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Diamond-studded meteorites came from the collision of a lost planet

When our solar system was in its infancy 4.5 billion years ago, a swarm of protoplanets swirled around the sun—some of which coalesced into larger and larger masses, while others were blasted to smithereens in a demolition derby of planetary proportions. Those collisions would have produced innumerable fragments of cosmic shrapnel, some of which orbited the sun as carbon-rich asteroids. Now, a new analysis of the remains of one such asteroid, which broke apart in our atmosphere and fell to Earth, bolsters the idea that they are, in fact, the remnants of one of our solar system’s lost planets—some of which may have been the size of Mercury, or larger.

The Almahata Sitta meteorites, a few hundred rock fragments that rained down on Sudan’s Nubian Desert in 2008, included a number of coarse-grained, carbon-rich fragments known as ureilites. Inside were tiny diamonds that likely measured up to 100 micrometers across when they originally formed. That’s at least 100 times larger than the nanodiamonds that form when planetary objects collide, and it’s far larger than diamonds that form by condensing from carbon vapor inside clouds of interplanetary gas and dust. Excluding those possibilities, a new study proposes that the diamonds in the Almahata Sitta meteorites grew deep inside a large protoplanet before it suffered the collision that turned it into cosmic shrapnel (artist’s concept, above).

Just how large would the planet have been? Small blebs of iron-rich sulfides inside these meteorites’ diamonds provide key clues. Because the minerals could only have formed at pressures about 200,000 times those of Earth’s atmosphere at sea level, the diamonds would have formed near the center of a Mercury-or-larger-size protoplanet, the researchers report today in Nature Communications. Another alternative: They could have formed just outside the metal-rich core of a Mars-or-larger-size body.