Extra-solar massive planets with small semi-major axes? S. Fern´andez, D. Giuliodori and M. A. Nicotra Observatorio Astron´ omico. Universidad Nacional de C´ordoba. 5000 C´ ordoba, Argentina. Monograf´ ıas de la Real Academia de Ciencias de Zaragoza. 25: 115–120, (2004). Abstract In this paper we explore the perturbing forces between two giant planets as a possible mechanism for the observed depletion of massive planets with small semi- major axis and high eccentricities in recently detected extra solar systems. The orbital evolution of simulated systems composed by a solar like central star and two massive planets is performed by the numerical integration of the classical equations of motion using a Bulirsch-Stoer integrator. Our results show strong dependence on initial conditions and the adopted mass for planetary companions. Key words and expressions: Exoplanets, numerical simulations, orbital evolu- tion. MSC: 37M05, 70F15, 70M20, 85A99. 1 Introduction The detection of extrasolar planets during the last years is one of the greatest discoveries in the history of Astronomy. In fact, the possibility of existence of other worlds beyond our solar system has attracted the human mind since ancient epochs. For astronomers interested on celestial mechanics problems, these indirectly detected objects show un- expected dynamical features and open a rich field for exploration with a new series of challenges. More than one-third of the detected exoplanets have significantly elliptical orbits, with eccentricity e> 0.3 and about two-third seem to be orbiting their host star much than the Mercury-Sun mean distance. The progress in the understanding on formation and properties of the extrasolar plan- ets is in evolution but it is far from being considered as a complete theory. 115