By spectroscopically analyzing the light from extrasolar planets, the compositions of their atmospheres may be determined. There are certain gases that are so reactive, they will not remain long in an atmosphere without a source continually replenishing them. Earth has, for example, short-lived gases like molecular oxygen and methane in its atmosphere because its biosphere constantly produces them. Scientists say that Earth's atmosphere is in chemical disequilibrium, and this disequilibrium is a direct result of life. The discovery of a planet with an atmosphere in chemical disequilibrium that cannot be explained by known non-biological processes would be a hugely important find.
Molecular oxygen is key gas New Worlds Observer will search for. A good fraction of life on Earth uses it, but more importantly, O2 in an atmosphere cannot be produced in any detectable abundance by any known abiotic (i.e. non-life) processes. The oxygen in our atmosphere is a direct result of plant life – before Earth had photosynthetic plants to produce oxygen, our atmosphere lacked the gas. Free molecular oxygen is very reactive, and it requires a constant replenishing source (like plants) to keep it in an atmosphere. The discovery of a planet with free molecular oxygen in its atmosphere would therefore be extremely interesting, as it would be strong evidence for the presence of life. New Worlds Observer will look in particular for an O2 absorption line known as the “A band” at a wavelength of 760 nm since this is the strongest O2 absorption line it will be able to detect.
All life as we know it uses water, and we have reason to believe that life elsewhere might be inclined to use it, too, because water has many unique properties that make it especially useful to life. Water is not a “biosignature” in the way O2 is, since the water in our atmosphere does not result from life. It is, however, difficult to imagine a habitable planet without water, so H2O is therefore a high priority compound for detection. Water has several strong absorption bands in the spectral window New Worlds will analyze.