The formation of gas giant planets is assumed to be preceded by the emergence of solid cores in the conventional sequential accretion paradigm. This hypothesis implies that the presence of Earth-like planets can be inferred from the detection of gas giants. A similar prediction cannot be made with the gravitational instability model, which assumes that gas giants formed from the collapse of gas fragments analogous to their host stars. We propose an observational test for the determination of the dominant planetary formation channel. Based on the sequential accretion model, we identify several potential avenues that may lead to the prolific formation of a population of close-in Earth-mass (M) planets around stars with (1) short-period or (2) solitary eccentric giants and (3) systems that contain intermediate-period resonant giants. In contrast, these close-in Earths are not expected to form in systems where giants originated rapidly through gravitational instability. As a specific example, we suggest that sequential accretion processes led to the formation of the 7.5 M planet around GJ 876 and predict that it may have an atmosphere and envelope rich in O2 and liquid water. Assessments of the ubiquity of these planets will lead to (1) the detection of the first habitable terrestrial planets, (2) verification of the dominant mode of planet formation, (3) an estimate of the fraction of stars harboring Earth-like planets, and (4) modification of biomarker signatures.

为解释天王星窄环的长期存在现象，研究了天王星ε环在其两颗牧羊犬卫星作用下的动力演化。采用平面椭圆型限制性三体问题模型，其中三体分别为天王星、卫星及环粒子。基于描述环粒子演化的常微分方程，得到了一个映射系统。对该映射系统的数值研究表明，只有那些初始位于卫星共振有序平动区的环颗粒才能被卫星控制，从而得以长期存在。而有序平动区的大小主要取决于卫星的质量和其运动轨道的偏心率。对于胡斑比环的外牧羊犬卫星Ophelia，只有在其质量约为目前所假设值的1/3时，多数环粒子才能被控制。据此推断0phelia具有比目前所假设值更小的质量。

We obtain a 2-dimensional area-preserving map to study the dynamical evolution of comets. The presence of singularities in the energy-increment function leads to the L

Kuiper带是指太阳系内位于离太阳30到50天文单位一个区域。1992年来该区域陆续发现了一群半径在几十到几百公里的小天体。这些小天体在Kuiper带的分布是极其不均匀的。Kuiper带小天体的发现对人们认识太阳系形成与演化有重要的意义。本文回顾了近年来国际上在Kuiper带小天体动力演化方面的研究，着重分析了目前国际上几种用以解释其非均匀分布的动力学机制，并提出目前该领域的一些尚未解决的问题。

To date, more than 130 cxtrasolar planets around retain sequence stars are revealed mainly by the Doppler radial velocity measurements. Duc to the obscrvational biases, most of the detected planets are moving in orbits closing to the host stars, with some in highly eccentric orbits. Dynamical processes during the late stage of planet formation are important to account for the present orbital properties. These processes include: planet migrations and rcsonance trappings causcd by gravitational interactions between protostcllar disk and planets, dynamical scattering due to interactions between planets, etc. In this paper, we review the major effects of these dynamical processes on the orbital characteristics of the planet systems.

. The applications of the criteria to the other multiple planetary systems are discussed.

We propose the Hamiltonian description of the streamline formalism, which includes: (a) the canonical elements for the unperturbed epicyclic motion and (b) the perturbation equations when the streamlines are perturbed both by conservative and dissipative forces. As an application to planetary ring dynamics, we use the Hamiltonian description to derive a mapping system for the self-gravity model of elliptical rings. The canonical epicyclic elements are also useful in satellites dynamics when the oblateness of the central body is the main perturbation.

A systematic method is presented to construct a mapping model for a near-conservative system, based on that of Hadjidemetriou for a Hamiltonian system[1]. The mapping we constructed preserves the basic features of the actual system. We apply this method to the forced oscillating system and to the 3:1 resonant grain motion in the presence of Poynting-Robertson drag.

With the help of the Laplace-Lagrange solution of the secular perturbation theory in a double-planet system, we study the occurrence and the stability of apsidal secular resonance between the two planets. The explicit criteria for predicting whether two planets are in apsidal resonance is derived, which shows that the occurrence of the apsidal resonance depends only on the mass ratio (m1/m2), semimajor axis ratio (a1/a2), initial eccentricity ratio (e10/e20), and the initial relative apsidal longitude (e20/e10) between the two planets. The probability of two planets falling in apsidal resonance is given in the initial element space. We verify the criteria with numerical integrations for the HD 12661 system and find they give good predictions except at the boundary of the criteria or when the planet eccentricities are too large. The nonlinear stability of the two planets in HD 12661 system are studied by calculating the Lyapunov exponents of their orbits in a general three-body model.We find that two planets in large-eccentricity orbits could be stable only when they are in aligned apsidal resonance. When the planets are migrated under the planet-disk interactions, for more than half of the studied cases, the configurations of the apsidal resonances are preserved. We find the two planets of the HD 12661 system could be in aligned resonance and thus more stable, provided they have 2-1 180. The applications of the criteria to the other multiple planetary systems are discussed.

In the framework of the circular restricted three-body problem (the Sun-planet-comet), we generalize the map of Liu &Sun (1994) to study the transfer of comets from near-parabolic to Halley-family ones, under the perturbations of Jupiter. Numerical esults show that the transfer is effective for comets on both the direct and retrograde orbits crossing the planet's orbit, and on the direct non-crossing orbits but with the perihelion distances close to the semi-major axis of the planet's orbit. The dependence of the transfer probability and of the average transfer time on the perihelion distances of the cometary orbits and on the planet mass are determined. The probabilities of comet transfer from the Oort Cloud are found to be 0:12 and 0:067 for comets on direct and retrograde orbits, respectively. This leads us to expect an average flux of 0:23 new Halley-family comets per year interior to Jupiter's orbit. The calculation shows that vabout86%of the flux comes from comets on the Jupiter-crossing orbits.