Planetary Systems and the Formation of Habitable Planets

Abstract

As part of a big national scientific network ’Pathways to Habitable Worlds’ the formation of planets and the delivery of water onto these planets is a key question because water is essential for the development of life on a planet. In the first part of the paper we summarize the state of the art of planet formation – which is still under debate in the astronomical community – before we show our results on the formation of planets. The outcome of our numerical simulations depends a lot on the choice of the initial distribution of planetesimals and planetary embryos after the disappearance of gas in the protoplanetary disk. In addition we take into account that some of these planetesimals of sizes in the order of the mass of the Moon already contained water; the quantity depends on the distance to the Sun - too close and the bodies are dry, but starting from a distance of about 2 AU they can contain substantial amounts of water. Our assumption is that the gas giants and the terrestrial planets are already formed when we check the collisions of the small bodies containing water (in the order of a few percent) with the terrestrial planets. We thus are able to give an estimate of the respective contribution to the actual water content (of some Earth-oceans) in the mantle, in the crust and on the surface of Earth. In the second part we discuss how the formation of larger bodies after a collision may happen in detail, because the outcome depends on different parameters like the collision velocity, the angle of the impact and the materials involved. The detailed results were accomplished with the aid of SPH (Smoothed Particle Hydrodynamics) simulations, a sophisticated numerical method to simulate these events. We briefly describe this method and show different scenarios with respect to the formed bodies, possible fragmentation and the water content before and after the collision. In an appendix we discuss the detection methods for extrasolar planets around other stars for which the current number of known ones is already close to 2000.