The issue here is the definition of the "habitable zone" of a star. Right now, scientists think that they are most likely to find life on other planets within the habitable zone of a star, and that definition is based on the Earth. Scientists are looking primarily at Earth-like planets (rocky, with surface liquid water) to find life. But Manasvi Lingam and Abraham Loeb posit that this is too narrow a search. In their study, they look closely at the concept of habitable zone and how it's not necessarily a predictor for habitability. For example, Mars and Venus are within the sun's habitable zone.
However, planets outside this area are capable of supporting liquid oceans underneath a crust of ice. We see this in our own solar system, thanks to worlds such as Europa, Titan and possibly even Pluto. Lingam and Loeb calculate the advantages and disadvantages life would have on these planets, as well as the likelihood life could even exist on them.
They conclude that these types of icy planets with liquid oceans could exist across a wide range of conditions, and that they are much more common (around 1,000 times) than rocky planets within a star's habitable zone. "As these worlds are likely to be far more abundant than the standard paradigm of rocky planets in the HZ of stars, we suggest that more effort should focus on modeling and understanding the prospects for life in subsurface oceans," the study says. After all, life may face more challenges on these worlds than on an Earth-like planet, but if it's just a numbers game, chances are that at least a few of them will support life.