Dynamical Evolution of Planets in the Habitable Zone

One decade after the famous discovery of the first extra-solar planet orbiting a Sun-like star by M. Mayor and D. Queloz from the Geneva Observatory, astronomers strive to find additional terrestrial-like planets outside the Solar System. The discovery of an "exo-Earth" will be a milestone in the very ambitious scientific research of extra- solar planetary systems and an important step to answer the famous question, whether we are alone in the Universe or not. However, the detection of such a planet in a "proper distance" to the host-star, which is called habitable zone does not imply the habitability of the planet. This is an interdisciplinary venture, which includes 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. The latter can be considered as a crucial aspect for long-term stability of the terrestrial-like planets and therefore for their habitability not only from the dynamical point of view, since the evolution of a biosphere is a process over a very long time. This fact underlines the importance of dynamical stability studies for the extra-solar planetary research. Therefore, the present project will be a valuable contribution to this scientific field , for which we expect interesting results about: 1. The application of the resonance crossing mechanism to extra-solar planetary systems, that are similar to the Jupiter-Saturn configuration of the Solar System. Additionally, the influence for the orbital stability of terrestrial- like planets in the appropriate habitable zone will be analyzed. 2. The influence of an accompanying star on the motion of Earth-like planets in the habitable zone of a main- sequence star - based on a stability study of planetary motion in the binary HD4100AB. Such stellar perturbation on this region can be recognized by the appearance of secular resonances, depending on (i) the distance between the HD41004 B and a giant planet outside the habitable zone, (ii) the masses, (iii) the eccentricities. 3. The influence of close-in planets on habitable zones of K- and M-star (assumed that the inward migration has not fully destroyed this region, or terrestrial-like planets have migrated to this region afterwards.) In addition , we will compile a data-base for planetary motion around one stellar component in a double star systems, which will be very helpful for future observations, to verify if a planet in a double star system is in the stable zone or not. Therefore, I believe that the successful completion of the described research program will advance our knowledge on the diversity, formation mechanism and dynamical stability of extra-solar planetary systems.

This project analyzed the stability of Earth-like planets moving in the habitable zone of a Sun-like star. Besides Sun- Jupiter-Saturn like systems planets in double star systems have been investigated. Studying the Sun-Jupiter-Saturn configuration it was found that a small variation in the distance between Jupiter and Saturn influences the habitability of the Earth significantly. In the Solar System Jupiter moves in a distance more than 5 times the Earth-distance (=1 Astronomical Unit = 1AU) and Saturn is at about 9.5 AU. In case Saturn would have reached only a distance of 8.7 AU during the formation phase of the Solar System the two giant planets would strongly perturb the orbit of the Earth. Instead of the nearly circular Earth-orbit a highly eccentric motion around the Sun would be the result. Then the Earth would be outside the HZ for most of the time since its closest distance to the Sun would be next to Mercury`s orbit and the outermost distance is near Mars. In this case the Earth`s surface would have strong variations in temperature, so that the conditions for life would be quite complicated. A similar result was found for systems with a more massive Saturn. If we increase Saturn`s mass by a factor of three - so that Jupiter and Saturn have nearly the same mass - we observe perturbations for the Earth which leads to an increase of its orbital eccentricity up to 0.3 In that case the Earth`s orbits reaches from 0.7 AU (i.e. inside Venus) to 1.3 AU and it would be most of the time inside the HZ and leave this zone only near its perihelion. From these studies we have learned that the planetary constellation of our Solar System seems to be a great fortune and that the habitability of our Earth can be affected considerably by small changes in the planets constellation. Another interesting result from this study is the fact that Venus moves only in a nearly circular orbit because of the Earth-Moon system otherwise it would have an eccentric orbit, where the distance to the Sun varies from 0.5 AU to 0.9 AU. For planetary motion in double stars, where the planet orbits one star, an area has been found which seems not to be influenced neither by the giant planet nor by the perturbing star of the system. If this area coincides with the HZ the conditions for dynamical habitability would be very good. All results are of great importance for the exo-planetary research and the interdisciplinary field of habitability.