To date, there are more than 4,000 exoplanets, which are surprising in their diversity and that of their systems. Their detailed observations reveal that their atmospheres are very varied. It is commonly accepted that the origin of this diversity is linked to the formation processes and the history of planetary systems.
The atmospheres of exoplanets are a mixture of different gases : those present from the moment of their formation, those ejected from their interior in the form of volcanism, or finally those ″deposited" by comets. A study led by an astronomer from the Paris Observatory - PSL at the Laboratory of Space Studies and Instrumentation in Astrophysics (LESIA : Paris Observatory - PSL / CNRS / Sorbonne University / University of Paris) shows that a fourth process could well dominate all the others : late gas accretion.
Using the ALMA array of antennas that observe submillimetre radiation, astronomers have indeed discovered disks of late gas around a very large number of stars, a gas that appeared after their planets had formed in their initial cocoons. This gas will enrich the atmospheres of the planets for several tens of millions of years.
This gas is probably released by collisions between planetesimals - rocky bodies several tens of kilometres across - that orbit at the periphery of planetary systems. The gas produced is composed mainly of carbon monoxide (CO), and probably water and more complex molecules. Analysis of the distribution of the gas shows that it is carried towards the interior of the system and the study reveals that it can be captured by the planets in this way.
This very efficient process can accumulate up to several million times the mass of the Earth’s atmosphere and therefore appears to be sufficient to form atmospheres as massive as those of the Earth or Venus, but also much thicker atmospheres such as those of mini-Neptunes (name given to exoplanets of intermediate size between the Earth and Neptune and surrounded by a thick atmosphere). Molecules generated in the outer regions of planetary systems and which are driven inwards and captured by the planets could potentially be favourable for the development of the first building blocks of life.
These hypotheses could be tested with the future James Webb Space Telescope and, on the ground, with the European Extremely Large Telescope.
Reference
This research was the subject of a paper entitled "Formation of secondary atmospheres on terrestrial planets by late disk accretion", by Q. Kral et.al., to be published on 6 April 2020 in the journal Nature Astronomy.
DOI : 10.1038/s41550-020-1050-2
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