Nearly isotropic comets with very long orbital period are supposed to come from the Oort Cloud. Recent observational and theoretical studies have greatly revealed the dynamical nature of this cloud and its evolutionary history. However, many issues are yet to be known. Our goal is to understand current structure of this cloud as well as its dynamical origin. For estimating the current structure of the Oort Cloud, key information lies in the original orbit of the Oort Cloud new comets (OCNCs) that are defined at a distance where these objects do not receive gravitational perturbation from major planets (such as at rg = 250 au from the Sun before comets enter into the planetary region). There have been several attempts to obtain OCNC’s original orbits, but it never has been an easy task. This requires numerical orbit propagation of the observed comets with high accuracy including perturbation from major disturbing bodies. In addition, non-gravitational forces often play significant roles here. First and foremost, the orbit determination of OCNC includes substantially large uncertainty because of limited number of observational arcs and very large eccentricity of the comets (~1). Here we show our preliminary result of comparison of various catalogues of OCNCs’ original orbital elements at rg = 250 au: So-called the Warsaw catalogues by Krolikowska, the ephemeris given by MPC (Minor Planet Center), that given by Horizons/JPL, and others calculated by a few individuals (Marsden, Kinoshita, and Nakano). The resulting orbits that these catalogues yield are overall similar, but sometimes they are starkly different by reasons yet to be known. Through a series of plots with a help of our own orbit propagation using numerical and analytic methods, we give considerations on which catalogue yields the information that is the most significant (or the most fundamental) for understanding structure, origin, and evolution of the Oort Cloud.

Many of the unusual properties of the dwarf planet Pluto's orbit are widely accepted as evidence for the orbital migration of the giant planets in early solar system history. However, some properties remain an enigma. Pluto's long term orbital stability is supported by two special properties of its orbit that limit the location of its perihelion in azimuth and in latitude. We revisit Pluto's orbital dynamics with a view to elucidating the individual and collective gravitational effects of the giant planets on its perihelion location. In this presentation we demonstrate with numerical experiments that, while the resonant perturbations from Neptune account for the azimuthal constraint on Pluto's perihelion location, the long term and steady persistence of the latitudinal constraint is possible only in a narrow range of additional secular forcing which arises fortuitously from the particular orbital architecture of the other giant planets. Our numerical investigations also find that Jupiter has a largely stabilizing influence whereas Uranus has a largely destabilizing influence on Pluto's orbit.

Recent observational and theoretical studies have greatly revealed the dynamical nature of the Oort Cloud and its evolutionary history. However, many issues are yet to be known. Our goal is to understand current structure of this cloud as well as its dynamical origin. For estimating the current structure of the Oort Cloud, key information lies in the original orbit of the Oort Cloud new comets (OCNCs) that are defined at a distance where these objects do not receive gravitational perturbation from major planets (such as at r = 250 au from the Sun before comets enter into the planetary region). There have been several attempts to obtain OCNC’s original orbits, but it never has been an easy task. This requires numerical orbit propagation of the observed comets with high accuracy including perturbation from major disturbing bodies. In addition, non-gravitational forces often play significant roles here. First and foremost, the orbit determination of OCNC includes substantially large uncertainty because of limited number of observational arcs and very large eccentricity of the comets (~1). Here I show our result of comparison of various catalogues of OCNCs’ original orbital elements at r = 250 au: So-called the Warsaw catalogue by Krolikowska, the ephemeris given by MPC (Minor Planet Center), and that given by Horizons/JPL. In particular, I pay attention to the difference of the original semimajor axis among the several different solutions that the Warsaw catalogue and the MPC ephemeris have in comparison with the solutions given by Horizons/JPL - such as the difference between the solution 1 in the Warsaw catalogue and the solution from Horizons/JPL, the solutions 1 and 2 in the Warsaw catalogue, the solutions 2 and 3 in the MPC ephemeris and so forth. The resulting orbits that these solutions yield look overall similar, but sometimes they show stark difference for some reason.

We describe the result of our numerical simulations that trace the dynamical evolution of the Oort Cloud new comets that fall down to the planetary region. These comets approach the planetary region (such as inside Neptune’s orbit) through the galactic tide and encounter with nearby stars, stay in the planetary region for a while, and eventually get scattered away. We combine two dynamical models (analytic and numerical) to follow the dynamical paths of the comets that leave the Oort Cloud during two different periods: One is the 1 billion years that include the present time when the outer comet cloud is likely nearly isotropic, and the other is the 1 billion years in the early solar system presumably with the comet cloud confined in a flat disk. Here are two major conclusions derived from our numerical result: (1) Typical dynamical lifetime of the comets is O(10^7) years. After traversing the planetary region for this timespan, most of the comets get scattered and ejected out of the system. (2) When orbital inclination of the comets is small, the so-called planet barrier works effectively, preventing the comets from penetrating into the terrestrial planetary region. Transition probabilities of the comets into other minor body populations, such as Centaurs or Jupiter-family comets, are also discussed.