First Planets Lived Fast and Died Young

Rough neighborhood. Miniplanets, called planetesimals, enjoyed only a brief existence in the early solar system.

ScienceNOW Daily News
30 October 2008

Surprising findings from some of the oldest known meteorites suggest that our solar system was once chock-full of miniature planets, complete with metallic cores and rocky crusts. The findings could force a rethinking of how the solar system and its constituent bodies evolved.

Some 4.568 billion years ago, our sun and solar system condensed out of a primordial cloud of dust and gas. Within about 3 million years, small, rocky objects called planetesimals were circulating in the nascent solar system. Fragments of these planetesimals remain today as meteorites called achondrites, which scientists have pored over for clues to how planets formed. Oddly, the meteorites are magnetic, which is strange because the planetesimals were supposed to be just large agglomerations of rubble.

A U.S.-Canadian team took a new approach to the problem, testing samples of three well-preserved achondrite meteorites with an extremely sensitive magnetometer. What they discovered stunned them: The meteorites showed evidence of ancient magnetic fields similar to those of rocks formed on Earth within the planet's magnetic field. In other words, the team reports this week inScience, the 4.565-billion-year-old meteorites once were part of bodies that were either big enough or hot enough to produce central, molten, metallic cores.

"The meteorites, therefore, are essentially magnetic recording tapes," says planetary scientist and lead author Benjamin Weiss of the Massachusetts Institute of Technology in Cambridge. The magnetic fields that they recorded were probably generated by molten metal swirling around inside the planet's core like a giant, rotating dynamo, as happens on Earth. Although most asteroids now are rocky through and through, the new findings suggest that back at the beginning of the solar system even planetesimals could melt at their cores and retain a rocky crust. These bodies could be as small as 160 kilometers in diameter, the research suggests. The planetesimals, which eventually merged to form the rocky planets, were more planetlike than previously thought, with cores that must have formed and melted within just a few million years of the formation of the solar system, Weiss says.

The paper "makes a good case," says planetary scientist David Stevenson of the California Institute of Technology in Pasadena. Although dynamos are thought to require large planetary cores, it's possible that the molten cores of these small bodies may have churned vigorously enough to generate magnetic fields, he says.