A breakthrough in the formation of exo-terrestrial atmospheres

Exoplanets could capture a late gas in their atmospheres for tens of millions of years, enriching them with carbon and making them potentially suitable for life. This is the conclusion reached by Kral et.al., a team from the Paris Observatory.

Abstract: Recently, gas disks have been discovered around mainsequence stars well beyond the usual protoplanetary disk lifetimes (that is, ≳10 Myr), when planets have already formed1–4. These gas disks, mainly composed of CO, carbon and oxygen5–7, seem to be ubiquitous3 in systems with planetesimal belts (similar to our Kuiper belt), and can last for hundreds of millions of years8. Planets orbiting in these gas disks will accrete9,10 a large quantity of gas that will transform their primordial atmospheres into new secondary atmospheres with compositions similar to that of the parent gas disk. Here we quantify how large a secondary atmosphere can be created for a variety of observed gas disks and for a wide range of planet types. We find that gas accretion in this late phase is very important and an Earth’s atmospheric mass of gas is readily accreted on terrestrial planets in very tenuous gas disks. In slightly more massive disks, we show that massive CO atmospheres can be accreted, forming planets with up to subNeptune-like pressures. Our results demonstrate that new secondary atmospheres with high metallicities and high C/O ratios will be created in these late gas disks, resetting their primordial compositions inherited from the protoplanetary disk phase, and providing a new birth to planets that lost their atmosphere to photoevaporation or giant impacts. We therefore propose a new paradigm for the formation of atmospheres on low-mass planets, which can be tested with future observations (James Webb Space Telescope (JWST), Extremely Large Telescope (ELT), Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL). We also show that this late accretion would show a very clear signature in sub-Neptunes or cold exo-Jupiters. Finally, we find that accretion creates cavities in late gas disks, which could be used as a new planet detection method, for low-mass planets a few to a few tens of astronomical units from their host stars.The exoplanet atmospheres we study may not be primary atmospheres. Models show that the accretion of gas late in a planetary system’s formation may completely replace the primary atmospheres of terrestrial planets. These secondary atmospheres are likely to have high metallicities and high C/O ratios.