Planetary formation

Every two weeks, a current scientific topic is explored by a researcher from the Collège de France.

How did the Earth form ? What processes shaped the other planets ? Exploring the dynamics of the Solar System aims to answer these questions. Through an approach that combines numerical modeling and geochemical analysis, it is possible to reconstruct the history of the Solar System.
Interview with Sarah ^Joiret*, astrophysicist at the Collège de France.

Before becoming gas giants or rocky planets, planets are first and foremost tiny grains of dust. These grains form in the disk of gas and dust surrounding young stars, known as the protoplanetary disk. " These are dynamic environments, where physical and chemical processes take place that cannot be reproduced in the laboratory ", explains astrophysicist Sarah Joiret. The scientist is now trying to understand why the Martian atmosphere seems to bear the trace of a gas of solar origin, while its solid mantle shows a rather asteroidal signature. What's more, there's no trace of cometary input caused by the impact of numerous comets, which is very surprising. A mystery that could be partly explained by the early formation of Mars, before the dissipation of the protoplanetary gaseous disk, and which planetary dynamics models could help to unravel.

Geochemical analyses are at the heart of this research. Using mass spectrometers, the researcher studies meteorites found on Earth, notably in the deserts of Morocco and Antarctica. These fragments of celestial bodies, often from asteroids, provide invaluable information on the first materials in the Solar System. " By determining the isotopic signature of the elements they contain, particularly rare gases, we can trace their origins back to ", explains Sarah Joiret. Measuring the quantities and proportions of isotopes in matter - that is, measuring atoms of the same element whose different numbers of neutrons give them different masses - enables us to trace their origins, their paths and their sources. Rare gases make ideal tracers. Because they do not react chemically with other elements, they retain a memory of initial conditions. On Mars, their distribution reveals a surprising dichotomy : a mantle enriched in rare gases of asteroidal origin, and an atmosphere marked by solar rare gases. To gather this data, the meteorites are melted under precise conditions to extract sufficient quantities of gas for analysis. A long and painstaking process, but not the only tool available to researchers.

Simulating cosmic chaos

Alongside geochemistry, Sarah Joiret uses numerical models to reconstruct the dynamic evolution of the Solar System. These simulations are based on Hamilton's equations, which describe the motion of bodies under the action of forces, and enable a fictitious planetary system to evolve from initial conditions. " We place the objects where we think they were, make them evolve, and see if we find the current state observed ", explains the researcher.

Among the most influential models, the " Nice model " proposes that a major instability between the giant planets, during the first millions of years of the Solar System, disrupted the orbits of the comets and provoked a massive bombardment of the inner planets. This hypothesis could explain the presence of certain chemical elements on Earth or Mars. The simulations thus enable us to test the consistency of this scenario with geochemical data. However, as these models are chaotic, hundreds of simulations are required to obtain reliable statistical trends.

Interdisciplinarity at the heart of tomorrow's discoveries

Combining modelling and isotope analysis is not an easy task. " These are two very different tools ", admits Sarah Joiret. " Each tells a story. And our job is to see if these two stories are compatible. " This dual expertise, between physics and chemistry, is the fruit of a desire for dialogue between disciplines. The astrophysicist recounts how she had to learn a new language by working with geologists and chemists. A constant challenge, but also an epistemological asset.

Research in this field advances thanks to a collective dynamic, nourished by exchanges, colloquia and collaborations. The arrival of new data from space missions, exoplanet observation and samples brought back from other celestial bodies will refine existing models. This should lead to a better understanding not only of our own history, but also that of other planetary systems.

*Sarah Joiret is an astrophysicist on the  Planetary Formation: from Earth to Exoplanets chair held by Prof. Alessandro Morbidelli.