A team of scientists identified the first definitive evidence of a lost protoplanet which orbited the Sun at the dawn of the solar system before being destroyed in a catastrophic collision. The discovery, published in the journal Earth and Planetary Science Letterscame from an unexpected place: a meteorite fragment recovered in the sahara desert.
The key piece was the meteorite NWA 12774classified within a group known as angrites. These are volcanic rocks among the oldest in the solar system, formed just a few million years after it began to exist, approximately 4.56 billion years ago. Its rarity is extreme: of more than 80,000 meteorites discovered on Earth, only 68 belong to this category, indicated the University of Colorado at Boulder.
What has historically baffled the scientific community is the chemical composition of angrites. Unlike Earth, Mars and other rocky planets, these rocks contain very little silicon dioxide —silica—, a central ingredient in virtually all known terrestrial planets. Therefore, for decades it was assumed that angrites must come from small asteroids, with a radius of less than 200 kilometers.
The work of Aaron Bella research assistant professor in the Department of Earth Sciences at the University of Colorado at Boulder, completely reversed that hypothesis.
The pressure that revealed a lost world
When analyzing NWA 12774, Bell and his colleagues found a mineral called clinopyroxenecommon in the Earth’s crust and mantle. What was striking was that this clinopyroxene had an exceptionally high concentration of aluminum, an unequivocal sign that the rock was formed under enormous pressure, at great depth.
The researchers reconstructed the pressure conditions necessary for this chemistry to be possible and obtained a number that changed the picture: at least 17.5 kilobars. To measure it, the pressure at the deepest point of the Mariana Trench – the deepest place in the Earth’s oceans – is just around 1 kilobar. Such pressure is impossible to achieve inside a small asteroid.
The calculations indicated that the body from which the angrites came must have a minimum radius of 1,000 kilometers. But the analysis went further. The crystals inside the meteorite retained sharp edges and delicate chemical patterns that would have been erased if the rock had crystallized at great depth. This suggested that they formed in relatively superficial layers of the parent body, implying that this world was even larger.
Under that scenario, the angrites’ progenitor planet could have had a radius greater than 1,800 kilometerscomparable to that of the Moon, and possibly approached the size of Mars, whose radius reaches 3,300 kilometers.
«It’s incredible to think that a world that size ever existed,» Bell said. «We only know that it existed because some of its fragments reached Earth. These meteorites preserved evidence of a completely different path by which early planets developed,» he added.
It is unknown how that world ended. One possibility is that a catastrophic event in the early solar system destroyed it, and that its fragments later became constituent material of other terrestrial planets, including Earth.
Bell also warned that the discovery could be just the beginning. «There are many meteorites stored in drawers that have not been studied in depth, so there are probably more of these protoplanets that we don’t know about,» he said.
«The materials that formed the angrites parent body are fundamentally different from the ingredients on Earth and Mars. It points to a separate and distinct evolutionary trajectory in planetary formation during the early history of our solar system,» Bell said.
