The nature of life
My contribution has been to throw away the rulebook.
Rule number one of astrobiology has been that life requires water. Water is a superb solvent, and any biochemistry needs a solvent. Liquid rock is too hot - any chemicals would flash-burn. So what else is there? This has lead nearly all researchers to say that habitable planets = planets with water on (or in) them. From this, we can conclude that carbon chemistry has to be central for life, because other chemistry either does not have the flexibility, or breaks down in water. This has lead the more conservative to suggest that terrestrial biochemistry - proteins, nucleic acids, sugars - is likely to be pretty much Universal.
I disagree, and to illustrate why, I wrote up how biochemistry might work in completely different environments. The formal write-up can be found in Astrobiology, vol 4, pages 137 - 167. This is the short version.
Firstly, what liquids are there in the Universe? Water we know about. Liquid rock is too hot, as is the metallic hydrogen in the centre of Jupiter. Recently, lakes of liquid ethane have been found on Titan. And geysers of what is probably liquid nitrogen were spotted by the Voyager probes as they passed Triton. Some complicated (and not very reliable) calculations suggest that liquid methane and nitrogen might be quite common in our region of the galaxy.
But surely such liquids a) cannot act as excellent solvents, and b) are too cold to allow any useful chemistry to happen? Not so. There are other options, which it would take a lifetime to work through but which we can glimpse from our existing knowledge of chemistry. My favourite is the idea of a life-form based on silicon. Unlike the Asimov/Star Trek versions, this is not based on rocks, but on organosilicon chemistry. Silicon can form as dazzling as diverse set of chemicals as carbon, with the only drawback from a biochemical point of view that most of them break down in water. This is not a problem if your organism is not made of water. And silicon compounds apart from rocks and sand are found in nature - silanes in space, silicones and silicon nitride on Earth and in meteorites. But ... why bother? Why not just use carbon?