So this brings up my question, why do we believe there to be no life on any of our solar system's planets besides Earth? (As far as I know.) But then I ask, why not? I heard once that the environments aren't supportive to life, but that doesn't make any sense, because the common belief is that we tolerate temperatures, breath oxygen, etc., because we needed to to form here, so?
Although it's possible that life does exist elsewhere in the Solar System, there are some prerequisites that make life (as we know it anyway) more or less likely. The presence of liquid water is one. That rules out Mercury (too hot/no water at all) and possibly Venus, and the outer planets/minor planets (too cold) as well as many of their moons. There are also other things that could make it more likely that life could arise - like solid surfaces (more specifically, phase boundaries where liquid and solid surfaces interact). That rules out the gas giants (Jupiter, Saturn, Neptune and Uranus) as
likely places for life to arise (although it is still conceivable), since they have no solid surface to speak of. From what we know about evolution and the origins of life on Earth, other factors such as 'continuity' are important - that a certain environment exists for long enough to allow life to originate. The surface of Io, for example, is constantly being reshaped by volcanism and as such, no surface features are likely to survive on Io for the massive (millions of years) time periods suitable for the origin of life. Even Earth has this problem, but the Earth's outer crust is relatively stable compared to somewhere like Io. Then there is also the consideration of the availability of energy and organics (or equivalents) to serve as the building blocks of life. Our Moon, for example, simply doesn't have the environments or the building blocks (organics, liquid water etc.) to make the origination of life even remotely possible. However, we know for a fact that simple organics - including amino acids - exist in space (evidence from comets and meteorites has proven this beyond doubt), so we know that the basic building blocks of life are abundant. However, the building blocks alone are not enough - they also need an appropriate crucible to interact. Other places in the Solar System may have had the right conditions for life for atleast some time, but no longer have - Mars is a possible example. But the Earth does have all of these components, and has had them for a very long time - and it has life. Other likely candidates are Europa (one of Jupiter's largest four moons) and Titan, Saturn's giant moon. Both have organics, surfaces, liquid/surface interfaces, heat and energy supplies (i.e. geothermal energy) and Europa is believed to have liquid water beneath it's 'surface' layer of water ice...
Although there is always the possibility of completely different forms of life that would have a completely different set of prerequisites for their formation, we are limited to considering what is likely based on the realities of what types of matter exist and the laws of physics etc. The fact that even the most remote galaxies appear to be made from the same stuff as we are, and obey the same laws of physics as we do, I think it is a safe assumption to say that any consideration of 'possible' forms of life should be limited to what is physically and chemically possible. In other words, although life could be very different to Earth-based life in major ways, I don't think it will be so radically different that we wouldn't recognise it as life.
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< seriously big safety wink.
