It is tempting to think that a planet is a planet is a planet. But one word-planet-surely covers a multitude of entities. Just as satellite does. Planets vary all over the place, not only in their size and density and composition. In our own solar system, variability is great. Earth is the densest of the nine planets (I still think of Pluto as a planet). Jupiter is the largest. Venus is the hottest. I suppose we shouldn't be surprised. After all, stars vary a great deal with regard to their size, temperature, age and composition. They vary more than people do. Shouldn't the same processes pretty much apply everywhere concerning planets? Apparently not.
展开▼
