Terrestrial Planets in Extrasolar Planetary Systems

In this hapter we deal with the dynamical stability of fictitious terrestrial planets in exosolar planetary systems, and concentrate on possible motions in the habitable zones of these systems (see Chapters 4, 6 and 11). Up to now, all planets found outside our solar system are of the size of several earth masses and therefore cannot be regarded as terrestrial planets (which are the interesting ones for they can build up a biosphere). Theoretical studies of fictitious objects - a widely used technique in astrodynamics - are very important to help observers in their search for "a second Earth". The conditions to be fulfilled to allow for a terrestrial planet are reviewed fromthe dynamical point of view: the interesting regions of motion around a host star have been studied by many authors with the aid of extensive numerical integrations. Besides the stability of an orbit itself, its dynamical evolution, changes of its semi-major axis, its inclination as well as its eccentricity are studied. These investigations were accomplished in different dynamical models, with the aid of numerical integration methods of the equations of motion and with different methods of analysis (e.g., chaos indicators). Three types of "habitable" regions for terrestrial planets are the subject of investigation: a) the Jupiter-like planet is outside its star's habitable zone, b) there is a hot Jupiter in such a system, c) the giant planet moves inside the habitable zone. Many studies have been undertaken for systems with only one giant planet, and with more than one gas-planets present in a system respectively. We present the results of several investigations of different authors and discuss also global results of attempts to compile catalogues where the dynamics of planets in habitable zones for most systems can be estimated according to the characteristics of the extrasolar planetary systems. An interesting conclusion can be drawn: the chance for terrestrial planets to move well inside habitable regions around a host star on stable orbits is not too small. The respective estimates from different authors are between 10 and 60 percent for the presently known extrasolar planetary systems!