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.
Both Venus and Mars have good odds of having had liquid water oceans in the past. From what I understand, Venus would possibly be habitable today if it had a carbon cycle and planetary magnetic field to prevent the runaway greenhouse effect.
it allows to imagine that something could be a "life" in Venus like conditions as well:
"Scientists once thought that no living thing could survive the harsh combination of toxic chemicals, high temperatures, high pressures, and total darkness at these vents. "
"Vent bacteria use hydrogen sulfide as their energy source instead of sunlight. "
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.