Planet formation is a fundamental question in astronomy and planetary science. Understanding how they form and evolve is crucial to deciphering the solar system and beyond. Despite advances in science, we are still far from understanding all the details of planetary formation.
It is well known from astronomical theory and observations that planets form in disks of gas and dust around very young stars. It therefore makes sense that the planets of our solar system also formed in the protoplanetary disk of our sun. This happened 4.5 billion years ago.
However, perhaps the most obscure phase of planetary formation is how dust accumulates in a protoplanetary disk to give rise to planetesimals, the constituent objects of the planets.
Classical models were based on progressive collisional coagulation, a process where small dust grains agglomerate little by little to form increasingly larger particles through collisions, ranging from a few micrometers to the size of planets. However, laboratory tests have shown that these models do not work, because they encounter a number of obstacles, for example, dust grains cannot grow by successive collisions from a submillimeter size to a kilometer size.
A new theory
New models suggest that planetesimals form from dust clouds that reach densities high enough to support themselves by gravity. This is called the Streaming Instabilitya theory that has gained momentum in the planetary science community. Various studies have shown that this theory well reproduces the size distribution of large asteroids and trans-Neptunian objects.
Yet it was not clear how far the size distribution predicted by this theory extended, that is, whether it produced many small planetesimals or whether there was a minimum size for them.
On the other hand, different studies find a typical size of around 100 km in diameter for planetesimals, thus suggesting that small particle aggregates do not necessarily give rise to small planetesimals. Indeed, it appears that small aggregates disperse due to gas turbulence in the protoplanetary disk before having the opportunity to form planetesimals. Were these formed, therefore, on a large or small scale? A definitive answer was not yet established.
In search of surviving planetesimals
Origins, a research project that I direct and funded by the National Research Agency (ANR), has made an original contribution to this field using an innovative methodology. This is based on the analysis of observations and astronomical data to identify the planetesimals still surviving among the asteroid population, in order to measure their size distribution.
These planetesimals are located in the inner part of the asteroid belt, between 2.1 and 2.5 astronomical units, the latter being the average Sun-Earth distance. This made it possible to provide strict observational constraints to models of planetesimal formation.
The founding idea of our project is based on the concept that asteroids represent the remains of the era of planet formation, but that not all asteroids observed today are survivors of this primordial era. . It is known that many asteroids are fragments resulting from collisions between larger parent bodies. These collisions have occurred throughout the history of our solar system. The age of these asteroid fragments corresponds to the time from the collision event that produced them to the present. Although these fragments still retain the original composition of their progenitors, their dimensions and shapes provide no information on the accretion processes that led to the accretion of planetesimals, and consequently, planets. The methods developed in our project effectively distinguished the original asteroids, having accumulated as planetesimals in the protoplanetary disk, from families of fragments resulting from collisions. Subsequently, they also made it possible to study the dynamic events which contributed to sculpting the current structure of the solar system.
Our team developed and used a method to discover, locate and measure the age of the oldest collision fragment families: each member of a fragment family moves away from the center of the family due to a thermal force non-gravitational known as the Yarkovsky effect.
This drift occurs in a manner dependent on the size of the family member, with smaller asteroids drifting faster and farther than larger ones. Our project’s innovative method was to look for correlations between size and distance in the asteroid population. This made it possible to reveal the shapes of the oldest fragment families.
Asteroids 3 to 4.5 billion years old
Using this technique, four important and very old families of asteroids were discovered by our project researchers. These are among the most widespread families of asteroids. Their extension is about half an astronomical unit and they are located in the more inner part of the asteroid belt and have ages between 3 and 4.5 billion years.
However, a new method for identifying asteroid families still requires verification. One of these checks consists of determining the direction of rotation of each family member.
To do this, researchers launched an international observation campaign called “Ancient asteroids”, involving professional and amateur astronomers. They obtained photometric observations of the asteroids in order to measure their variation in brightness as a function of their rotation (light curve). Using lightcurve inversion methods, our project team was able to determine the three-dimensional orientation of asteroids in space and extract the rotation direction. This revealed that retrograde asteroids are generally found closer to the Sun than the center of families, while prograde asteroids are found beyond the center of families, consistent with theoretical expectations. This research made it possible to confirm that several asteroids belong to the very old families identified by our team.
A major scientific question that arose after the identification of the most primordial asteroids was what their composition was. To answer this question, ANR Origins scientists returned to their telescopes to spectroscopically study these bodies. Their spectroscopic investigation of planetesimals from the inner main belt confirmed that silicate-rich bodies dominated this region. However, almost all spectral types of asteroids are present, with the notable exception of olivine-rich asteroids. Their absence among planetesimals could be due to the rarity of these types among large asteroids.
Some types of asteroids are very rare
But why are asteroids rich in olivine so rare? The oldest asteroids are expected to be rich in olivine due to the differentiation process. This process causes a body to be organized into layers of different densities and compositions, due to the heat generated by the decay of radioactive elements.
Researchers have discovered, for the first time, a family of asteroid fragments rich in olivine, likely formed by the fragmentation of a partially differentiated parent body. This family could also come from the fragmentation of an olivine-rich body, perhaps from the mantle of a differentiated planetesimal which could have broken up in a different region of the solar system, and one of its fragments could have been dynamically implanted in the main belt.
Indeed, the idea that asteroids could have been implanted in the main belt by dynamic processes has been widely studied in recent decades. One of these dynamic processes could be the orbital instability of giant planets. This suggests that Jupiter, Saturn, Uranus and Neptune formed in close orbits before migrating to their current positions. This migration could have triggered gravitational interactions with planetesimals, moving them into the early solar system. Determining the epoch of this instability is a major question because it is crucial to understanding its impact on the destabilization of populations of small bodies, the disruption of the orbits of terrestrial planets, and possibly its role in their evolution.
