Venus and Mars are the most like Earth, but in different ways. In terms of size, average density, mass, and surface gravity, Venus is very similar to Earth. But Mars is the planet that is most similar to Earth in other ways. A Martian day is just over 24 hours, and its rotation axis is tilted by about the same amount as Earth's. Most interestingly, however, it is thought that at some point in Mars's past, it may have had liquid water on its surface, just as Earth does now.