Astronomers know our solar system better than any other, but they're still learning new ways in which it doesn't seem to be particularly normal. Right now, the leading theory of planetary formation, called the "core accretion model," is tailored to explain what we see in our solar system — the only one we knew much of anything about when the model was developed. But the more planets we identify in other solar systems, the more we find they don't match the patterns of mass and orbital distances found here on our own. Take, for example, the staggering size gap between...