Late-stage accretion and habitability of terrestrial planets

The final stage in the formation of terrestrial planets consists of the accumulation of ∼1000 km "planetary embryos" and ∼1 km planetesimals via collisional accretion., under the mutual gravity of other solid bodies and the gas giant planets (if any). Water is delivered to planets via collisions with volatile-rich bodies that condensed past the snow line, beyond about 2.5 AU.; We present results of a large number of relatively low-resolution simulations, designed to assess the predictability of systems of terrestrial planets as a function of "observables" such as the orbit of gas giant planets. These show that a variety of terrestrial planets can form, from small, dry, Mars-like worlds to planets with similar properties to Earth, to >3 Earth mass "water worlds" with ≥30 times as much water as the Earth. The terrestrial planets are largely shaped by the influence of the giant planets and the surface density of material. We have uncovered trends between the terrestrial planets and (i) the mass, (ii) the orbital distance and (iii) the orbital eccentricity of a giant planet, (iv) the surface density of the disk, and (v) the disk's density profile.; Five simulations with 1000--2000 particles reveal new aspects of the accretion process Water is delivered to the terrestrial planets as a few large planetesimals in a "hit or miss" process, and as billions of planetesimals in a robust way. The water delivery process is therefore more robust than previously thought, implying that the range of water contents of extra-solar Earths is less stochastic than indicated in previous studies; most planets accrete water-rich bodies.; We simulate terrestrial accretion in the presence of close-in giant planets (e.g., "hot jupiters"), assuming these form and migrate quickly. Potentially habitable planets can form in these systems, but are likely to be iron-poor. Asteroid belts may exist between the terrestrial planets and hot jupiters in these systems. We have also tested the accretion process in four known extra-solar planetary systems. In 55 Cancri, terrestrial planets form relatively easily, and may have orbits in the habitable zone and significant water contents.