Formation of secondary atmospheres on terrestrial planets by late disk accretion

Abstract

Recently, gas disks have been discovered around main sequence stars well beyond the usual protoplanetary disk lifetimes (i.e., 10 Myrs), when planets have already formed 1-4 . These gas disks, mainly composed of CO, carbon, and oxygen 5-7 seem to be ubiquitous 3 in systems with planetesimal belts (similar to our Kuiper belt), and can last for hundreds of millions of years 8 . Planets orbiting in these gas disks will accrete 9, 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 significant 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 sub-Neptune-like pressures. Our new 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 1 arXiv:2004.02496v1 [astro-ph.EP] 6 Apr 2020 impacts. We therefore propose a new paradigm for the formation of atmospheres on lowmass planets, which can be tested with future observations (JWST, ELT, 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 au to a few tens of au from their host stars.The discovery of large amounts of gas around main-sequence stars is recent with most detections occurring in the last few years 3, 11 . These late gas disks are observed in systems that have planetesimal belts, which are older than 10 Myr and can last for hundreds of millions of years 8 .It is thought that the observed gas is released from volatile-rich planetesimals when they collide with each other in the system's belts 6, 12 . The gas then viscously evolves 4 , spreading inward and outwards 10, 15 . Hence the observed gas is likely secondary (rather than of primordial origin) and this late disk (main-sequence) phase is different from the younger (<10 Myr) protoplanetary disks that are much more massive, hydrogen-rich and in which giant planets form within a few millions of years 16 .These late gas disks are nearly ubiquitous around A-type stars; Gas has been detected around more than 70% of systems with bright planetesimal belts 3 . As for other stellar type stars or lower mass systems the statistics are still based on too small a sample as these gas disks are harder to detect but gas evolution models 6 predict that all stars surrounded by planetesimal belts s...

Unveiling the β Pictoris system, coupling high contrast imaging, interferometric, and radial velocity data

“…There can, therefore, be two additional planets with masses between 0.3 and 2.5 M Jup between 10 and 50 au that could be explored further observationally using more SPHERE data and combining them as in this present work, or more sensitive instruments on the JWST or on the ELT. We note that gas observations in β Pic (Matrà et al 2017;Cataldi et al 2018) show that the atomic gas does not extend in the inner regions, as expected from previous models (Kral et al 2016(Kral et al , 2017, and it is mainly colocated with the outer planetesimal belt, which may be a sign that viscous spreading is halted by a planet inward of 50 au, as demonstrated in Kral et al (2020).…”

Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b

“…Statistical analysis of Kepler data suggests a transition between these two populations at ∼1.8 R Earth (Fulton et al 2017), which is compatible with models of photo-evaporation of H 2 -dominated atmospheres surrounding rocky cores (Owen & Wu 2017;Lehmer & Catling 2017), or a water world formation model as proposed by Zeng et al (2019). Following the observed trend in giant planets of the Solar System and a prediction of planetary formation model, one would expect the fraction of heavy elements (metallicity) in primary atmospheres to decrease with planetary mass (Kreidberg et al 2014a;Fortney et al 2013;Kral et al 2020). Sub-Neptunes are thus expected to be enriched in heavy elements reaching typically 100-1000× solar metallicity.…”

A molecular wind blows out of the Kuiper belt

“…However, we note that in the past, when the KB was much younger and heavier, the release of CO would have been orders of magnitude larger (above the solid line in Fig. 1) and the gas dynamics would have also been much different (e.g., in the fluid regime); this could have potentially led to some interesting effects, such as delivering some CO mass from the KB to planetary atmospheres, as proposed recently for extrasolar systems (Kral et al 2020). Indeed, in more massive disks, gas becomes optically thick to the SW and does not get pushed outwards.…”

“…Gomes et al 2005) and could have led to a CO outgassing rate close to the dashed line in Fig. 1, hence providing CO that falls onto the young planets in a greater quantity than from other potential sources, such as impacts (see the comparison between different CO sources in Kral et al 2020). This is a whole new study that emerges naturally from this work and will be tackled in a different paper.…”