I believe that life is ubiquitous in the Universe. Not only on Earth, but on Mars, the moons of Jupiter, may be Jupiter too. In fact life exist everywhere there is an energy gradient (a source and sink) and suitable chemistry.
We must look at what chemistry is required. Well carbon certainly, water and some other chemicals in smaller amounts. It is possible that ammonia could replace water but water will also be present in such systems.
Details of the biochemistry will vary vastly. A DNA-RNA-PROTEIN system is not likely to be the same or even related in unconnected biochemistries.
The external structure of living things is going to depend on the niche that it lives in, as here. So it is likely the convergent structures analogous across the whole of life. So living things living similar lives will have similar structures.
An intelligent nervous system has a very high cost but a very high survival value. Thus intelligent beings are likely to abound in the universe, everywhere where there is sufficient available energy to generate a rich environment of living things.
So as Men discover the rest of the Universe there will be unlimited opportunities to find other species, totally different but all united in their struggle to survive.
Although we may meet competitors we are not likely to find edible species outside our Earth and other species are unlikely to have a taste for a Man.
That is not to say that we will not meet species that are dangerous and even deadly.
We may find that we are the cleverest species anywhere, but I think this is unlikely and I am sure we will discover very clever exobiology. Even on Jupiter. That struggle for survival in such a dangerous place. Is the red spot on Jupiter the torn up remains of animals killed by a giant storm? Will we find giant flying animals in that dense atmosphere. Will we find small green plants on Mars with attendant grazing animals? On a planet with a very low pressure of oxygen. Where did all the oxygen to make those oxidised minerals all over Mars? From photolysis of water? I doubt if there would ever be enough pressure of oxygen from this process to oxidise all that iron. The oxygen must have come from photosynthetic organisms (plants) and thus there would be oxygen consuming organisms to live on the plants (animals).
Life on Jupiter is likely to depend on a thermal gradient rather than light. Living things will use the redox potential available from a compound generated low in the atmosphere and rise to be reduced (or oxidised) higher in the atmosphere on a convection current. An organism with the right enzymes could tap this energy source and use it to synthesise its body parts. Other organisms would eat this "plant" and reverse the redox process to break down the components and make its own body parts and generate motion. With such a vast planet there is likely to be huge numbers of competing species which will give rise to intelligence.
Some of Jupiter's beings are likely to be very clever. There would be no distinction in likely form between the oxidisers and the reducers all would be mobile, with adaptation for the predator and their pray. The only sessile forms will live on, or in, the bodies of mobile organisms.
In a place like Europa with animals living on the reducing power of a volcanic vent there will be oxidisers living on the rocks surrounding the vent and numerous reducers feeding off them swimming in the surrounding water.
Living things will occur in every star system with planets and may exist on some lone "planets" not connected to a star if there is enough energy being generated there.
Living things (based on Carbon) are unlikely to be found above 300 degrees C (approx 600K) or below the freezing point of ammonia. The low point is based on the idea that Ammonia can be a solvent at low temperatures.
The conclusion: God is Unlimited and can make life anywhere. We cannot be alone in the wonderful Universe.
Chris. 4-7-2002.