Around 99.8 percent of meteorites found on Earth’s surface came from asteroids orbiting the sun. The other tiny fraction of the 50,000 meteorites scientists have recovered so far originate on the moon and Mars. But researchers have discovered that one particular meteorite recovered from the Sudanese desert in 2008 is unique. As Sarah Kaplan reports for The Washington Post, it likely comes from a protoplanet that formed in the early days of our solar system.

The space rock, called Almahata Sitta, is extraordinary in several ways. An astronomer tracked the meteorite before it entered Earth’s atmosphere, enlisting other sky watchers to help keep tabs on it. When it finally exploded over the Nubian desert, volunteers recovered 600 bits of the rock, making it the first meteorite ever traced from space and recovered on the ground.

As Ian Sample at The Guardian reports, two things immediately stood out about the space rock. First, it was a ureilite, a meteorite of unusual composition whose origin is disputed. Second, the flecks of diamonds inside the rocky meteorite were much larger than those often found in other space rocks. Typically, these diamonds form in asteroids when they bang into other rocks in space, creating powerful shockwaves that roll through the orb conver some of its carbon into tiny diamonds, just millionths of a millimeter across. According to a 2015 study, however, the diamonds in Almahata Sitta are much, much larger and were likely not produced by shock waves. Instead, the researchers suggested the diamonds were produced inside an unknown planetary body.

Now, a new study published in the journal Nature Communications supports that idea. “I thought, if there were diamonds forming inside a planet, inside a parent body, they could have trapped some material from their environment,” Farhang Nabiei, materials scientist at the Swiss Federal Institute of Technology in Lausanne and lead author of the study, tells Kaplan. “And indeed, they did.”

The researchers looked at the meteorite through an electron microscope, finding traces of chromite, phosphate, and iron-nickle-sulfide inside the diamonds, minerals that only form under extreme pressure. According to the Associated Press, the pressure needed to produce the impurities in the Almahata Sitta diamonds is 2.9 million psi. Only a planet the size of Mercury or Mars could produce that much pressure, the researchers conclude.

The finding jibes with what astronomers hypothesize about how the solar system formed. As Sarah Gibbens at National Geographic reports, in the first 10 million years of our planetary family, swarms of protoplanets composed of rock, dust and gas likely pinballed around the sun. Eventually those bodies collided and flung material out into space. It is this debris that eventually coalesced into our current lineup of planets.

The Almahata Sitta meteorite is the first evidence of this early stage. “What we're claiming here is that we have in our hands a remnant of this first generation of planets that are missing today because they were destroyed or incorporated in a bigger planet," senior author Philippe Gillet tells the AP.

While the claim that we have a chunk of missing planet seems like sci-fi, James Wittke, director of the meteorite laboratory at Northern Arizona University, tells Sample that the study is sound. “We think that there were probably many larger ‘parent’ bodies in the early solar system, which have since been destroyed, so a since-destroyed body the size of Mercury is reasonable,” he says. “One as large as Mars seems a little surprising, but this paper presents the best, and perhaps only, type of evidence for determining the sizes of these parent bodies.”

As Gibbens reports, researchers have collected 480 pieces of the unusual ureilite meteorite. And Nabiei plans to examine them more closely to see if they too show signs of these long-vanished protoplanets.