A zircon crystal embedded in sandstone found on a sheep ranch in Australia is the oldest piece of the Earth's crust to be discovered, shedding new light on our planet's formation.

The zircon, described in the journal Nature Geoscience, is about 4.4 billion years old and smaller than a single grain of rice. But the tiny crystal carries an outsize significance: It is evidence that by that point in its history, Earth had gone from a superheated ball of molten rock to a congealed surface eventually capable of supporting life.

"One of the main goals of the space program is to understand if there's life elsewhere in the universe," said John Valley, a University of Wisconsin professor who led the study.

By studying how the conditions of life came together on our planet, scientists believe we will learn what to look for on other planets. But the earliest rocks and first evidences of life have been subject to dispute over the years. Until now.

The age of the zircon described by the Valley team does not appear to be in dispute. The team used a new technique called atom-probe tomography, which allowed them to confirm the accuracy of the crystal's age. The new instrument, made in Wisconsin, is so sensitive that researchers were able to identify the atomic number and mass of each atom in the sample.

Samuel Bowring of the Massachusetts Institute of Technology said, "It's only one grain, mind you, but it's very significant."

Dating of the zircon helps clarify an early chapter in Earth's history. Scientists have theorized that one of the crucial early events occurred when an asteroid about the size of Mars struck a glancing blow to the Earth, vaporizing the mantle and crust. Dust from the collision merged to form the moon. The energy from the collision transformed the surfaces of Earth and moon into oceans of molten rock. Zircon was one of the minerals formed when the planet cooled. "We like to say that zircons are forever," Valley said. "They really persist in the rock record."

Milwaukee Journal Sentinel