Venus is about the same size as Earth. Mars has seasons and ice-capped poles, just like Earth. Plus, they’re in our neighborhood—not far from the Sun. But Venus has a surface hot enough to melt lead, and hurricane-force dust storms whip around Mars. From sunrise to sunrise on Mercury takes six Earth months. Both the differences and the similarities tell scientists a lot about Earth. Looking at our neighbors is like asking over and over, “What if Earth were...?”