Exoplanetary systems: architecture, evolution, habitability

The search for other habitable worlds in the universe is certainly a venture of great scientific and public interest. Space missions like CoRoT, Kepler, Herschel, Gaia and others will hopefully help us to detect an "exo-Earth". Recent discoveries of smaller planets (with some Earth-masses) raise our hopes to find such an exo-Earth, which may be situated in the so-called habitable zone (HZ) of a Sun-like star. In this region surrounding a star, the surface-temperature of the planet might be similar to that of our Earth, so that similar conditions for this planet can be expected. Certainly, the detection of such a planet in a "proper distance" to the host-star, does not imply the habitability of the planet. Defining habitability is, of course, an interdisciplinary venture. Astrophysical studies like the evolution of the host-star and of the planet, the existence of a planetary atmosphere or the orbital behaviour of all celestial bodies in the system are vital in this respect. Since the evolution of a biosphere is a process encompassing large timescales, long-term stability of a planetary system is a necessary requirement for the habitability. In this project we will focus on the long-term stability of (fictitious) Earth-mass planets moving in the HZ of Sun- like stars. We study the architecture, evolution and habitability in (i) single-star multi-giant planet (S-MP)systems and in (ii) double star (DS) systems. From our studies we expect detailed information about * Possible spacing of Earth-mass planets in the HZ of S-MP systems that have similarities with our Solar System; * The stability of Earth-mass planets in the HZ, when the giant planets move on eccentric orbits (since many giant planets have been found in such highly eccentric orbits); * The possibility to predict the existence of additional giant planets in a planetary system at large distances from the host-star via dynamical studies; * The stability of highly inclined multi-planet systems; * The planetary formation in inclined DS systems using N-body simulations. Due to the practical applicability of these topics to observational target assessments, the realisation of this project will be a valuable contribution to the extra-solar planetary research.

The FWF project Exo-planetary systems: Architecture, Evolution and Habitability studied research related to a current topic in Astronomy and Astrophysics. Using numerical simulations, we showed how the architecture of planets and the long-term orbital evolution affects the habitability of an Earth-like planet in the so-called habitable zone. Moreover, this project was not restricted to planets orbiting single stars like the Sun. Therefore, we studied also multi-planetary configurations in binary stars. The reason for such studies was raised from observations in the solar neighborhood which showed that about 60 % of all star in this region form such stellar systems. Therefore, it is interesting to study whether binary stars provide appropriate conditions for habitability or not. Of course, in such stellar systems we are faced with additional perturbations as it is quite usual that a higher number of massive bodies cause stronger perturbations. However, dynamical studies have shown that resonant orbital motion can also provide stable conditions. This means that the discovery of an additional planet in a system does not lead immediately to chaotic motion as resonances can be stabilizing under certain conditions. In the studies of this project we distinguished between mean motion resonances (i.e. MMRs caused by the orbital periods of the planets) and secular resonances (due to the motion of the peri-center) where periodical perturbations might change the shape of the orbit. Locations of MMRs can be calculated easily for given positions of planets. In this project, we developed a new semi-analytical method which allows us to calculate the location of a secular resonance without a lot of computational effort. This is of great importance, especially for multi-planetary systems in binary stars as the perturbations of the secondary star can easily cause such a secular resonance which might vary the orbital eccentricity and therefore the shape of the orbit. In case such a perturbation affects the habitable zone, the motion in this area could be highly eccentric so that a planets orbit would not lie as a whole in this zone which provides best conditions for habitability concerning insolation on a planet. Moreover, we showed in our studies that strong variations in the orbit of our Earth could be possible for certain Jupiter-Saturn configurations. A small change in the architecture of these giant planets (i.e. in case Saturn is 0.8 AU closer to Jupiter) can lead to strong perturbations of a planet in the habitable zone which may change the conditions of habitability significantly. We have found that Earth orbit could extend from 0.3 to 1.7 AU for a special Jupiter-Saturn configuration. In that case the existence of the other terrestrial planets would be questionable.