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Shards of a lost planet from the early solar system may give us evidence of the chaos that surrounded the birth of the terrestrial planets, including Earth. Researchers may have found the first relics of an enormous protoplanet that was smashed up into a type of meteorite we see today called a ureilite.

Most of the meteorites we have found on Earth are thought to be bits of asteroids that never grew to be planet sized. Now, at the 脡cole Polytechnique F茅d茅rale de Lausanne in Switzerland and his colleagues used an electron microscope to study a ureilite meteorite, and found signs that it may have come from something bigger.

Ureilites all came from a single parent body, which we can tell from their similar compositions 鈥� they tend to have more carbon than other meteorites. Some of this carbon is in the form of tiny diamonds. Diamonds are a result of high pressure, so these could have formed in the centre of a larger, planet-sized rock, Nabiei says.

If we could prove that they did, it would be the first direct evidence of the family of large rocks that formed the inner four planets of our solar system.

Diamonds are forever

Nabiei and his team think they have found some proof. They examined the diamonds and found other minerals trapped within them. Because diamonds are so tough, they can preserve materials for millennia, giving us a window into their environment when they formed.

The team found mostly iron sulfides, which were trapped as solid crystals. These crystals could only have formed at pressures above about 20 gigapascals, they say. That is similar to the current pressure at the centre of Mercury and about half that at Mars鈥檚 core.

So, depending on where the diamonds formed, Nabiei says this indicates that the ureilite parent body was somewhere between the size of Mercury and Mars before it collided with another large object and was smashed into fragments. 鈥淭here were whole planets that were destroyed, and now we know it from hard evidence, not just from simulations,鈥� says Nabiei.

鈥淚f they鈥檙e right about this, then maybe some of the meteorites we have were originally residents of the early solar system,鈥� says at the Southwest Research Institute in Boulder, Colorado. 鈥淭hen we could learn more about planet formation because we鈥檙e directly sampling things that lived there when they were forming.鈥�

Planet seeds

But other investigations have suggested that the object that shattered into ureilites was much smaller, and some researchers say this is not hard enough evidence that it was really planet-sized.

鈥淭his meteorite came from the asteroid belt, and the total mass of the asteroid belt is about 4 per cent the mass of the moon,鈥� says at the University of Arizona in Tucson. 鈥淚f the parent body was so big, where鈥檚 the rest of it? You鈥檙e talking about a factor of 100 times more material than currently exists in the asteroid belt.鈥�

Another issue is the difficulty of breaking up such a large object without melting all the pieces. It would require an enormous amount of energy to bust up an object as large as the moon or even bigger, says at the Carnegie Institution in Washington, DC. 鈥淲hether the rocks would survive intact or melt or vaporise, I don鈥檛 know.鈥�

It鈥檚 hard to figure out for sure, because you can鈥檛 exactly smash together two moon-sized objects in the lab. But if the ureilite parent body really was among the giant seeds that formed Earth and the other terrestrial planets, this shard of it could be a window into the time when our world was just forming.

Read more: Giant loner asteroids suggest baby planets formed quickly