner solar system in its early days, the team reports in the Aug. 9 Science.

The study "does seem to suggest that aluminum-26 was widespread," says Conel M. O'D. Alexander of the Carnegie Institution of Washington (D.C.).

In the ordinary chondrites, MacPherson's team studied both tiny, gray- white inclusions and small, silicate droplets known as chondrules. They found that a higher proportion of aluminum-26 appeared in the inclusions than in the chondrules. Inclusions and chondrules formed independently of meteorites and were later incorporated into them.

These findings suggest that chondrules formed from 2 million to 5 million years after inclusions appeared. This, in turn, indicates that the swirling disk of gas and dust that orbited the infant sun and provided the raw material for asteroids and planets might have lasted for at least 5 million years.

Thus, some asteroids, and perhaps planets, may not have formed until 5 million years after the sun's birth.

One perplexing aspect of this scenario, notes Alexander, is the proposed formation of inclusions and the earliest chondrules about 3 million years before asteroids. It's unclear, he notes, how these tiny amalgams of solid material could have remained within an orbiting disk of material for so long without getting dragged into the sun.