If you look at the diagram for the habitable zone in our own solar system you can see that Nasa would class both Mars and Venus as habitable planets. They already place pretty generous bounds around the habitable zone.

Given the very real possibility that there could have been (still is?) life on mars that makes a lot of sense. (Nasa just sent Curiosity on its way to Mars and one of its main goals is to look for signs of life.)

Still, the only life we currently know of is here on earth. There could be life on Europa, maybe even some other weird places in the solar system but it’s certainly sensible to start with what we know.