scientists can't define life. Last November, NASA launched Curiousity, a rover designed to look for life on Mars. However, according to the article:
When NASA says it wants to find out if Mars was ever suitable for life, they use a very circumscribed version of the word. They are looking for signs of liquid water, which all living things on Earth need. They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive. In other words, they’re looking on Mars for the sorts of conditions that support life on Earth.Of course, the claim of the article, that scientists can't define life, is absurd. The standard definition of life used by biologists is in no way restricted to carbon-based water loving life forms. But we do have to use some criteria for looking for life, and in fact, I think looking for places that we think are like Earth is a good strategy. Certainly we stand a greater chance of finding life if we look in places like those where we know life exists.
But there’s no good reason to assume that all life has to be like the life we’re familiar with. In 2007, a board of scientists appointed by the National Academies of Science decided they couldn’t rule out the possibility that life might be able to exist without water or carbon. If such weird life on Mars exists, Curiosity will probably miss it.
But what the article fails to mention is how we could look for the kinds of life proposed. If we make no assumptions about the chemistry used or the conditions required, we literally have to look at everything, everywhere. That is not something we have the technology to do. It's easy to criticize someone else's methodology as being too narrow, but harder to come up with an alternative approach.
A few years ago I attended a lecture by Freeman Dyson. He has an interesting idea, which I like to explain this way. There's an old joke about a drunk looking for his keys under a street lamp. A passerby stops and helps him look and finally asks if he's sure the keys are here. The drunk replies that he thinks he lost the keys in the alley. Then why have we been searching here? Because the light is better, says the drunk.
Dyson's idea is not to look for life where we think it is, but to look where it is easy to look. Hunters find animals in the dark by shining a light out into the darkness, and when the animals look back, observing the reflections from their eyes. The method is called "pit lamping" and it is illegal in most places.
According to Dyson we could search for life in the solar system without ever leaving Earth. We simply (OK, it's not that simple, but easier than sending robotic probes) illuminate parts of the solar system with powerful beams of light and using ground-based telescopes observe the reflection. By analyzing the spectrum of the returning light we could look for chlorophyll or other chemicals associated with photosynthesis. This assumes the life gets its energy from light, which is not necessarily true either, but as I said some assumptions have to be made or the search is too hard.
Dyson gives a similar lecture in the TED talk entitled Let's look for life in the outer solar system.
At about 3:50 into the video he begins speaking about the search for life. It sounds like a viable way to search, for someone who had the desire and funds. Is it a likely way to find life? Dyson himself admits it is not, but it covers a large amount of real estate very cheaply.