Asteroids formed early on in solar history.

Bits of dust and gas gathered into clumps, and boulder-size bodies careened into each other, forming asteroids and ultimately planets. A lot went on in the infant solar system, and new research suggests that some of it happened in a hurry.

A study published this week supports theoretical models in which some planetesimals--asteroids and other building blocks of planets--had already formed, heated up, and partially cooled a mere 5 million years after the birth of the solar system. The analysis identifies the radioactive isotope aluminum-26 as the heat source that melted these primitive rocks.

A highly accurate method for radioactive dating of meteorites underlies the findings. Gopalan Srinivasan and his colleagues at the Physical Research Laboratory in Ahmedabad, India, describe their work in the May 21 SCIENCE.

The researchers focused on a meteorite called Piplia Kalan, named for the village in western India where it crashed 3 years ago. The rock belongs to a group of meteorites known as eucrites and considered to be fragments chipped from the giant asteroid 4 Vesta.

To study processes in the early solar system, Srinivasan and his colleagues needed to trace a radioactive isotope with a half-life shorter than 1 million years, so they chose aluminum-26. Theorists have long suspected that the heat emitted by this isotope could have melted some of the solar system's first solid bodies.

Srinivasan and his colleagues wanted to determine when in the solar system's history a fragment of Piplia Kalan had cooled enough to solidify. To do so, they compared the rock's aluminum-26 abundance with that in the solar system's oldest known solids, primitive grains found on some meteorites. Although the aluminum-26 present in the early solar system has by now disappeared, researchers can infer its presence by measuring its decay product, magnesium-26.

The team calculated that the fragment of Piplia Kalan they analyzed contained about one-hundredth the abundance of aluminum-26 in the ancient grains. This result, along with the half-life of aluminum-26, allowed them to determine that the meteorite's parent asteroid--most likely Vesta--assembled, melted, and at least partially cooled within 5 million years of the solar system's formation.

As the third-largest asteroid known, Vesta would have taken a long time to even begin cooling and so must have formed considerably earlier than the 5-million-year benchmark.

That conclusion supports models that theorists have recently developed. They propose that bits of dust within the disk of material that surrounded the infant sun assembled "into planetesimals many kilometers across over a time interval of order 1 million years or less," says Jack J. Lissauer of NASA's Ames Research Center in Mountain View, Calif.

Alan P. Boss of the Carnegie Institution of Washington (D.C.) notes that the findings corroborate the results of previous, less precise radioactive-dating studies, which relied on isotopes of manganese and chromium. The presence of aluminum-26 in a meteorite whose parent asteroid is known to have undergone early melting "helps buttress the case for aluminum-26 having been a major source of energy for planetesimal heating," he adds.

Other eucrites show no evidence of aluminum-26. Harry Y. McSween Jr. of the University of Tennessee in Knoxville says these may have originated from parts of Vesta that took many millions of years to cool, so they didn't crystallize until long after the isotope had decayed.