One of the hottest topics in modern astronomy is exoplanets — worlds that orbit stars other than our own. That’s not surprising, because there’s a lot to study — almost 2,000 confirmed planets, and thousands more suspects. The variety among these worlds is astounding. There are big planets and little planets, rocky planets and slushy planets, hot planets and cold planets. There are planets around faint stars and bright stars, and even planets around dead stars. Despite the diversity, there are enough known planets to start drawing a few conclusions about planets in general. For one, several studies suggest that most of the stars in the galaxy should have planets. Another study concluded that more than half of host stars are binaries — two stars bound by their mutual gravitational pull. And yet another says that billions of stars should have planets in the habitable zone — the zone where temperatures are just right for liquid water. But there are still plenty of points of contention. On
