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.