崔辰州2006

黄天衣教授翻译的IAU大会修改后的决议5草案以及由此产生的两个新决议草案
　

现代的观测正在改变着我们对行星系统的认识。天体的命名应当反映这些最新的知识，这一点特别适用于行星这个名词。名词“行星”源自描画“漫游者”，那时只知道它们是天空中移动的光点。最近的发现使我们能用新得到的科学信息创建新的定义（这里不关心区分“行星”和“恒星”的上界，也不涉及恒星和棕矮星的区别）。

（1）行星（planet）是一个具有如下性质的天体：（a）有足够大的质量来克服固体应力以达到流体静力平衡的形状（近于球形）¹，（b）在围绕一颗恒星的轨道上运行，而且既不是恒星也不是行星的卫星。

（2）在我们的太阳系中，区分8个“经典行星（classical planet）”和“矮行星（dwarf planet）”，前者在各自的区域中是占统治地位的天体，后者则不是。

（3）所有环绕太阳运行的非行星的自然天体统称为为“太阳系小天体（small solar system bodies）”，包括现在太阳系大多数的asteroids（译注：原通用译名为小行星），近地天体（NEOs），火星、木星和海王星的特洛央族asteroids，大多数半人马族（Centaurs），大多数海外天体（TNOs）和彗星。在新的命名系统中不再用“minor planet（译注：原通用译名为小行星）”这个词。

¹ 一般指质量超过5×10²⁰千克，直径大于800公里的天体。IAU将建立一个程序，来对接近这个边界值的行星候选者进行评估。

按上面的科学定义，冥王星是一个矮行星，最近发现的一个或更多的大海外天体也是矮行星。与经典行星相比较，这些天体的轨道周期大于200年，一般有大偏心率和大倾角轨道。我们把以冥王星为原型的这类行星天体归为新的一类，称为“XXXXXX”。建议小行星中心（MPC）为这些天体和他们的轨道特征建立一个新的和专门的目录，并把冥王星列为第一个成员。

对于2个或更多天体组成的多天体系统，如果主天体满足行星的定义，定为行星。如果在轨道周期的大多数时间里系统的引力中心位于主天体之外时，满足这些条件的次天体也是行星。不满足这些准则的次天体定为“卫星”。

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation ‘planets’. The word ‘planet’ originally described ‘wanderers’ that were know only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information. (Here we are not concerned with the upper boundary between ‘planet’ and ‘star’, nor the distinction between stars and brown dwarfs.)

The IAU therefore resolves that planets and other bodies in planetary systems be defined in the following way:

(1)A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape^{^[i]}, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

(2) In our Solar System we distinguish between the eight “classical planets,” as the dominant objects in their local population zones, and “dwarf planets,” which are not.

(3) All natural non-planet objects orbiting the Sun, currently including most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter-, Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets, shall be referred to collectively as “Small Solar System Bodies”. In the new nomenclature the term “minor planet” is not used.

[1] This generally applies to objects with mass above 5x10²⁰ kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary.

Pluto is a dwarf planet by the above scientific definition, as are one or more recently discovered large trans-Neptunian objects. In contrast to the classical planets, these objects have orbital periods in excess of 200 years and typically have highly inclined orbits with large eccentricities. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call ‘XXXXXX’. We propose that the MPC create a new and separate catalog for these objects and their orbital characteristics, with Pluto as the first entry.

For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the definition of planet. A secondary object satisfying these conditions is also designated a planet if over most of the orbital period the system’s center of gravity resides outside the primary. Secondary objects not satisfying these criteria are satellites.

[i] This generally applies to objects with mass above 5x10²⁰ kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary.