The origin of diversity of extrasolar planets formation of giant gasous planets and orbital evolution

自1995年以来，已经发现了大约30颗系外行星。它们可能是太阳系中像木星和土星这样的巨行星，因为目前的探测方法限制了可以探测到的行星质量。其中大约一半的轨道周期非常短（比水星短），而另一半的轨道离心率很大。我们试图通过推广太阳系的标准理论来解释太阳系外行星的多样性。今年，我们更深入地研究了巨行星的形成及其轨道稳定性。巨行星是这样形成的：一个大约有10个地球质量的固体核心从星子中吸积，原行星盘的气体被核心的强大引力捕获，形成巨大的包层。我们研究了(1)固体核的形成与原行星盘质量的关系；(2)气体落向原行星盘的时间尺度；(3)原行星盘中巨行星的轨道稳定性。对于(1)，我们发现在大质量原行星盘中，内部区域可以形成气态巨行星。在(2)中，我们发现气体的进入可以在比目前考虑的更小的核心质量开始，但气体进入的时间尺度相当长。对于(3)，我们发现气体阻力通常会稳定轨道，但当行星之间的分离距离小于某个临界值时，效果较弱。根据这些结果，我们得出结论，将太阳系的标准理论推广到不同质量的原行星盘将解释太阳系外行星的多样性。

About 30 extrasolar planets have been discovered since 1995. They would be giant planets like Jupiter and Saturn in the Solar system, because the present detection method limits planet mass that can be detectable. About half of them have very short orbital period (shorter than that of Mercury), while the other half have large orbital eccentricity. We have tried to explain the diversity of the extrasolar planets by generalizing the standard theory of the Solar system.This year, we investigated the formation of giant planets and their orbital stability more deeply. Giant planets are formed as follows a solid core with about 10 earth masses accretes from planetesimals and gas of a protoplanetary disk is captured by strong gravity of the core to form massive envelope. We investigated(1) the dependence of formation of a solid core on mass of a protoplanetary disk,(2) time scale of gas infall onto the core, and(3) orbital stability of giant planets in the protoplanetary disk.Regarding (1), we found that in a massive protoplanetary disk, gas giant planets can be formed in inner region. Regarding (2), we found that gas infall can start at less massive core mass than so far considered, but the time scale of gas infall is rather long. Regarding (3), we found that gas drag generally stabilizes the orbits, but the effect is weak if separation distance between the planets is smaller than some critical value. From these results, we conclude that generalization of the standard theory of the Soalr system to different mass protoplanetary disks would account for the diversity of the extrasolar planets.