Discovering the original planetesimals of our Solar System – ORIGINS

The question of how planets form has always been a challenge in planetary science. After decades of progress, perhaps, the most obscure phase of this process is how dust accretes in a gas-dominated protoplanetary disk to form the planetesimals, the building blocks of planets. Did planetesimals form directly big as 100 km-sized asteroids or much smaller? What were their compositions? In the case of our Solar System, how did they get scattered and perturbed by the growing planets and implanted in the region now populated by the asteroids? What was the planetesimal role in the sequence of the events that sculpted the orbital distribution of Solar System bodies and when did this happen? Two French laboratories, one expert in the study of planetary formation and dynamics, and the other in the compositional characterisation of minor bodies join forces to seek answers to the questions above. The problem of planetesimal formation is typically approached in the community from theoretical aspects, but even same models predict different planetesimals size distribution depending on the numerical resolution that is used!

Here we propose a different line of attack: we directly observe the surviving planetesimals of our Solar System: We know that asteroids are leftovers from the planet-formation era. But we know that not all asteroids are survivors from the primordial times. Their vast majority are collisional fragments of larger parents, which broke during the eons. Although they still carry the original composition of their progenitors, the sizes and shapes of collisional fragments do not provide any information on the accretion process that led to the formation of planetesimals and, by consequence, of the planets.

We need to do a “cleaning” of all the asteroids that were created by collisions in the Main Belt in order to reveal the original planetesimals. To do so, we developed a novel technique to identify the oldest and largest families of collisional fragments. Those families were missing in catalogs, which were based on conservative asteroid family membership. We demonstrated our technique in a restricted zone of the Main Belt, by discovering a primordial and vast collisional family; by cleaning the area from its members, we pinpointed some of the original planetesimals and surprisingly found that none is smaller than ~30km. We want to apply this technique to the total asteroid population.

How many more Gyr-old families will we find? The ages of these families, their number and dynamical properties, which could be different in different regions of the asteroid belt, will give us constraints to the phases of planetary orbital instabilities, perhaps helping us to determine the epoch of these upheaval in the orbits of our Solar System. We use the search and study of the families as a Trojan horse to understand the big picture of the planetary formation and evolution.

Our approach needs an interdisciplinary intertwining of method based on orbital dynamics, astronomical observations and massive data acquisition and exploitation. The latter will include literature data and the valorisation of asteroid spectroscopic observations of ESA’s Gaia mission, in which France has a leading role. Other scientists across the world join our effort. We also open our project to collaboration of the community of amateur astronomers, who will follow and contribute live to ORIGINS. The ANR support is asked to fund this ambitious project, developing a new generation of scientist and to endure a leading role of French planetology in the fields of planetary formation and studies of the minor bodies of our Solar System. We expect ORIGINS to have an impact on studies devoted to asteroids, planet formation, and by pinpointing the original planetesimals, to the design and preparation of novel scenarios of space missions devoted to find, study, and sample, the original matter in our Solar System, and the bodies that can potentially still harbour it.