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| The eight planets, plus Pluto |
Regarded as a group, the planets of our local star range from solid rock and metal to fluffy gas balls. Compared to many of the extra-solar families that have been observed, ours is a rather sedate and orderly system. Each of our worlds goes around the sun in a nearly circular path, and each keeps its distance from the others. Around foreign stars we have seen planets on wildly oval tracks, plunging from the oven to the deep freeze every cycle. Others have been spotted spiraling ever closer to their suns. So we are lucky that our neighbor planets are so placid and predictable in their ways.
The planets of the solar system, according to the official definition, are massive enough to be rounded by their own gravity, but are not massive enough to cause thermonuclear fusion. And also they have cleared their nearby regions of planetesimals. Because Pluto is way out in the Kuiper Belt, which is full of these planetesimals, it is no longer considered a planet.
