Forming Hospitable Rocky Worlds with Some (But Not Too Much) Water

Planetesimal-based models frequently result in the formation of small planets (R < 4REarth) with water mass fractions > 1 % (e.g. Alibert & Benz, 2017). Such planets are thought less likely to promote the biochemical origins of life than drier ones (Noack et al. 2016). Some pebble accretion models can form small planets within 106 yrs (Chambers, 2016), in the presence of a gas-rich disk, and gather a primordial atmosphere of mass fraction 2 % (Ginzburg et al. 2016). If the inner disk is wet due to the migration of volatile-rich planetesimals (Ciesla & Cuzzi, 2006), such a planet could accrete one Earth ocean of water, along with concomitant amounts of molecular hydrogen and other species. Though young Sun-like stars have enhanced EUV, FUV, and x-ray fluxes capable of driving photoevaporative flows, the species-dependent mass loss is insufficient to remove enough of the H and He rendering such a planet inhospitable (Howe et al. 2020). These processes could impact the final atmospheric composition of such a planet, although erosion-driven mass loss is a major factor (Biersteker & Schlichting, 2019). One way to desiccate planetary building blocks is through 26Al and 60Fe decay-driven heating, yet numerous water worlds still result (Lichtenberg et al. 2019). What if the outer, volatile rich, circumstellar disk material, could be separated from terrestrial planets forming near the liquid water zone of their host stars? Gas giant planets formed near the ice-line could have a dramatic impact on the water content of small temperate planets (Bitsch et al. 2021). If ice-line gas giants are found around 10 % of Sun-like stars, and rocky planets near the liquid water zone are found around about 10 %, is the probability of having both uncorrelated? We explore recent occurrence rate estimates for inner rocky planets, as well as gas giants near the ice-line, including stability criteria for multi-planet systems (cf. Bowens et al. 2021) to assess the likelihood of having both, and compare to observational constraints. Bryan et al. (2019) have found that the probability of having a gas giant near the ice line around low mass stars, is more than x3 greater (compared to typical field stars) if the system has an inner super-Earth. Given global occurrence rates, this implies that most gas giants near the ice-line also possess an inner super-Earth.