Giant impact pulverized atmosphere of exoplanet to give birth to moon

In forming planetary systems, pileups between young planets are said to be rife, colliding with each other until each stabilizes in its cruising orbit. This is how most scientists explain the birth of the Moon. The latter is said to have formed about 4.5 billion years ago from debris resulting from a collision between proto-Earth and another protoplanet the size of Mars called Theia. Astronomers call them giant impacts. Except that beyond the theories none had ever been observed. It is now done … or almost! Indeed, an international team of researchers, including a Frenchman, has just uncovered the first concrete proof of the occurrence of such a cataclysm.

Scanning their favorite object of study, the dust and planetesimal belts that surround the stars (like the Kuiper belt in the solar system), these researchers spotted one that looked really different from the others around the star HD 172555. A star located 95 light years from Earth in the constellation Peacock and only 23 million years old (compared to 4.6 billion for the Sun). “Not only was there way too much dust there compared to what we are used to seeing in a basic planetesimal belt, but the size distribution of these dust grains was not ordinary either. », Explains Quentin Kral, astronomer at the Paris Observatory (Lesia) and co-author of the study published in the highly regarded journal Nature.

But that’s not all ! Upon closer inspection and delving into the archives of the Alma Telescope (Atacama Large Millimeter-Submillimeter Array), researchers, intrigued by the unusual nature of this circumstellar disc, also discovered significant amounts of gas. In particular carbon monoxide (CO), very bright and therefore easy to detect. However, at only ten astronomical units (i.e. ten times the distance between the Earth and the Sun) of a star like HD 172555, the lifespan of such a gas is relatively short since the photons it emits break the molecules. carbon monoxide (CO) by “photodissociation”. Astronomers estimate, however, that the disc observed around the star contains as much as 20% of the CO present in the atmosphere of Venus. So what is he doing there?

After exploring several hypotheses to explain its presence, the authors of the study believe that this dust and this gas are the products of a giant impact that would have torn its atmosphere from an exoplanet close to that of Earth. 200,000 years ago. “This is the only plausible explanation,” confirms Quentin Kral. Dust is the smallest debris resulting from the collision, while gas comes directly from the atmosphere of the impacted planet. This is therefore a first: this type of phenomenon has never been documented before!

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