We are used to thinking of life based upon the distinctive properties of carbon and water. It does not have to be like that, according to astrobiologists. Life probably requires solvents, and a way for complex molecules to be built up: a prerequisite for self-replication that is one of life’s essential features. However, out on Titan (a moon of Saturn) there is liquid methane in which life could arise and whole new biochemical pathways could be imagined. A National Research Council report concludes: “If life is an intrinsic property of chemical reactivity, life should exist on Titan.”

But why stop there? On Triton (a moon of Neptune), where temperatures are so low that gases liquefy, a metabolism based on strange silica compounds and liquid nitrogen has been posited. Or again, astrobiologists have proposed life in the clouds surrounding Venus, and based on sulfuric acid. Why not life around red dwarfs if one tweaks the elements in a different fashion? Or in the multiverses that allegedly exist alongside our own universe, but through an impenetrable curtain? Or near absolute zero? Life — or at least theoretical life — seems to be possible almost everywhere and at all times by Toomey’s account. Even on the edges of black holes.

There is a real problem with these speculations if they are to be regarded as scientific hypotheses, and Toomey gives them far too easy a ride. He does know about Karl Popper’s distinction between science and metaphysics, because it appears in a footnote in his book: science, Popper holds, is testable and falsifiable. Virtually all the fun speculations of the astrobiologists would fail these stringent but necessary criteria. Provided the chemistry and physics are plausible, just about any bio-creation is presented as scientifically justified. The trouble is that they are not yet capable of being falsified, or indeed testable by experiment. It must be a tremendously enjoyable intellectual exercise paddling around in the seas of faraway worlds imagining life as a mist, or life that takes three millenniums to form a thought, or life based on liquid nitrogen. But it is not science. It is playful imagination.

This leads to the interesting notion that the only difference between science and science fiction in astrobiology is who says what. If the creature from outer space comes from the pen of Ursula K. Le Guin, it is science fiction, whereas if it is Carl Sagan (he of the Jovian “floaters”), it must be science. Where predictions have been made of weird life in an earthly system — like claims of arsenical life in Lake Mono — they have quickly been brought to account and sent packing. Provably wrong ideas in science get banished to an amnesiac world. A playful imagination might suggest that they are free to interbreed there and generate new life-forms. Science, Jim, but not as we know it.