Since the 1970s, a number of unusual organisms have been discovered in environments that are hostile to other living things. These organisms, called extremophiles because of their ability to survive at the extremes of typically hospitable conditions, thrive where other life would perish. For example, microbial life has been found deep underground in tiny spaces within rocks, in the frozen environment of Antarctica, in the searing hot waters of deep sea vents and hot springs, and in the harsh environments of dry deserts. Life has also been found in extreme conditions of high radiation, pressure, acidity, alkalinity, or salinity. With the discovery of such organisms, some scientists have broadened their definition of what a habitable environment might be to include a greater range of potential environments for extraterrestrial life.

All life has three main requirements for existence: elements, a source of energy, and a medium that allows for chemical reactions to occur. The first two requirements are readily available throughout the universe. The third requirement causes more controversy. On Earth, liquid water is that essential medium, but is it the only solvent that could sustain life?

Liquid ammonia could possibly fulfill the role of water. In our solar system, liquid ammonia is common, and it is similar to water as a solvent. However, it would be difficult for life to survive in an ammonia environment because the temperature range in which it exists as a liquid, -78°C to -33°C (-108°F to -27°F), is very small and extremely cold. At such low temperatures, chemical reactions happen incredibly slowly, which makes it unlikely that life could arise. Based on our experience on Earth, water appears to be a crucial ingredient for life.

Nevertheless, The National Academy of Sciences has organized a group of scientists to address the limits of organic life of planetary systems. Rather than restrict the search for life to carbon and water-based organisms, the committee is evaluating the possibilities for "weird life." When stretched outside the rules of standard biology, the search for extraterrestrial life opens up even more.

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