Astronomers have studied asteroids for centuries: initially they thought that these «space rocks» represent debris of an exploded planet. Later they realized that the these bodies are planetesimals, the building blocks of planets including our Earth, that for some reasons never accreted to form a planet between Mars and Jupiter. As such, the asteroid Main Belt offers us today a unique view of how our Solar System was 4.6 billions of years ago. However, as more and more asteroids were discovered, astronomers realized that not all of them are planetesimals that formed together with the planets. Scientists have noticed many tight clusters of similar-looking asteroids on closely related orbits, which they interpreted as families of fragments that were created during catastrophic collisions between larger asteroids and occurred throughout the entire history of our Solar System. As time passes, the orbits of family members move away from each other and clusters become more diffused; members of different families overlap with each other and with a special type of asteroids that are not member of any families: true survivors since the birth of our planets.

«Asteroid families always represented a problem for those who wants to find which are those asteroids that are not fragments of other asteroids and as such are not members of any family. These original asteroids are the “holy grail” of planetary formation. They allow us to understand what were the sizes and the composition of the planetesimals that formed our planets», comments Alessandro Morbidelli, research director from the CNRS at the Observatory of Cote d’Azur, Nice, France, and co-author of this study.

A team led by Marco Delbo’, also researcher at the Observatory of Cote d’Azur, has used a novel technique, developed by Bryce Bolin, PhD student at the University of Nice Sophia Antipolis, to find very old families, and just discovered one of the oldest ones known so far. Its members are dispersed over a vast area covering nearly one-third of the Main Belt. «Instead of searching for cluster of orbits, our methods looks for the edge of an expanding family. We know that smaller asteroids moves away from the center of the family faster than the larger ones. So, if you look for correlations of size and distance, you can see the shapes of old families», says Bolin.

The family that Delbo’ and colleagues found is at least 4 billion years old. But it could be even older, and formed by an asteroid impact just in the beginning of our Solar System, before Jupiter and Saturn were on their present orbits. When these giant planets moved to their current orbits, it was not an easy moment for our Solar System. The enormous perturbation of these planets kicked many asteroids out of the Main Belt, also chaotically stretching and changing the inclination of the orbits of the survivors. «We were surprised to find members of our primordial family having orbits so far away from each other. But we carefully checked the properties of these bodies: we found their spectra - reflecting composition - to be very similar, an important clue that they are pieces of a common parent body. We also tested that their correlation between size and distance was preserved throughout the giant planet instability and the following 4 billion years of solar system evolution», says Chrysa Avdellidou, fellow researcher of the European Space Agency in Noordwijk, Netherlands, and co-author of the study.

The family identified by Delbo’ and colleagues was indeed the largest missing piece of a gigantic puzzle: «Now that we have discovered this large and missing family, we have the genealogical tree of the asteroids in that portion of the Main Belt that is closest to the Earth. The asteroids that are not family members and thus must have been born as planetesimals are very few. They are all larger than 35 km, confirming Morbidelli’s theory that asteroids were born big», continues Delbo’. Using these techniques across the entire Main Belt should continue to reveal the history of Solar System by disentangling which asteroids are primordial and which are just fragments of subsequent collisions.

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