Meteorites Provide Clues to Planetary Birth

Author:  Jessica Kloss
Institution:  Princeton University
Date:  December 2008

The study of the origin of planets is appealing for many reasons; after all, understanding the origins of Earth makes it easier to find other systems likely to form Earth-like planets. But since it is unlikely to catch a planet in the process of forming, researchers in this field require a certain amount of creativity, always needing to find new ways to "observe" planet formation billions of years after the fact. Benjamin Weiss, associated with the Department of Earth, Atmospheric and Planetary Sciences at MIT, found such a way. With a fascinating study of meteorites, he and his five co-authors are giving answers to long-standing questions about the very early solar system.

In an innovative new approach to planetary science, Weiss and his team used three pieces of meteorites known as angrites, among the oldest rocks known, to trace the magnetic fields that existed during the formation of the solar system. Like old disk drives, meteorites hold information about past magnetic fields that scientists can read with the right tools.

With this method scientists can look back to the formation of the solar system, when the Sun was surrounded by a disk of dust and rubble that collided and stuck together, until eventually, the planets were formed. Until recently, these "planetesimals," or rocks that combined to form the planets, were believed to be "just homogenous, unmelted rocky material, with no large-scale structure," as Weiss says. It was previously believed that once these rocks combined together and became sufficiently large, such as planet-sized, they would "melt" and the constituents of the rock would separate out, forming a molten iron core and a silicate crust.

But now, researchers have made the surprising discovery that rocks much smaller than planets, just 160 kilometers across, could "melt" almost completely. This means that the constituents of these smaller rocks could separate and form a molten iron core and a silicate crust, and basically become like miniature planets, complete with their own magnetic fields. The meteorites, which are the remaining fragments of these molten rocks, showed that the magnetic fields of these planitesimals were up to 20 to 40 percent stronger than the Earth's magnetic field today – a far cry from the previously expected cold, magnetically neutral planetesimals.

The realization that some of these rocks already had molten cores when they slammed together to form planets could "significantly change our understanding" of the processes that took place in the early years of the nascent planets, Weiss says. For instance, the distribution of minerals in the Earth's crust, mantle, and core might be different from what was previously thought.

In any case, it's an exciting time to be a planetary scientist. "In the last five or 10 years, our understanding of the early history of the solar system has undergone a sort of mini-revolution, driven by analytical advances in geochemistry. In this study we used a geophysical technique to independently test many of these new ideas," Weiss explains. In light of these advances, more exciting discoveries are surely on the way.

Written by: Jessica Kloss

Edited by: Jeffrey Kost

Published by: Hoi See Tsao