The study of extrasolar planets has led to some astounding discoveries, many of which have defied the expectations of astronomers and challenged our notions about the forms planetary systems can take. For example, the discovery of Jupiter-sized planets that orbit closely to their stars ("hot Jupiters") defied what astronomers suspected about gas giants. Previously, the general consensus was that gas giants form beyond the "frost line"—the boundary beyond which volatile elements (like water) freeze solid—and remain there for the rest of their lives.
