Scientists have found the first hard evidence of a large and ancient protoplanet embedded in extraterrestrial diamonds that fell to Earth about 10 years ago.

To be clear, the diamonds did not fall to Earth on their own. Instead, they were discovered inside a small asteroid that slammed into the atmosphere over the Nubian Desert in northeastern Sudan in October 2008.

And in case you are imagining large twinkling space gems, let’s also be clear that these diamonds are extremely tiny — the biggest are about the width of a human hair.

And yet, in a paper published in Nature Communications, researchers say that within these small diamonds are chemical clues that suggest they could only have formed deep within a Mercury- or Mars-sized almost-planet that formed in the chaotic early days of the solar system.

“What makes this study so exciting is that it is direct evidence from an actual rock that there was a large protoplanetary body that is no longer around,” said Meenakshi Wadhwa, who studies meteorites at Arizona State University and who was not involved in the new work.

To come to this conclusion, the research team led by Farhang Nabiei of the Ecole Polytechnique Federale de Lausanne in Switzerland, used a high-powered electron microscope to study the tiny diamonds inside a thin section of the meteorite.

Their first line of evidence that the space rock was a chunk of what was originally a large planetesimal came from the size of the diamonds themselves.

“Shock waves produce very tiny diamonds,” said Paul Asimow, a geochemist at Caltech who was not involved in the study. “The diamonds they found are too big to have been produced that way.”

That means that the diamonds had to be formed toward the center of an object large enough to produce the requisite amount of pressure and heat.

When the study authors pointed their microscopes on the inclusions in the diamonds, they discovered evidence that the diamonds formed under an extremely high-pressure environment that could be found only in a planetary body between the size of Mercury and Mars.

“A commonplace theory for the origins of planets is that there were a lot of small objects that ran into each other to make larger objects,” Asimow said. “It is not surprising at all that there were Mars-sized objects that are now gone.”

However, he added, this is the first time that a meteor has revealed any evidence that it formed inside one of these large objects.

And that, he said, makes this a very cool discovery.