San José State University
Thayer Watkins
Silicon Valley
& Tornado Alley

Comparisons of the Sizes
of the Bodies of the Solar System

The classification of the bodies of the solar system into planets, moons and asteroids is somewhat misleading in that planets are thought to be generally larger than moons. Some planets are very large compared to moons but, as the following diagram shows, there are some moons larger than some planets.

When one sees that two moons are larger than the size of Mercury it makes Mercury's status as a planet less significant. Since seven moons, including Earth's moon, are larger than Pluto it makes Pluto's classification as a planet quite dubious. However, the small size of the largest of the asteroids, Ceres, compared to the moons of the solar system indicates that the asteroids are in a different category from the planets and the moons. There are of course many moons which are smaller than Ceres and other asteroids.

The Earth is a convenient comparison object because it is the largest of the smaller planets. The Sun and the large, gaseous planets are so large that Earth seems a pin point in comparison as the following diagram shows.

Another comparison of interest is of the sizes of the asteroids relative to that of a familiar object, the Earth's moon.

There are thousands of objects that could be classified as asteroids, but only thirty which are 200 km or more in diameter. The smaller asteroids are decided not spherical in shape. There are probably millions of objects the size of boulders which could be called asteroids, but debris is a more appropriate label.

A final comparison of interest is of the sizes of the asteroids relative to that of Pluto. If Pluto is not a planet then it would be the largest of the asteroids. The following diagram shows that that would be an awkward classification as well.

Probably a more appropriate label for Pluto would be one that put in the category of the moons, but moon implies that a body is associated with another larger body. Pluto is more or less a loose moon, a moon-object that did not get captured.

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