National Geographic has a great visualization of what we know about exoplanets and their habitability. This image shows that thus far, we’ve found fifteen planets (including Mars) that are a) the necessary distance from their star to have the right surface temperature, and b) are the right size to be considered “Earth-like”.
You can view the image better here.
You know, it’s got to be an exciting time to be an astronomer. Then again has there ever been a boring time to be an astronomer? Why aren’t I an astronomer?