For a habitable planet, location is everything. You don't want to be too close to the sun, or water would boil away. Too far away, and water would freeze. A stable, more circular orbit is also important, because wild swings toward and away from the sun wouldn't support a stable supply of liquid water over time. The Origins census of nearby stars will look for planets in this comfort zone, giving us a sense of how many other life-supporting worlds might exist.
Life changes a planet's condition as it takes in food and energy and releases waste products.
Nowhere is this relationship more apparent than in the observed characteristics of an atmosphere, especially combined with the planet's temperature and orbital location. Origins will particularly look for the presence of carbon dioxide, ozone, water, and chemical combinations that wouldn't tend to occur in nature without biological activity. Of course, it's not enough to look for chemicals that only indicate the presence of life as we find it throughout Earth's history. The Origins Program will therefore be developing a catalog of all the possible chemical signatures for habitable planets and for life on them. In this research, it will be very important to identify how atmospheric gases produced by geological activity differ from those produced by life.