第一百日（1）作屎的老貓（第3/8頁）

In addition to the vagueness of the concept of stability， the planets in our Solar system show a character typical of dynamical chaos (Sussman & Wisdom 1988， 1992). The cause of this chaotic behaviour is now partly understood as being a result of resonance overlapping (Murray & Holman 1999; Lecar， Franklin & Holman 2001). However， it would require integrating over an ensemble of planetary systems including all nine planets for a period covering several 10 Gyr to thoroughly understand the long-term evolution of planetary orbits， since chaotic dynamical systems are characterized by their strong dependence on initial conditions.

From that point of view， many of the previous long-term numerical integrations included only the outer five planets (Sussman & Wisdom 1988; Kinoshita & Nakai 1996). This is because the orbital periods of the outer planets are so much longer than those of the inner four planets that it is much easier to follow the system for a given integration period. At present， the longest numerical integrations published in journals are those of Duncan & Lissauer (1998). Although their main target was the effect of post-main-sequence solar mass loss on the stability of planetary orbits， they performed many integrations covering up to ～1011 yr of the orbital motions of the four jovian planets. The initial orbital elements and masses of planets are the same as those of our Solar system in Duncan & Lissauer's paper， but they decrease the mass of the Sun gradually in their numerical experiments. This is because they consider the effect of post-main-sequence solar mass loss in the paper. Consequently， they found that the crossing time-scale of planetary orbits， which can be a typical indicator of the instability time-scale， is quite sensitive to the rate of mass decrease of the Sun. When the mass of the Sun is close to its present value， the jovian planets remain stable over 1010 yr， or perhaps longer. Duncan & Lissauer also performed four similar experiments on the orbital motion of seven planets (Venus to Neptune)， which cover a span of ～109 yr. Their experiments on the seven planets are not yet comprehensive， but it seems that the terrestrial planets also remain stable during the integration period， maintaining almost regular oscillations.

On the other hand， in his accurate semi-analytical secular perturbation theory (Laskar 1988)， Laskar finds that large and irregular variations can appear in the eccentricities and inclinations of the terrestrial planets， especially of Mercury and Mars on a time-scale of several 109 yr (Laskar 1996). The results of Laskar's secular perturbation theory should be confirmed and investigated by fully numerical integrations.

In this paper we present preliminary results of six long-term numerical integrations on all nine planetary orbits， covering a span of several 109 yr， and of two other integrations covering a span of ± 5 × 1010 yr. The total elapsed time for all integrations is more than 5 yr， using several dedicated PCs and workstations. One of the fundamental conclusions of our long-term integrations is that Solar system planetary motion seems to be stable in terms of the Hill stability mentioned above， at least over a time-span of ± 4 Gyr. Actually， in our numerical integrations the system was far more stable than what is defined by the Hill stability criterion: not only did no close encounter happen during the integration period， but also all the planetary orbital elements have been confined in a narrow region both in time and frequency domain， though planetary motions are stochastic. Since the purpose of this paper is to exhibit and overview the results of our long-term numerical integrations， we show typical example figures as evidence of the very long-term stability of Solar system planetary motion. For readers who have more specific and deeper interests in our numerical results， we have prepared a webpage (access )， where we show raw orbital elements， their low-pass filtered results， variation of Delaunay elements and angular momentum deficit， and results of our simple time–frequency analysis on all of our integrations.

In Section 2 we briefly explain our dynamical model， numerical method and initial conditions used in our integrations. Section 3 is devoted to a description of the quick results of the numerical integrations. Very long-term stability of Solar system planetary motion is apparent both in planetary positions and orbital elements. A rough estimation of numerical errors is also given. Section 4 goes on to a discussion of the longest-term variation of planetary orbits using a low-pass filter and includes a discussion of angular momentum deficit. In Section 5， we present a set of numerical integrations for the outer five planets that spans ± 5 × 1010 yr. In Section 6 we also discuss the long-term stability of the planetary motion and its possible cause.