Researchers at the University of Colorado Boulder have identified a lost protoplanet using a Sahara meteorite designated Northwest Africa (NWA) 12774. The analysis, published in Earth and Planetary Science Letters, suggests a massive world—potentially the size of Mars or the Moon—existed 4.5 billion years ago before colliding with another celestial body.
The study, led by Aaron Bell and published in Earth and Planetary Science Letters, represents a significant shift in how cosmochemists interpret the chemical signatures of the 68 known angrites. Bell’s team focused on the specific chemical nuances that had previously been overlooked in the broader study of this rare group.
The Angrite Mystery
Angrites are the outliers of the meteorite world. Out of more than 80,000 meteorites discovered on Earth, only 68 belong to this rare group of volcanic rocks. These stones are among the oldest in the solar system, forming within a few million years after the system began its assembly roughly 4.56 billion years ago.
For years, the scientific consensus on Angrites was limited by their chemistry. Unlike Earth or Mars, they contain very little silicon dioxide, a primary building block of almost all known terrestrial planets. This chemical anomaly led researchers to believe that Angrites must have originated from small asteroids with a radius of less than 200 kilometers.
The assumption was simple: without the mass of a planet, you don’t get the pressure required for certain geological processes, but you can still get the specific volcanic chemistry seen in these rare stones.
The distinction between angrites and other basaltic meteorites lies in their extreme depletion of silica. While most volcanic rocks on Earth or Mars are silica-rich, angrites are characterized by high levels of calcium and magnesium alongside their low SiO2 content. These rocks represent an extremely early stage of planetary differentiation, occurring within the first few million years of the solar system’s formation. This specific chemical signature led previous researchers to conclude that the parent bodies were small, differentiated asteroids. Under those previous models, the lack of sufficient mass meant the objects could not have generated the high-pressure environments required for advanced mantle differentiation, effectively capping their estimated radius at less than 200 kilometers.
High-Pressure Evidence in NWA 12774
The discovery changed when a team at the University of Colorado Boulder examined the specific composition of Northwest Africa (NWA) 12774. They found clinopyroxene, a mineral crystal frequently found in Earth’s crust and mantle.
The clinopyroxene in this specific sample was exceptionally rich in aluminum. This is a chemical fingerprint of extreme pressure. According to the study published in Earth and Planetary Science Letters, the formation of this aluminum-rich crystal required a minimum of 17.5 kilobars of pressure.
To put that figure in perspective, that level of pressure does not exist even in the Mariana Trench. It requires the crushing mass of a planetary interior.
To achieve these results, the researchers employed electron probe microanalysis (EPMA) to perform high-resolution elemental mapping of the NWA 12774 sample. This allowed them to measure the precise weight percentages of various oxides within the clinopyroxene crystals. The calculation of 17.5 kilobars is derived from thermodynamic modeling of this specific aluminum-to-silicon ratio. In mineral physics, the amount of aluminum that can be incorporated into the clinopyroxene lattice is a direct function of the ambient pressure, meaning the high aluminum concentration serves as a geobarometer—a physical record of the depth at which the rock originated. A pressure of 17.5 kilobars corresponds to depths found deep within a planetary mantle, far exceeding the capabilities of any known asteroid.
The Fate of the Lost Protoplanet
The pressure readings prove that NWA 12774 did not come from a small asteroid. Instead, it is a fragment of a massive protoplanet. This world was likely as large as the Moon or even Mars.
“It is incredible to imagine that there was once a world of this size. We only know that it existed because a few fragments of it happened to land on Earth. These meteorites preserved evidence of a completely different evolutionary path for early planets.” Aaron Bell, Assistant Professor of Geosciences at the University of Colorado Boulder
This protoplanet did not survive the chaotic early years of the solar system. The most likely scenario is a catastrophic collision with another celestial body, which shattered the world into fragments. A few of those shards eventually drifted through space to land in the Sahara.
The existence of this lost world challenges the linear narrative of planetary formation. If a Mars-sized body could form and then be completely erased, the early solar system was far more volatile than previously modeled. It suggests that the current layout of planets is not the only version of “success” in planetary evolution, but rather the result of a violent winnowing process.
This finding shifts the scientific understanding of the early solar system from a relatively orderly accretion process to one defined by high-energy, stochastic events. The existence of a Mars-sized protoplanet that was subsequently destroyed suggests that the “building blocks” of the solar system were subject to intense competitive destruction. The study places these findings within the broader context of the “planetary embryo” stage, where numerous large-scale bodies were in constant competition for mass. This implies that the current terrestrial planets are merely the survivors of a much more crowded and violent era of planetary assembly.
The discovery of NWA 12774 transforms these 68 rare Angrites from geological curiosities into the only surviving witnesses of a dead world. The next phase of research will likely focus on whether other “lost” planets left similar fingerprints in the rare meteorite samples currently stored in labs worldwide.
