Back in 2008, an asteroid known as 2008 TC3 fell through the skies, exploded in mid-air, and birthed hundreds of meteorites, all of which crashed into Sudan’s Nubian Desert. Known as the Almahata Sitta meteorites, most of them fell into an enigmatic category known as “ureilites”, whose origins – unlike many space rocks – remained entirely ambiguous and sometimes controversial.
Now, in a stunning Nature Communications paper, an international team of researchers have ruled out all other possibilities to leave one, absolutely stellar hypothesis. Based on the alien diamonds present within these ureilites, it’s probable that they were once found within a lost planet in our Solar System, one that was destroyed billions of years ago.
“It came as quite a surprise to me too – I’m quite new to planetary science,” lead author Farhang Nabiei, a PhD student currently at the Federal Polytechnic Institute of Lausanne (EPFL), told IFLScience. “This is like a side project for me, it’s not even my main thesis topic!”
Explaining how it took many progressive, interdisciplinary steps to reach the striking conclusion, the materials scientist effused: “I couldn’t believe it turned out to be something so cool.”
The idea of lost planets or proto-planets isn’t new, of course. The formation of the Moon required that a massive, Mars-sized object (dubbed Theta) slammed into the young, molten Earth, creating a trail of embers that would go on to form our pale guardian. In fact, during the birth of the Solar System, and long after, computer models strongly suggest that massive objects were constantly ricocheting around and crashing into each other.
The idea that these particular meteorites came from a parent body that met its end during the first 10 million years of our galactic neighborhood isn’t too surprising, however, but it is incredibly significant. Space debris falls from the skies all the time, but this evidence – if corroborated – is “the first compelling evidence for such a large body that has since disappeared,” as the authors note in their paper.
This lost planet would have been anywhere between the size of the Moon and Mercury. Annihilated in collisional events, it took around 4.5 billion years for its remnants to reach Earth. The question is, how do we know that? Well, it’s all down to those extraterrestrial diamonds.
Diamonds form under intensely high pressures. It’s a common misconception that they’re formed when coal is extremely compressed, but the leading theory is that they’re made when carbon-rich deposits deep within the mantle are squashed and cooked, before being jettisoned up to the surface via quite violent volcanic eruptions that no longer take place.
Ureilites are known to have a surprising number of diamonds in them compared to other meteorites. It’s been previously suggested that energetic impact events can replicate those high-pressure conditions needed to make diamonds (and some other minerals) – something known as “shock-driven transformation”, which is observed in the geological records of other impact events on Earth.
Although possible, shock diamonds are normally small, and those found in the Almahata Sitta meteorites are, on average, fairly large. This suggested a slower, gradual growth process took place, but that's not all.
A little probing by the team also revealed that these diamonds had brought something else along with them: inclusions from what was clearly an alien world’s mantle.
“People hadn’t seen inclusions in extraterrestrial diamonds before,” Nabiei added.
Effectively deconstructing this mantle’s geochemical pathways in a laboratory, the team concluded that the larger diamonds, and their inclusions, could only have formed within said mantle under extreme pressures, those around 20 billion pascals, or 200,000 times our atmospheric pressure.
The only reasonable place this could have come from was a planet, but it didn’t match the chemistry of any we know of. The only conclusion? These meteorites came from a “planetary embryo” that no longer exists.
Nabiei expects there to be considerable debate over their considered conclusion, which finally gives ureilites a parent body, but he welcomes it. He was going to discuss it at the most recent annual gathering of the American Geophysical Union (AGU) but, because he’s Iranian, he wasn’t allowed to go because of the Trump administration’s travel ban.
Fortunately, this team’s work on the Almahata Sitta meteorites is likely to make a huge impact regardless.