In the early days of space exploration it was the rocky planets, particularly Mars and Venus that held some hope of significant life. Though those with the tools of observation and analysis were pretty negative and life in the rest of the solar system looked impossible. But as our knowledge and tools expanded the icy moons quickly became of interest because as cold region natives know, ice is not a bad insulator and a couple of miles of it would protect a lake. These days it seems pretty clear that Icy Moons often have oceans, seas or lakes inside, and the heat that melts the ice from underneath (from orbital stresses and or radioactive decay) could quite conceivably support life.
The article linked discusses model based research based on data from earlier orbiters and flybys. It shows that notionally their are several mechanisms that could be feeding nutrients and energy sources into the ocean of Enceladus, at a rate suffient to support a significant biome.
There are lots of other interesting articles on space at universe today website, take a look.