They discovered a mysterious planet in the solar system: it is the remains of a lost world

Four and a half billion years ago, a huge globe, probably the size of the Moon or even Mars, orbited our Sun before colliding with another celestial body and shattering into pieces.

Now, in a paper published in the journal Earth and Planetary Science Letters, scientists present the first definitive evidence of the existence of this lost planetary embryo (protoplanet). Its unique geological structure challenges long-held assumptions about planetary evolution.

It's amazing that such a large world once existed. We only know of its existence because several pieces of it accidentally landed on Earth. These meteorites preserved evidence of a completely different path by which early planets developed, said Aaron Bell, of the Department of Earth Sciences at the University of Colorado at Boulder.

The mystery of the lost world was revealed by a fragment of its remains discovered on Earth, in the Sahara, known as the Northwest Africa (NWA) 12774 angryite meteorite. It weighs 454 g and was found in 2019.

Small stone, big story

Angrites are among the oldest known volcanic rocks in the Solar System, formed just a few million years after the formation of the Solar System, approximately 4.56 billion years ago. They are also extremely rare. Out of over 80 thousand meteorites discovered on Earth, only 68 are angryites.

What makes angrytes particularly mysterious is their chemical composition. Unlike Earth, Mars and other rocky planets, angrites contain very little silicon dioxide (silica), which is the main component of almost every known rocky planet in the solar system.

For this reason, scientists believed that angryites must always come from an asteroid whose radius is less than 200 km.

When Bell and his colleagues examined NWA 12774, they found that the meteorite contained clinopyroxene, a mineral commonly found in the Earth's crust and mantle. The clinopyroxene contained in NWA 12774 was exceptionally rich in aluminum, which clearly indicated that this rock was formed under enormous pressure deep underground.

The researchers then recreated the pressure conditions that might have existed when NWA 12774 formed.

To their surprise, the aluminum-rich clinopyroxene needed a pressure of at least 17.5 kilobars. By comparison, the crushing pressure at the bottom of the Mariana Trench, the deepest point on Earth, is only about 1 kilobar.

Such pressure could not exist inside a small asteroid. Calculations, however, suggested that the body from which the angrites came must have had a radius of at least 1,000 km.

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Reconstructing the size of the lost world

Other clues in the meteorite pointed to an even more surprising possibility. The crystals inside NWA 12774 still retain sharp edges and delicate chemical patterns that would be obliterated if they formed deep underground. This suggested that the crystals probably formed at a relatively shallow depth inside the parent body, so the world must have been even larger.

Under this scenario, the angrite parent body could extend to a radius greater than 1,800 km, making it comparable in size to the Moon, and could potentially approach a planet the size of Mars, which has a radius of 3,300 km.

Many meteorites sit in drawers and haven't been thoroughly studied, so there were probably more protoplanets that we don't know about, Bell said.

It is unclear how the protoplanet ended its life. One possibility is that a catastrophic event in the early history of the solar system shattered it, and its fragments later became the building blocks of other rocky planets, including Earth.

The materials from which the angryite core was formed are fundamentally different from those on Earth and Mars. This indicates a distinct and distinct evolutionary path in planet formation in the early history of our solar system, Bell said.

NWA 12774 is a fragment of an ancient world that we will not see in the night sky. Its surface, mountains, craters, and orbit no longer exist. All that remained was the rock, in which the minerals recorded information about the pressure, depth and size of the parent body.

This was enough for scientists to reconstruct the history of a planet that did not survive the youth of the solar system. The most moving thing about this discovery is that such a huge story is contained in a piece of meteorite found in the desert.