Question: Rambam states in [Hilchot Yesodei HaTorah, Chapter 3, Section 9](https://www.chabad.org/library/article_cdo/aid/904969/jewish/Yesodei-haTorah-Chapter-Three.htm#9) that each of the planets have a soul, knowledge, and intellect.

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> כל הכוכבים והגלגלים כולן בעלי נפש ודעה והשכל הם והם חיים ועומדים ומכירין את מי שאמר והיה העולם
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> All of the stars and spheres possess a soul, knowledge, and intellect. They are alive and stand in recognition of the One who spoke and [thus brought] the world into being. 
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What are we to understand from this? It's a rather strange concept indeed when you get to thinking about it.

Answer: 
Rambam discusses this at length in Book II of *Guide for the Perplexed*. Particularly chapters 4-5 would be informative on this topic. You can read them online in the Friendlander English translation [here](http://www.sacred-texts.com/jud/gfp/gfp091.htm). Chapter Four in fact opens by noting that you might find it strange at first:

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> THE enunciation that the heavenly sphere is endowed with a soul will appear reasonable to all who sufficiently reflect on it; but at first thought they may find it unintelligible or even objectionable; because they wrongly assume that when we ascribe a soul to the heavenly spheres we mean something like the soul of man, or that of an ass, or ox. We merely intend to say that the locomotion of the sphere undoubtedly leads us to assume some inherent principle by which it moves; and this principle is certainly a soul. 
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A little later he states:

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> The circular motion of the sphere is consequently due to the action of some idea which produces this particular kind of motion; but as ideas are only possible in intellectual beings, the heavenly sphere is an intellectual being.
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And a little later:

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> It follows that the heavenly sphere must have a desire for the ideal which it has comprehended, and that ideal, for which it has a desire, is God, exalted be His name! When we say that God moves the spheres, we mean it in the following sense: the spheres have a desire to become similar to the ideal comprehended by them. This ideal, however, is simple in the strictest sense of the word, and not subject to any change or alteration, but constant in producing everything good, whilst the spheres are corporeal: the latter can therefore not be like this ideal in any other way, except in the production of circular motion: for this is the only action of corporeal beings that can be perpetual; it is the most simple motion of a body; there is no change in the essence of the sphere, nor in the beneficial results of its motion.
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These quotes should give you a taste of the general idea, but again you may want to read the chapters at length for the full experience.
Rambam's reasoning is [seemingly] simple (to elaborate Alex's quote *"...leads us to assume some inherent principle by which it moves; and this principle is certainly a soul."*): 

1. Nature is divided into animated and unanimated.
2. Animated world possesses the lowest form of a soul - the Nefesh (out of נרנח"י). Nefesh is one that allows it to move.
3. Unlike the Earth and plants that stand still, celestial bodies move. Moreover, they seem to move on their own (nobody pushes them) and they move in circular (nonlinear) trajectories.
4. Therefore they must possess Nefesh.
5. Therefore, like all animals, they are self-aware and knowledgeable (see שירת הבריאה).
6. Because they belong to the upper world beyond the firmament and close to G-d, they know G-d very much.