This ancient asteroid family reveals clues about the birth of the solar system

Illustration by NASA's Goddard Space Flight Center Conceptual Image Lab

Asteroids are more than just dinosaur missiles. They're the remaining clues to the birth of the solar system.

The solar system's origins have been a point of controversy among planetary scientists. But, an international team tipped the debate Thursday in Science with the discovery of one of the oldest known asteroid families. Their work uncovers how some of the first asteroids formed 4 billion years ago and point to how planets like Earth came to be.

"This family is like a big missing piece of a puzzle that we found," Marco Delbo, a planetary scientist at the Observatoire de la Côte d'Azur in Nice, France, and the study's lead investigator, said.

In 2012, Delbo and his team launched an asteroid belt treasure hunt. They wanted to learn as much as possible about dark asteroid families — fragments from asteroid collisions that tend to orbit as a collection — in the region of the belt closest to Earth and Mars. NASA's OSIRIS-REx mission will visit this region, when it stops at the near-Earth asteroid Bennu and collects a sample. Bennu is almost certainly a member of one of these dark asteroid families, Delbo said.

Because smaller chunks of asteroids drift from the point of collision faster than larger pieces do, these asteroid families become shaped like the letter "V." Smaller pieces spread out far and wide from the original impact site, while larger fragments remain condensed at the point. The legs of the V can resemble the straight wings of a swift or the narrow angle of a vulture in flight.

Older asteroid families are harder to find, but this V signature can expose the age of these clusters. The longer it's been since the initial collision, the more time smaller pieces have had to spread out into the belt. But this pattern also makes it more difficult to know which pieces belong to a certain family.

Delbo and his team focused on the inner side of the asteroid belt, the one closer to Mars. Then, rather than looking at all of the asteroids in this region, they narrowed their search to dark, carbon-rich asteroids, which are not as common as bright asteroids in this part of the belt.

Using this specific homing method, Delbo and his team identified a collection of asteroids with a unique V-shape, a new family. Given the angle of this family's V-shaped leg, they estimate this primordial asteroid family is 4 billion years old.

"We discovered this family that is more ancient than anything we know," Delbo said.

The OSIRIS-REx probe is headed to Bennu, a roughly spherical asteroid measuring about 1,614 feet (492 meters) in diameter. Photo by NASA's Goddard Space Flight Center Conceptual Image Lab

They double checked their discovery by paging through old science papers. Members of an asteroid family, which originate from a single asteroid, have similar traits, like how dark or bright they are. Their curation found every astral body in the new family looked alike.

Beyond the edge of the newly identified family, however, is a void. A few orphan asteroids populate this area.

"This is the holy grail of the asteroids," Delbo said. These orphans must have formed in different manner than those that belong to the new family. The orphans are the original settlers, the report found, they existed in the inner belt before anything else.

These orphan asteroids are large, ranging from 21 to around 93 miles across. Their size matches up with predictions from theoretical models of how large original asteroids might have been 4 billion years ago, when they initially formed.

Their size suggests the solar system was likely formed by gravitational collapse, according to a recent proposal from the Max Planck Institute. This hypothesis posits that the solar system began 4.5 billion years ago with grains of space dust that pooled into eddies. After half a billion years, gravity had rapidly pulled them together into large objects, around 62 miles in diameter. In the past, people thought this space dust had aggregated over a much longer period of time to create the myriad-sized objects in the solar system.

Delbo's study provides evidence for the Max Planck Institute's gravitational collapse hypothesis by suggesting the oldest asteroids started out large, and then became smaller through collisions and other destructive forces happening in the ancient solar system. Planetary scientists have been debating this hypothesis for nearly a decade.

"It's strong evidence, but it's just one piece of the puzzle," Francesca DeMeo, a planetary scientist at the Massachusetts Institute of Technology, who was not involved in the study, said. "It's probably not the final say in asteroid sizes."

In particular, DeMeo points out that Delbo and his team were looking at one type of asteroid family — the dark asteroids on the inner side of the belt. Whether their results can also apply to bright asteroids or dark asteroids found in other regions of the asteroid belt remains to be seen.

For Delbo's part, he dreams of tying his discoveries of the asteroid belt to the piece of Bennu due to be delivered by OSIRIS-REx. He is also grateful for the scientists who came before him and without whom his discovery would not have been possible.

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