During a recent panel discussion on water in the universe, NASA chief scientist Ellen Stofan stated that she thinks we’ll have strong indications of life beyond Earth within the next ten years, and definitive evidence within 20 to 30 years. She was referring to the possible discovery of microbial life, given recent indications that liquid water is common in many places in the Solar System (and probably beyond).
While Stofan’s enthusiasm is encouraging, and she very well might be correct, her statement has to be taken with a grain of salt. The presence of life on some other planet or moon requires not only that life can survive, but also that it originated there in the past. We expect that Earthlike, water-based life could find many habitable planetary bodies in the universe. But conditions for the origin and evolution of life are likely to be more rare.
While liquid water is a key ingredient for life here on Earth, this might not necessarily be true elsewhere. Water even has some properties that make life difficult—such as its volume expansion during freezing, which commonly leads to the piercing of cellular membranes. Also, any origin-of-life researcher can tell you about the difficulty of using water to achieve the organic synthesis of larger macromolecules critical for life.
Perhaps we should view the water issue the other way around. Water is used as a solvent for life on Earth because it is so common here, and life had no other choice but to use it in order to establish itself on our planet. Methanol would be a better solvent based on its physical and chemical properties, but it is simply not around on Earth, at least not in sufficient quantities.
Habitable worlds appear to be far apart, and for any of them to contain life, the more stringent conditions for the origin of life have to be fulfilled, which we don’t understand well at all. One place where this might not apply is Mars, which is close enough to Earth that panspermia—the transport of organisms between neighboring planetary bodies—is conceivable, particularly in the Solar System’s early history, when Mars still had large water bodies on its surface and Earth already teemed with life. But in that case, finding life on Mars could be viewed as re-discovering our long-separated, ancient “cousins” instead of discovering true extraterrestrial life (although one could argue about the semantics).
Thus, as much as we should applaud the enthusiasm of NASA’s chief scientist (and as much as I personally hope her prediction comes true), some healthy skepticism is in order. Proof of life with remote sensing technology alone will be very difficult, even more so outside of our Solar System.