All this talk about organisms on other planets (rather than green aliens) brought to mind a presentation my friends did few years back on one of Jupiter's moons Europa. Why, you might ask.
Unlike anything in our solar system, Europa has a smooth surface that is covered by a thin layer of ice. The images of Europa's surface strongly resemble images of sea ice on Earth. It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities.
Galileo (a space craft) has found that Europa has a weak magnetic field. And most interestingly, it varies periodically as it passes through Jupiter's massive magnetic field. This is very strong evidence that there is a conducting material beneath Europa's surface, most likely a salty ocean.
There are traces of oxygen in Europa's atmosphere (yes, it has an atmosphere) but unlike the oxygen in Earth's atmosphere, Europa's is almost certainly not of biologic origin. It is most likely generated by sunlight and charged particles hitting Europa's icy surface producing water vapor which is subsequently split into hydrogen and oxygen. The hydrogen escapes leaving the oxygen.
Though the probabilty is very low, Europa might be a life bearing body, even if that life is that of a tiny speck of bacteria that thrives in these conditions.
Here is the source of my information:
http://www.nineplanets.org/europa.html