I keep reading that scientists are looking for life on other planets. However almost always their premise to determine the same is based on life on Earth, i.e. need for water, air, etc. Why do we always think of life on Earth?
Isn't it conceivable that life on other planets may take a different course? Instead of water they may need something else. Instead of breathing air, they might need some other means of life support.
Have these questions not been analysed? Or already been discounted?
We simply would not have a good idea what to look for if looking for non-water-based life.
We know that life using aqueous chemistry can occupy a wide range of temperatures, pH values, light and dark, using photosynthesis, chemosynthesis - even microbes that "eat and poo electricity" have been discovered in a wide range of environments. (Despite the cute title, the article is a good review of the subject - bacteria and other microbes that derive their metabolic energy directly from external chemical potentials.)
This large variety gives us a sound basis for choosing signs or markers of life processes. Things we should look for that would suggest life.
Since we have no examples of life processes without water, and not even any good theories of what it might be like, it is much harder to propose what signs or markers are worth spending $10,000,000 or 100,000,000 or more for a space probe to look for it.
SETI - the Search for ExtraTerrestrial Intelligence - looks for signs of intelligence, with no bias on chemistry. They certainly don't have a bias towards water, and actually they don't even have a bias towards life. Just intelligence.
Looking for water-based life is a little bit of a case of looking for your keys under the streetlight because that's where the light is, but at least here you know you have lost at least some of your keys under the light.
This has been thought about a great deal in the field of astrobiology, and is the basis of the idea of biosignatures:
The usefulness of a biosignature is determined, not only by the probability of life creating it, but also by the improbability of nonbiological (abiotic) processes producing it...
Perhaps the best example of biosignatures that don't depend on the presence of anything associated with life as we know it is atmospheres in chemical disequilibrium:
Over billions of years, the processes of life on a planet would result in a mixture of chemicals unlike anything that could form in an ordinary chemical equilibrium. For example, large amounts of oxygen and small amounts of methane are generated by life on Earth.
The example is of chemicals associated with life as we know it, but any chemical that should be short-lived and yet is present in large quantities would count as worth investigating as a possible sign of life. We still can't detect much about exoplanet atmospheres, but scientists have been able to say a few things about them. The next generation of infra-red space telescopes will improve matters.
Theory for detection of this kind has been worked on for over 20 years now, and a framework has been established for how to determine whether an observed phenomenon could be life. A system out of equilibrium is an initial sign, the work of Christoph Adami goes from there to examine how to take such data and find out if there is a pattern associated with that which looks like life:
Using such analysis does require a lot of data, though. It could be applied to places like Europa, Titan, or Enceladus, but it will be a while before we could apply it to any exoplanets.
Yes, these questions have been analysed extensively. Life based on compounds other than carbon and water is theoretically conceivable, but unlikely:
It is often hypothesized that life forms elsewhere in the universe would, like life on Earth, be based on carbon chemistry and rely on liquid water. Life forms based on ammonia (rather than water) have been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.[24]
About 29 chemical elements are thought to play an active positive role in living organisms on Earth.[25] About 95% of this living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts.[26] The unique characteristics of carbon made it unlikely that any other element could replace carbon, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—to the chemical composition of life on other planets.[27]
Life on Earth requires water as its solvent in which biochemical reactions take place. Sufficient quantities of carbon and the other elements along with water, may enable the formation of living organisms on other planets with a chemical make-up and temperature range similar to that of Earth.[28] Terrestrial planets such as Earth are formed in a process that allows for the possibility of having compositions similar to Earth's.[29] The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can provide structural elements for life. Plants derive energy through the conversion of light energy into chemical energy via photosynthesis. Life, as currently recognized, requires carbon in both reduced (methane derivatives) and partially oxidized (carbon oxides) states. Nitrogen is needed as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer.
(emphasis mine)
