Les astéroïdes du Système solaire ont subi une phase de collisions intense, durant ses premiers millions d'années. © Nasa, JPL, Caltech, T. Pyle

The birth of the Solar System was much more chaotic than we thought

We know that solar system formed about 4.5 billion years ago, from the protoplanetary disk that surrounded our little ones star. He gas and the dust that makes it up grew with each other, forming larger and larger bodies: first planetesimals, then protoplanets, and finally planets. Not all rocks fuse, as evidenced asteroid belt which houses many fragments and small rocks. To track the exact course of this process, researchers analyze asteroids primitive, almost intact, remnants of the first solar system, as a recent study in the journal shows. Astronomy of nature.

Fossils of the birth of the solar system

Some asteroids it crashed on Earth only a few thousand years after its formation, and then contains precious traces of our past. “Previous scientific studies have shown that asteroids in the Solar System have remained relatively unchanged since their formation billions of years ago.”, explains Alison Huntfirst author of the study and researcher at ETH Zurich. “Therefore, they are an archive, in which the conditions of the first solar system are preserved. »

A. Hunt and his team analyzed it that way asteroid nuclei having landed on Earth almost 4 billion years ago. Specifically, these are samples of 18 meteorites of iron, extracted from the metal nuclei of asteroids. They then dissolved each sample, with the aim of establishing isotopic abundance for the palladiumme ‘payment and the platinum. In fact, during the first million years of the Solar System, these asteroid nuclei were heated by radioactive decay, in particular creating a specific silver isotope from the palladium, which then accumulated during cooling. By measuring its proportion among the other elements, it is possible to know when this cooling began, but also in what speed to happen. And the results showed, according to previous studies, a rapid cooling due to numerous collisions: they exposed the asteroid core to the freezing vacuum of space. But what brings the study back is the time when this cooling occurred.

“Our additional measurements of platinum isotope abundance allowed us to correct silver isotope measurements for distortions caused byirradiation cosmic samples in space. So we were able to date the time of the collisions more accurately than ever.explains A. Hunt. And to our surprise, all the asteroid nuclei we examined were exposed almost simultaneously, between 7.8 and 11.7 million years after the formation of the Solar System. » In fact, taking platinum into account has allowed researchers to quantify the influence of cosmic rayswhich can alter the proportion of silver isotopes: thus they have drastically reduced their uncertainties.

Chaotic beginnings for the Solar System

Remember that the solar system it is more than 4.5 billion years old, so a range of 7.8 to 11.7 million years represents only a brief moment in its history. A particularly unstable time, according to researchers. “It simply came to our notice thenasks A. Hunt. And we wanted to know why. ». Several hypotheses could explain this period: these are numerical simulations they have decided. “The theory that best explained this first energy phase of the Solar System indicated that it was mainly caused by the dissipation of the solar nebula.”says Maria Schönbächler, co-author of the study and professor of cosmochemistry at ETH Zurich. “This solar nebula is the rest of the gas it left behind cloud from which the cosmic was born Ground. For several million years it still revolved around the young Sun until it was dragged by the Sun. winds and solar radiationshe adds.

In fact, the stars are made up of collapse gravity of a cloud of gas called a nebula. But not everything collapses: a part of it revolves around the newly formed nucleus, which will later become the famous protoplanetary disk. At beginning of the solar systemthis disk was especially dense: but as the Sun lit up, more and more solar winds they are emitted accompanied by radiation. These winds “blew” much of the isolated dust from the disk, making it less and less dense and reducing the drag of the objects that make it up. By then, all the rock bodies would have accelerated, and then the famous unstable phase would have begun. “Our work illustrates how improvements in laboratory measurement techniques allow us to infer key processes that took place at the beginning of the Solar System, such as when the solar nebula was likely to disappear. Planets like Earth were still “Ultimately, this can help us better understand what our own planets were like, but it can also give us a glimpse of others outside of our Solar System.”concluded Schönbächler.

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