Scientists Find Meteorite Evidence of Moon-Sized Lost Protoplanet from 4.5 Billion Years Ago

Approximately 4.5 billion years ago, a massive celestial body, possibly comparable in size to the Moon or even Mars, orbited our Sun. This world, however, did not last long as it collided with another cosmic object and fragmented into scattered pieces across space.

Scientists now present the first conclusive evidence of this missing "protoplanet" in a study published in the journal Earth and Planetary Science Letters.

The revelation of this mysterious world did not come from advanced spacecraft or giant telescopes but rather from a small rocky fragment discovered in the Sahara Desert, specifically in Mauritania.

This meteorite, named Northwest Africa 12774 (NWA 12774), belongs to a very rare group known as angrite meteorites. Out of more than 80,000 meteorites found on Earth, only 68 are classified as angrites.

Angrite meteorites are notable for being among the oldest volcanic rocks in the solar system, having formed just a few million years after the solar system's birth 4.56 billion years ago. More intriguingly, their chemical composition has puzzled scientists for a long time. These meteorites contain an extremely low amount of silica (silicon dioxide), which is the primary component in nearly all known rocky planets such as Earth and Mars.

Due to this unusual composition, scientists had long assumed that angrites originated from small asteroids with radii no greater than 200 kilometers.

However, a team led by Aaron Bell from the University of Colorado Boulder conducted a detailed analysis of this meteorite and discovered that it contains a mineral crystal called clinopyroxene, a common mineral in Earth's crust and mantle. What caught their attention was that this crystal was highly enriched in aluminum, a clear scientific indicator that the rock formed under immense pressure deep within a planetary body.

When the researchers calculated the pressure required to form such an aluminum-rich crystal, they determined it must have been at least 17.5 kilobars. For context, the pressure at the bottom of the Mariana Trench, Earth's deepest ocean point, does not exceed one kilobar. Such an extraordinary pressure level could not exist inside a small asteroid. Their calculations indicated that the parent body of these meteorites must have had a radius of at least 1,000 kilometers.

The evidence extended further. Researchers observed that the crystals within the meteorite still retained sharp edges and very precise chemical patterns. If these crystals had formed deep under extreme pressure, those delicate edges would have been destroyed or faded. This suggests the crystals actually formed at relatively shallow depths within the parent body, leading to a remarkable conclusion: for these crystals to form near the surface, the parent body itself had to be much larger than previously estimated.

Based on these findings, the team concluded that the lost protoplanet likely had a radius exceeding 1,800 kilometers, making it roughly the size of the Moon and potentially comparable to Mars, which has a radius of 3,300 kilometers.

The fate of this large world remains uncertain. Scientists believe a catastrophic event early in the solar system's history shattered it into fragments. Some of these fragments then contributed to the formation of terrestrial planets, including Earth.

Aaron Bell emphasized that "the materials that formed this parent body differ fundamentally from the components of Earth and Mars, indicating a separate and distinct evolutionary path in planetary formation during the early history of our solar system."