A team of Austrian scientists have recently taken their colleagues to task for limiting their extraterrestrial search to carbon-based, water-reliant life. In response, they've begun experiments to see whether, water aside, any other solvents have the necessary properties to support extraterrestrial life.
Recently we spoke to Dr. Seth Shostak, senior astronomer at SETI, on just why it is we think extraterrestrial life requires water and carbon. Is it simple scientific fact? Not quite. A lack of humility? Not that either. The answer, as is often the case in the annals of science, is one of practicality.
Q: So why do we limit our search to carbon-based, water-reliant life like we have here on Earth?
A: Dr. Seth Shostak, Senior Astronomer, SETI:
It's true: most researchers who search for extraterrestrial biology look for "life as we know it" – which is to say, carbon-based organisms dependent on liquid water. To some extent this is just pragmatism; after all, it's not easy to design an experiment to search for life-as-we-don't-know it, unless you expect it to be large and complex enough to wave at our cameras.
But the idea that water is essential to life isn't just silly provincialism. After all, earthly organisms are built up from complex chemistry, as anyone who's taken a course in molecular biology knows. The building blocks of your cells are not simple (think of DNA). So in order for life to get started, nature has to cook up some pretty intricate chemical structures. That kind of bottom-up construction project can only succeed if atoms and molecules frequently interact – something that happens best in a liquid medium. A liquid facilitates chemical interactions the way parties facilitate social interaction.
Since water is both plentiful throughout the cosmos and is liquid over a wide range of temperatures, many scientists regard it as the indispensable prerequisite for life. Sure, you can think of other compounds that might work – Saturn's moon, Titan, has lakes of ethane and methane, for example, and who knows? Maybe they've produced something that's alive. But Titan's lakes are far colder than the lakes of a watery ocean, and as a result chemistry is painfully slow there: perhaps too slow to have succeeded in spawning something that can thrive and reproduce.
It comes down to this: it would be hubris of a high order to expect alien life to be identical to what we find on Earth. Extraterrestrial biochemistry is bound to be different, and plants and animals from another world probably won't look much like the pictures in your biology textbook either, given their different evolutionary history. But carbon is able to form complex molecules far better than any other element, and water is the best liquid, as far as we know, for aiding and abetting intricate chemistry. So while alien life might not be life as we know it, there's a good chance that much of it will at least be recognizable.
Dr. Seth Shostak makes a good point, H2O and carbon (when speaking of evolution) are convenient for forming life as well as being abundant in the universe.
I believe I read something about silicon based organisms being possible instead of the normal carbon based ones.
Liquid is only necessary because it is non-static and contains sufficient material.
On a gasseous planet, however, the very air would be dense enough to contain sufficient material and motile enough for chemical reaction. Life, in the form of microbiological dust, floating arround inside of a gaseous giant plantet seems equally plausable from the chemical reactivity perspective.
Lovelock, however, has already elliquently laid out the neccessity that life be a planetary phenomenon. Planets are, in general, closed loops (apart from solar radiation). As such, their stability is based on diversity. Diversity equals exspansion. Exspansion will planetize life in any evolutionary scenario.
Could universes be diverse? Could another universe have a totally different configuration from that of ours? Lets keep our minds open to ideas.
I have thought the same thing forever. Everyone's been brainwashed by science-fiction. The term "alien," in its own context is essentially "foreign," when something is of that nature, it may be hard to qauntify. It's not what is alien, it is what perceptions are lacking...
I wonder if life forms could be organized at a non-chemical level. Chemistry is about electrons. Perhaps life structures could exist and replicate without being dependent on electron transfer. If so, they sure wouldn't look much like us.
There is not a shred of evidence to support the idea that life of any other kind other than one essentially similar to that observed on this planet exists.
Indeed, close consideration of the properties of the chemical elements and their compounds precludes any other than minor departures from the structures we observe on this planet at a comparable stage of evolution
It is, of course, as easy to fantasize about "non-chemical" life forms. Fred Hoyle's "Black Cloud" was one of the early versions efforts. But there is no more rational reason to make such general assumptions than the invention of the many gods so common in our past.
Nevertheless, there is very good circumstantial evidence that the current biological phase will transition, by a process of self-assembly, into an inorganic phase. And that this is likely to occur within a few decades.
Furthermore, when we consider the observed genetic/technological evolutionary vector it is reasonable to assume that a form having appropriate adaptations for extraterrestrial life will subsequently develop.
Given the size of the observed universe, and being aware the "scattergun" way that nature seems to do its evolutionary business we could expect similar trains of events to be occurring elsewhere.
This is all discussed more fully in my recent book “Unusual Perspectives”, the full electronic edition of which can be freely downloaded from the eponymous website.