We think one sun is normal because that's all we've got. In reality, solar systems with two stars are two times as common as single-star systems. And data from the Spitzer Space Telescope suggest that planets may in fact be more common in binary-star systems, like the one imagined below. About 60 percent of the binary systems studied have debris disks, suggesting the potential for planets, whereas only 20 percent of single stars have them. Astronomers have even found tentative evidence of a planet in a triple star system, HD 188753.