From Charles Q. Choi at Inside Science:
When it comes to looking for alien life, scientists mostly focus on where there is water. Now researchers suggest that looking at “bioessential” elements such as phosphorus and molybdenum could help judge a world’s potential for life.
But if we can’t find water, why should we consider these elements biosignatures?
To see what roles such bioessential elements might play in the evolution of alien life, the researchers focused on how accessible they might be on worlds with liquid oceans underneath their frozen surfaces, much like Jupiter’s moon Europa and Saturn’s moon Enceladus. “People suspect there may be life in liquid water under the ice in Europa and Enceladus, and both NASA and ESA [the European Space Agency] have plans to visit them with missions such as Europa Clipper 1,” Loeb said.
Yes. But we already know there is water there. Thus we would be justified in searching such spaces even without a search for “bioessential” elements.
…
The researchers cautioned that although bioessential elements might prove rare on average across a world, patches could exist on a world where levels of these elements, and the chances for life, are higher, Lingam said. And of course, the researchers only accounted for life as we know it — “life as we do not know it might follow different chemical routes than on Earth, which would be a more exciting finding than finding life as we do know it,” Loeb said. More.
Looking for “life as we do not know it”? In theory, something similar to what we call life might exist by depending on some substance other than water. But the question then becomes, what makes the search for the entity a science-based endeavour? A scientific search must begin with and stick to a rationale. So we would need to develop a model for non-water-based life before we know what to look for.
See also: At Forbes: Maybe fine-tuning doesn’t matter as much as some claim. Many planets might support life… It’s convenient for Siegel that no one has found life anywhere but on Earth. He is free to assume whatever he likes about the extraterrestrial conditions that imagined life forms could survive, and call it science.
and
What becomes of science when the evidence does not matter?