Abstract

Water content is one of the most fundamental properties of a terrestrial planet. Most moons and Kuiper belt objects in our Solar System have the solar composition of half ice, half rock. The Earth is extraordinarily dry in comparison, ~ 0.05% water by weight. Proximity to the Sun is not the sole cause. While high temperatures are possible in young, actively accreting disks, radiative transfer models (including my own I will present) routinely show temperatures were cold enough to let ice condense, well inside 1 AU, by the time Earth's building blocks were forming. The distribution of water in the disk was also affected by radial transport within the disk. Here I argue that the radial transport of water is sensitive to the star-forming environment in which the disk is born. Planets born in high-mass star-forming regions will be dry. Nearby massive OB stars photoevaporate disks and drive outward transport in them. This transport enhances outward diffusion of water vapor and frustrates inward migration of ice. The inner portions of these disks are dehydrated early in their evolution. In contrast, planets born in low-mass star-forming regions, I predict, are ice-rich. Without photoevaporation-driven outward transport, ice migrates into the inner portions of disks as accretional heating vanishes, before planets form. I will present model calculations of ice/rock fractions in planets formed in either type of star-forming region. Roughly equal fractions of planets should form in either region, possibly explaining the fraction of terrestrial exoplanets with low densities < 2 g/cc.

Presentation

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