The debate focuses on a group of primitive meteorites called enstatite chondrites. Because the ratio of different forms (isotopes) of oxygen in materials from Earth and the moon matches that of enstatite chondrites---but no other meteorites---it had been proposed that Earth formed from materials much like the enstatite chondrites.

These materials were thought to have come from a very localized region of the solar system. But other studies, such as simulations of how the planets formed, have cast doubt on the theory.

Der-Chuen Lee, a research fellow in the U-M Department of Geological Sciences, and Prof. Alex N. Halliday of the Swiss Federal Institute of Technology's Institute for Isotope Geology and Mineral Resources in Zurich, Switzerland, used a recently developed method that compares extremely small differences in tungsten isotope ratios.

If Earth and the moon did form from material similar to enstatite chondrites, their tungsten isotopic compositions, as well as their oxygen isotopic compositions, should match.

"To our surprise, enstatite chondrites do not have the same tungsten isotopic compositions as Earth," nor do they match any other meteorites that have been tested, says Lee.

Based on this evidence, "we propose for the first time with much certainty that there is no direct genetic relationship between Earth and enstatite chrondrites."

Instead, the data support the alternative idea that planets, as they formed, received materials from very broad regions of the solar system.