A rare meteorite discovered in the Sahara Desert proves that our solar system almost had at least one extra planet. In a study published in the journal Earth and Planetary Science Letters, astronomers say the chunk of space rock known as Northwest Africa (NWA) 12774 once belonged to a protoplanet possibly as large as Mars. That is, until a cosmic crash likely blew it to smithereens. 

The solar system includes eight known planets (sorry, Pluto). Barring interstellar catastrophe, this number will remain the same until the sun finally dies about 5 billion years from now. However, this total planetary count was never a guarantee.The solar system’s earliest era featured multiple embryonic protoplanets that had the potential to grow together into additional cosmic neighbors.

The remnants of these long gone celestial bodies are scarce, but traces still exist. That said, astronomers didn’t expect to find protoplanetary evidence in a meteorite like NWA 12774. Discovered in 2019, NWA 12774 is an angrite—one of the oldest known types of volcanic rock that was formed during the solar system’s era about 4.56 billion years ago. They’re also very rare. Of the roughly 80,000 meteorites discovered on Earth so far, only 68 are angrites.

Unlike rocky planets such as Mars and Earth, angrites do not have a lot of silicon dioxide. Because of this, astronomers have long assumed that angrites always originated in asteroids no larger than about 124 miles wide. NWA 12774 blows this theory apart..

While analyzing the meteorite, researchers at the University of Colorado Boulder detected clinopyroxene, a mineral crystal that exists throughout Earth’s mantle and crust. NWA 12774’s clinopyroxene was also heavy in aluminum, which directly points to formation under massive amounts of pressure underground. The team then calculated the conditions necessary to create an angrite like NWA 12774, and settled on at least 17.5 kilobars of pressure. To put that in perspective, the pressure experienced at the bottom of the roughly 35,875-foot-deep Mariana Trench is barely one kilobar.

Small asteroids simply don’t possess the conditions needed to generate a rock like NWA 12774. What’s more, the angrite’s sharp crystalline edges also indicate that it formed at comparatively shallow depths in its host body. Based on all of these factors, astronomers now believe NWA 12774 once belonged to a young protoplanet with a radius anywhere from 621 to 2,050 miles wide. This means that instead of an asteroid, the angrite may have existed inside something as big as Mars.

“It’s incredible to think there was once a world this large,” Aaron Bell, a UC Boulder earth scientist and study co-author, said in a statement. “We only know it existed because a few fragments of it happened to land on Earth. These meteorites preserved evidence of a completely different pathway through which early planets developed.”

Although it’s unclear how the protoplanet met its demise, some type of crash between early solar system denizens is definitely a possibility. Regardless, the ramifications are huge for astronomers’ understanding of our cosmic neighborhood’s history.

“The materials that formed the angrite parent body are fundamentally different from the ingredients of Earth and Mars,” explained Bell. “It points to a distinct and separate evolutionary path in planetary formation in the early history of our solar system.”

 

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