Here’s how I am tempted to answer that seemingly legitimate question: very, very little. Alone, a planet is really just an abstraction.
Mercury, for one quick example, is related to our curiosity – and some degree of curiosity exists in more or less everyone. But obviously there are people who are driven by curiosity, and people who barely feel it at all.
More to the point, what exactly are you curious about? Show me an article about human migration patterns as reflected in ancestral genetics or 19th century sailing vessels, and I will devour it. Seeing those same articles, you might skip to a piece about how to improve your golf swing – while I would have to be paid handsomely even to read the first paragraph.
Curiosity is clearly not a question of right or wrong. It’s more like different strokes for different folks. We all have Mercury in our charts, and we can make a few general statements about its archetypal nature. But what does Mercury actually mean for an individual? Who knows? . . . or rather, who knows – unless we give that Mercury a set of distinct motivations and interests by placing it in a specific sign. After that, we might give it an area of characteristic behavior by putting it in a house. Then we could further wire it into the larger framework of the birthchart by studying the aspects that it makes.
A planet in a specific sign and a specific house: for actual human beings, that is the ultimate indivisible quantum unit of astrological meaning. A planet alone is only a broad idea, about as “human” as a lecture on taxation algorithms.