Interview with Alessandro Morbidelli
Alessandro Morbidelli is an astronomer and planetologist with a passion for celestial mechanics. He is world-renowned for having formulated the Nice model, which shows that the current structure of the Solar System is the result of a phase of instability during its evolution. In 2023, he will be appointed Professor of Planetary Formation : from Earth to Exoplanets at the Collège de France.
How did your interest in science, and in particular celestial mechanics , come about?
Alessandro Morbidelli :I grew up in a techno-scientific environment. There were no researchers in my family, but my father was an industrial chemist and we were immersed in a certain culture of rationality. My passion for astronomy began when I was just five years old. My family owned a small, very rustic house in a remote corner of Italy, devoid of light pollution. In the middle of the night, I remember going out into the meadow to defy my fear of the dark and, staying outside, my eyes became accustomed to the darkness - a phenomenon I was unaware of at the time. Then, when the Milky Way appeared to me, I experienced a true encounter with the sky. It was a memorable moment. Later, I was given a telescope with which to observe the Moon and planets in Milan. Then, during my studies, I quickly became interested in the natural sciences, physics and mathematics : everything to do with describing the world, in short. During my doctorate in Belgium, I developed a theory on the dynamics of celestial objects in resonance with Jupiter. This led to a post-doctoral position at the Observatoire de Nice. In the early 1990s, we were beginning to use computers to run the first numerical simulations capable of tracking the evolution of celestial bodies over millions of years. But these simulations were showing some strange behaviours. It was up to me to analyze these data and establish models to explain them. In 1993, with the support of the Observatory, I joined the CNRS, and began work on the dynamics of the Solar System as it exists today - including asteroids, meteorites, comets, etc. Then, as we approached the year 2000, I made a kind of thematic leap by focusing on the origins of the Solar System : its formation and evolution.
What motivated this thematic leap towards the study of the origins and evolution of the Solar System ?
First and foremost, it was my great interest in the mystery of the origins of things that prompted me to make this leap. But also, and above all, the magnitude of the intellectual challenge it represented. When you're working on the current dynamics of the Solar System, you need to be precise, of course, but the framework is already in place : we know exactly where the planets are, and we can observe the Solar System directly. There's nothing to invent, just study. On the other hand, when you're interested in origins, you have to be imaginative. You can observe the end result - the state of the system today - but not the initial conditions. Thinking therefore requires an additional creative step, which consists of imagining scenarios to explain certain current properties of our system. Then we have to prove that our intuition is right. In concrete terms, this involves running numerical simulations, starting from assumed initial conditions, to see if we end up with the current observable system. This is a true virtual laboratory experiment. You can't simulate the Solar System from the present to the past, because entropy[1] is always increasing. I found myself in this different, intellectually challenging and stimulating process, which I liken to that of an investigator. Indeed, when a crime takes place, the investigation requires us to follow up the trail by determining a motive, a modus operandi and so on. It's fascinating, and a little more frustrating ; when you build a model on the current system, it's either right or wrong, and there's no room for ambiguity. On the other hand, when we develop a model of origins, the debate lasts a long time, we formulate approximations of reality and there are confrontations, until a form of consensus emerges.
What effect did the discovery of the first exoplanets in the 1990s have on your work ?
The effect was resounding. When something new is discovered, it causes a bit of a panic. You have to get out of your comfort zone and quickly enter a new field of investigation. Before exoplanets, there was the discovery of the Kuiper Belt in 1992 - i.e. the population of small bodies whose orbits lie beyond Neptune. This provided us with an enormous number of constraints on the evolution of the Solar System, which laid the foundations for the Nice model I was to propose a few years later. Then, the discovery of extrasolar planets had an impact on my work in two ways. We suddenly had a whole series of strange worlds orbiting in different systems. The planet 51 Pegasi b, a Jovian planet[2] that is very close to its star, enabled us to study the question of planetary migration, for example. Exoplanets have also brought about a paradigm shift in the study of the formation of the Solar System. Until then, it was thought that all planetary systems had to resemble our own, and modelers were expected to be able to reproduce the structure of the Solar System in an almost deterministic way. However, thanks to the discovery of extrasolar planets, we have come to realize the great diversity of planetary systems, and the idea of a single deterministic model that leads to the Solar System in 100 % of cases has become obsolete. In fact, without the discovery of exoplanets, the Nice model would have been rejected, because it only managed to reproduce the current configuration of the Solar System in 5 % of cases, which was attributed to chance. However, exoplanets have shown us that the configuration of our system is indeed naturally rare.
What else do exoplanets tell us about our Solar System ?
We've discovered over a thousand multi-planetary systems, and not a single one resembles our own. Moreover, many of them are completely different from what we know here ; we've seen hot Jupiters or Jupiters in highly eccentric orbits, super-Earths very close to their stars, and so on. The only planet in the Solar System whose analog can be detected around another star with current technology is Jupiter. So, if we want to make observational statistics, we need to determine the frequency of the Sun-Jupiter pair. However, only 10 % of stars are of the solar type ; 10 % of these stars have gas giants like Jupiter orbiting them, and only 10 % of these giant planets have orbits similar to that of Jupiter. In view of these values, I think we can write off a potential abundance of systems similar to our own in the galaxy. The formation of the Solar System, and its structure, is due to many contingencies. Even with similar initial conditions, we can arrive at a different result. By chance, our Solar System took the right directions, which enabled the Earth to form. So there is an anthropic bias[3]. Systems that " fail " to meet the parameters for the appearance of life have no direct observers. So it's not so surprising that the Solar System is atypical. This way of thinking is quite revolutionary, and I'm looking forward to discussing it with my humanist colleagues at the Collège de France. In a way, we're taking a step backwards from the Copernican movement. This showed that the Earth was not at the center of the Universe. From there, science was built up in stages of trivialization : the Sun is just one star among a hundred billion, in one galaxy among a hundred billion, in a Universe without a center. A trivialization that explains that we are nothing special. But now, the observation of exoplanets is debanalizing our Solar System and, for the first time in centuries, we're starting to think again that we're a bit special. From a philosophical and epistemic point of view, this is an interesting notion to ponder.
In 2005, in conjunction with international colleges, you formulated the Nice Model, which describes the formation and evolution of the Solar System. How does it differ from older models, now obsolete ?
The older models assumed that the planets formed in their current orbits, which are circular and coplanar[4]. A closer look at the Solar System shows that this is impossible. Firstly, because planetary orbits are not entirely circular or coplanar, and the populations of small bodies - both in the asteroid belt and in the Kuiper belt - are widely dispersed, with their orbits highly eccentric and inclined. Mass deficits have also been observed. We would expect to see many more objects in these belts, even though their total mass is less than that of the Moon or Mars. So something must have shaken up the whole system. However, exoplanet observation has shown that planets that form in a disk must migrate somehow - which explains how 51 Pegasi b got so close to its star. The result is that several planets move towards each other and lock together, forming a resonant chain. In other words : a situation where the orbital period ratios are ratios between whole numbers. This inevitably results in very compact systems ; the system of our giant planets should fit between 5 and 10 to 12 astronomical units (AU)[5]. But today's planets are much more spread out - between 5 and 30 AU, with slightly distorted, eccentric and inclined orbits, so something must have radically changed their orbits. Now, all populations of small bodies are shaken, so is there a link ? Work began on potential planetary dynamics that could have changed the planets' orbits and shaken the entire system. We imagined, we simulated, and we showed that when the planets are so close to each other in this chain of resonance, even small perturbations can generate a global instability of the planetary system, the planets temporarily acquire eccentric orbits, move apart, and, in this process of expansion of the planetary system, the small bodies are dispersed. We then had to look at where these small bodies went and see if this made it possible to reconstruct the structures we know about : the Kuiper belt, the Oort cloud, the asteroid belt, the trojans of Jupiter and Neptune, the irregular satellites... and we realized that instability could explain all this. Even, precisely, in a small fraction of possible evolutions ; we're back to those famous 5 %.
How did you feel about the paradigm shift triggered by the publication of your model ?
When you develop something like this, you inevitably wonder how it will be perceived. Since we were proposing a model that changed the existing paradigm, we might have feared that the community would be conservative. In fact, they like the idea of conceptual breakthroughs and are very open to new ideas, especially when they solve long-standing problems. Once our simulations were published, the community set about verifying them. There were detractors, of course, but soon others were successfully reproducing the simulations independently, and as reproducibility is crucial in science the community took a deeper interest in the model and began to study its implications and improve it. The Nice model took shape like an encyclopedic page on Wikipedia. We created the page and laid the foundations, then everyone added to it with their pens, making it more complex and refined. Today, the model we present with great confidence is quite different from the one we formulated in 2005. It has been consolidated thanks to community efforts.
This year, you are taking up the Chair in Planetary Formation : from Earth to Exoplanets. What are your expectations of this chair ?
I'm very grateful to the national community for this election to the Collège de France, which is accompanied by a great deal of emotion for me. It's also a great challenge, particularly in terms of teaching. I'd like to be able to interest the general public while being relevant to students and young researchers. It's a balancing act. What's more, this experience is going to transform the way I work, and I see this opportunity as a great chance. I'm approaching the final phase of my career, so it's a good time to take stock of my discipline, the subjects I've mastered and others that tend to orbit on the bangs of my work. My strategy will be not to go into too much detail, but to focus my approach on context, which should both speak to the general public and be useful to young researchers. It's an opportunity to complete an activity by taking stock and bringing order to a gigantic and sometimes somewhat contradictory literature. It's a great challenge, which I'm taking up with great determination !
Interview by William Rowe-Pirra
Glossary
[1] Entropy : in physics, entropy is the quantity that determines the degree of disorganization of a system, the disorder of matter.
[2] Jovian planet : a gas giant similar to Jupiter, composed mainly of hydrogen and helium.
[3] Anthropic bias : the principle that all observations of the Universe are made by a biological entity endowed with consciousness and culture.
[4] Coplanar : located in the same plane.
[5] Astronomical unit (AU) : unit of length corresponding to the distance between the Earth and the Sun, i.e. approximately 150 million kilometers.
