In 2008, astronomer Richard Kowalski looked into a telescope in Tucson, Arizona, and spotted an asteroid called 2008 TC3 barreling through the Earth’s atmosphere. Nineteen hours later, it exploded above Sudan, breaking into 600 meteorites that spread across the Nubian Desert. Many of the recovered meteorites, known as the Almahata Sitta, have been classified as ureilites — rare, stony meteorites that can contain nanodiamonds. On Tuesday, scientists revealed that some of these diamonds are evidence of a “lost planet” from the early days of the solar system.

Their findings, presented in Nature Communications, support the theory that large protoplanets, ranging from the size of the moon to that of Mars, served as the building blocks of the terrestrial planets we know today.

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Relics of these large protoplanets have never been found before, though some scientists consider the asteroids Ceres, Pallas, and Vesta to be remnants of surviving small protoplanets in the inner solar system.

“This study provides convincing evidence that the ureilite parent body was one such large ‘lost’ planet before it was destroyed by collisions,” the study authors write. “Although this is the first compelling evidence for such a large body that has since disappeared, their existence in the early solar system has been predicted by planetary formation models.”

asteroid, Nubian Desert
A fragment of 2008 TC3 in the Nubian Desert of Sudan.

Previous theories had already posited that ureilites could be potential remnants of protoplanets that, through collisions and radioactive decay, led to the creation of terrestrial planets like Earth and Mars. Previous examinations of ureilites, however, couldn’t prove that was the case.

In the new study, the researchers focus on the diamonds inside the ureilites to uncover evidence of these ancient planets. Because terrestrial diamonds trap minerals that reveal information about the environment in which they form, the scientists behind this study figured the meteorite’s diamonds could still hold clues to the past. It seemed reasonable that the diamonds inside the ureilite pieces would hold invaluable information about the size and composition of the larger entity from which they came.

Using techniques called transmission electron microscopy (TEM) and electron energy-loss spectroscopy, the team examined the meteorite’s kinetic energy, distribution of electrons, and inner structure of the crystal inclusions. The composition and mineralogy of the samples revealed that the diamonds must have formed at pressures above 20 gigapascals — a huge amount of pressure that “can only be explained” by the fact that these diamonds formed within a protoplanet that was in the Mercury-Mars size range.

meteroite
A different urelite meteorite, NWA 4231.

This discovery adds concrete evidence that ureilites, as the authors write, originated “from the mantle of the ureilite parent body that was disrupted following an impact in the first 10 million years of the solar system.” The scientists reason that protoplanets were common during the days of the early solar system and either accreted to form larger planets, collided with the sun, or were ejected altogether from the solar system. The protoplanet that the Almahata Sitta came from must have been catastrophically disrupted by a huge impact — the sort of violent collision like the kind that might have) created the moon.

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