“Perhaps we should think of the appearance of life as a planetary event”

Physicist in planetology, Stéphane Mazevet, 52, is piloting the interdisciplinary project “Origins and conditions of appearance of life” at the University of Paris, Sciences & Lettres. A theme that he has just developed in a book entitled Exoplanets and life in the Universe (Odile Jacob, April 2021, 296 p., 23.90 euros).

The first extrasolar planet, 51Peg-b, was discovered in 1995. What have we learned during this first quarter of a century of studying exoplanets?

First of all, that the exoplanets are not at all where we expected them. We had in mind the image of the Solar System and we thought that was the norm, with small rocky planets near the star and large gaseous planets, like Jupiter, very far away. However, the first planet that we discover is a Jupiter very close to its star and which circles it in a few days …

After twenty-five years, we realize that there is a much greater diversity than we had imagined, with a uniform continuum of planets ranging from Earth to bodies several dozen times the size. mass of Jupiter. When we look at this set of objects, we realize that the Solar System is not the norm but perhaps the exception. A real paradigm shift.

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How exotic is the Solar System?

To date, more than 4000 planets have been discovered in 800 systems, but it should be noted that there is a bias due to detection techniques: we can clearly see the planets which are close to their star. That said, statistics show that the Solar System analogues probably make up less than 10% of planetary systems. The discovery of all these giant exoplanets having migrated very close to their star has made one wonder why our giants have stayed so far from the Sun.

This led to a revolution in our vision of the formation of the Solar System: we went from a very static image where the planets were formed where we find them, to an extremely dynamic image where the planets are disturbed, migrate and are unstable. We also realized that the Solar System was chaotic, as Jacques Laskar, of the Paris Observatory showed, when, for three hundred years, we had produced mathematical demonstrations of its stability!

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What is the story of the Solar System being told today?

It is the story of a ballet. After its formation, Jupiter began to migrate towards the Sun. But Saturn had the good taste of being well positioned: it slowed the migration of Jupiter and brought it back outwards. This is called the big tack, the big tack. Another important phenomenon is the instability of the planets with the inversion of the places of Uranus and Neptune. When I talk about this model – which not everyone agrees on – people say to me: “Fine, but what does that change for us? “ Well, this is important because, by changing the history of the Solar System, we change the factors that have influenced the habitability of the Earth, such as the presence of water.

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“Perhaps we should think of the appearance of life as a planetary event”