Formation of aqua planets with water of nebular origin: effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>Recent detection of exoplanets with Earth-like insolation attracts growing interest in how common Earth-like aqua planets are beyond the Solar system. While terrestrial planets are often assumed to capture icy or water-rich planetesimals, a primordial atmosphere of nebular origin itself can produce water through oxidation of the atmospheric hydrogen with oxidizing minerals from incoming planetesimals or the magma ocean. Thermodynamically, normal oxygen buffers produce water comparable in mole number equal to or more than hydrogen. Thus, the primordial atmosphere would likely be highly enriched with water vapour; however, the primordial atmospheres have been always assumed to have the solar abundances. Here we integrate the 1D structure of such an enriched atmosphere of sub-Earths embedded in a protoplanetary disc around an M dwarf of 0.3$\, \mathrm{M}_\odot$ and investigate the effects of water enrichment on the atmospheric properties with focus on water amount. We find that the well-mixed highly enriched atmosphere is more massive by a few orders of magnitude than the solar-abundance atmosphere, and that even a Mars-mass planet can obtain water comparable to the present Earth’s oceans. Although close-in Mars-mass planets likely lose the captured water via disc dispersal and photoevaporation, these results suggest that there are more sub-Earths with Earth-like water contents than previously predicted. How much water terrestrial planets really obtain and retain against subsequent loss, however, depends on efficiencies of water production, mixing in the atmosphere and magma ocean, and photoevaporation, detailed investigation for which should be made in the future.</jats:p>

Formation of Venus, Earth and Mars: Constrained by Isotopes

“…One should also note that the production of nebula-based water depend on the nebula properties such as the dust grain depletion factor, etc. (Lammer et al, 2014;Kimura and Ikoma, 2020), which can be different on other star systems. Recently, Kimura and Ikoma (2020) showed that primordial atmospheres could be highly enriched in water vapor, if one does not assume solar abundances for the gas disk properties.…”

Hidden water in magma ocean exoplanets

“…In global chemical equilibrium, mantle oxygen may be able to produce water in mole number comparable to or more than hydrogen (Schlichting & Young 2021). Thus, primordial H/He atmospheres may be enriched with water vapour (Kimura & Ikoma 2020). In consequence, the water will in large parts dissolve into the mantle.…”

“…For now, there is no comprehensive interior model that accurately accounts for the chemical reactive atmospheremagma ocean boundary as well as the partitioning of water and other volatiles in the deep interior of sub-Neptunes. Only few studies have investigated individual aspects of it Lichtenberg 2021;Schlichting & Young 2021;Vazan et al 2020;Kimura & Ikoma 2020;Olson & Sharp 2018;Chachan & Stevenson 2018). As volatile partitioning in the deep interior has been neglected for inference studies of exoplanets, previous interior predictions have thus far generally underestimated the amount of water and hydrogen for sub-Neptunes.…”

Exploring the link between star and planet formation with Ariel

“…(right panel ) migration. Here we have carried out those calculations by adding the effects of atmospheric accumulation and loss (Kimura and Ikoma, 2020) in the population synthesis models (Miguel et al, 2019). The symbols for radii of 1-4 R ⊕ are shown with different colours and sizes, indicating that the highdensity planets are diverse in bulk composition; namely, they have different ice-to-rock ratios and different atmospheric masses.…”