One problem with researching the formation of the solar system is finding material that’s existed for more than four billion years. Much of our understanding of this time period comes from studying asteroids and the meteors that fall to Earth. On this planet, plate tectonics and surface weathering have destroyed, sunk, or remodeled most of the rock that constituted Earth’s crust during the Hadean period. But there are asteroids still floating in space that formed in the earliest days of our solar system — and every now and then, one of them crashes to Earth.
In 2008, one such asteroid, Asteroid 2008 TC3 landed in the Nubian Desert. Unlike most meteorites, which are classified as chondrites — a type of primitive, non-metallic, stony asteroid — 2008 TC3 is a rare type of asteroid known as a ureilite. Ureilites contain mostly olivine and pyroxene as well as nanodiamonds, which are believed to form when the rocks are exposed to high pressure and stress, most likely when their parent bodies collide with other asteroids. But the same characteristics that give diamond its strength also allow the formation process to trap other minerals inside the diamond when it forms. By studying the materials inside these nanodiamonds, scientists can peer back in time to an era when the planets within the solar system were still forming from the sun’s protoplanetary disk.
This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image Credit: Wikipedia
Our current theory of planetary evolution holds that in the beginning, a gas cloud of molecular hydrogen condenses into a protostar, while grains of ice and dust within the protoplanetary disk slowly accrete together, forming what are called planetesimals. Eventually, the planetesimals become large enough to form protoplanets, sometimes called planetary embryos. Ceres, 2 Pallas, and 4 Vesta are all protoplanets that survived the formation of the solar system. Of the three, only Ceres is rounded by its own gravity, but both Vesta and Ceres have a differentiated interior — another mark of a protoplanet. Pallas is thought to have a partially differentiated interior. Theia, the giant impactor that may have smashed into Earth to form the Moon, would have also been a protoplanet, while the asteroid 16 Psyche is thought to be the exposed iron core of a protoplanet (16 Psyche’s surface is 90 percent metallic iron).
Now that we’ve discussed early models of solar system formation in a bit more detail, what’s significant about the findings regarding this specific meteorite? When researchers found 2008 TC3, they theorized that the grains of diamond could have come from three sources — chemical vapor deposition, via the shock of impacts, or from the constant, steady pressure applied to a planetesimal fragment buried near the core of a protoplanet. By examining the elements trapped within the nanodiamonds and comparing the diamonds themselves to equivalent formation structures on Earth, the researchers determined that the diamonds within the asteroid were formed by constant, steady pressure of 20 gigapascals (GPa) — not a sudden impact or through CVD. The sample rock came from the mantle of a differentiated body and was formed at a temperature of ~1350K. These specific diamonds were likely formed in a protoplanet between the size of Mercury at the lower bound and Mars at the upper.
This find is the first direct evidence that the solar system once contained protoplanets that no longer exist today. Don’t mistake me — there’s a great deal of circumstantial evidence suggesting that these bodies once existed. The impact theory of the Moon’s formation posits that the Earth was struck by Theia. Uranus orbits very nearly on its side and is believed to have been knocked into that position by a pair of impacts. Asteroids like 4 Vesta lost 1 percent of their mass in a series of enormous impacts, and Mars’ entire North Polar Basin is thought to have been created by a colossal impact event that may have also formed the planet’s moons, Deimos and Phobos (Phobos may be a rubble pile as opposed to a solid structure). There is, in other words, plenty of evidence that the early solar system was an exceedingly dangerous place, with protoplanets and planetesimals smacking into each other like pinballs.
We may never know if 2008 TC3 vanished into the Sun, was scattered by its gravitational interactions with a gas giant, or accreted to form another planet. But its characteristics confirm that protoplanets themselves once existed in orbit around the Sun.