Ancient Asteroid Family Dates Back to Our Solar System's Early Days

Ancient family of asteroids
Researchers have found that a family of asteroids that dates back to the solar system's early days. Here is an artist's concept of a broken-up asteroid. The leftover building blocks of the solar system, called planetesimals, were similarly fragmented, and they drifted apart over time. (Image credit: NASA/JPL-Caltech)

A newfound ancient family of asteroids is nearly as old as the solar system, and it could shed light on how Earth and the other planets formed, a new study finds.

In the early days of the solar system, rocky material clumped together into bodies known as planetesimals. Some of these planetesimals collected to form the planets, while others mostly settled in the main belt of asteroids, between Mars and Jupiter.

Understanding what the planetesimals were originally like could yield insights into how the asteroids and planets formed, the new study's researchers told However, over billions of years, countless collisions have shattered nearly every ancient planetesimal into asteroids, they added. These fragments have drifted apart over time, making it difficult to identify families of related asteroids and learn about the planetesimals they came from. [The Asteroid Belt Explained: Space Rocks by the Millions (Infographic)]

Now, scientists have identified a previously unknown family of darkly colored asteroids that is "the oldest known family in the main belt," said study lead author Marco Delbo, an astrophysicist at the University of the Côte d'Azur in Nice, France.

This discovery came as part of research into the inner portion of the main belt. "Most of the asteroids and meteorites that hit the Earth come from this source region," Delbo told "Also, we and many other colleagues wanted to trace the asteroid Bennu, the target of NASA's OSIRIS-REx sample return space mission, back to its origin in the main belt and possibly to a parent asteroid."

The scientists used a computer model to look for v-shaped groups of asteroids. This shape is what one would expect from a single-parent body that fragmented into pieces and dispersed over time, the researchers said.

This newfound asteroid family consists of rocks averaging 7.15 miles (11.5 kilometers) in diameter that are found across the entire inner part of the main belt. This family has 108 members of which the researchers are certain, the largest of which is asteroid 282 Clorinde, which is about 26.1 miles (42 km) wide. 

"There are other asteroids in the region that could also be part of our primordial family," Delbo said.
"The largest of these is 51 Nemausa, with a diameter of 138 km (85.75 miles)."

Based on the asteroid family's size and shape, the researchers estimated that it is about 4 billion years old. In comparison, the solar system was born about 4.6 billion years ago. One other asteroid family potentially as old as this one is found among the Hildas, a group of asteroids located beyond the main belt, nearly as far away from the sun as Jupiter is, Delbo said. 

Asteroid families are usually named after their largest members, which are likely the parents of the rest of their families — smaller members are literally chips off the old blocks. However, "we would like to avoid to give a name to this family," Delbo said. This is because over the course of billion of years, gravitational tugs of war with the planets may have slung the parent of this family away from the rest of its members, he explained.

By analyzing this asteroid family and others, the scientists estimated the original planetesimals were all larger than 15.5 miles (35 km) wide. This supports the traditional view of asteroid formation, which says that most were born as fragments of large chunks of rock, in contrast to some recent models, which suggest that planetesimals were much smaller chunks of rock, Delbo said.

"The almost 1 million asteroids that we see in the main belt can be linked to a few progenitors — those that were born big," Delbo said.

The scientists said they plan to use their new method to find asteroid families in other areas of the main belt. "There are more families yet to be discovered," Delbo said. This research could go on to reveal "precious information about the orbital and mass distribution of planetesimals at the time when the planets were forming," he said.

The scientists detailed their findings online today in the journal Science.

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Charles Q. Choi
Contributing Writer

Charles Q. Choi is a contributing writer for and Live Science. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica. Visit him at