Earth as we know it today, is transient. The atmosphere has changed significantly since the earliest days. Soon after formation a dense atmosphere of carbon dioxide and water is suspected, though fortunately Earth was cool enough for the oceans to condense. After nitrogen levels rose and the carbon dioxide was mostly buried, the Earth was without free oxygen. The Sun was 25% less luminous, thus some sort of greenhouse gas kept Earth warm enough for liquid water rather than frozen oceans. Carbon dioxide, methane and hydrogen are suspected.
But beyond those what is the range of the possible?
William Bains discusses the issues and describes a possible silicon biosphere here: Many Chemistries Could Be Used to Build Living Systems. He discusses this in more detail on his web-page: The nature of life. Engagingly, doing polysilanol chemistry in liquid nitrogen sounds like fun, in a chilly, frost-bite prone way…
The National Academy of Science produced this book about 6 years ago, which discusses the issues of alternative atmospheres: The Limits of Organic Life in Planetary Systems
Also there’s this paper by Johnson Haas which discusses a biosphere based on halides as the active gases: The potential feasibility of chlorinic photosynthesis on exoplanets. While chlorine is “rare” in cosmic terms, there’s enough in our oceans to replace oxygen has the active gas in our atmosphere, thus “rare” should not be confused with “available”. Chlorine is definitely available.
A much older discussion, though still pertinent, is John Campbell’s discussion of hydrogen breathing life on Jupiter, from the 1930s: Other Eyes Watching. While our model of Jupiter has changed, there has been much discussion of biospheres on hydrogen rich planets in recent years – even Earth is suspected of quite high hydrogen partial pressures in the past. Hydrogen greenhouse planets could provide liquid-water conditions for photosynthetic life all the way out to Saturn. Past that point, the Rayleigh scattering of light makes photosynthesis too hard for life to pursue, so liquid water biospheres further out would need to run on more exotic energy sources. In the right parts of the Galaxy, capture of dark matter and its possible self annihilation could warm planets to provide more clement conditions for life.
Ammonia is often touted as a replacement for water as a biological fluid in cold conditions – there’s at least one astrobiology group experimenting with precursors to biomolecules in ammonia as the replacement solvent. Under pressure, the temperature range for ammonia becomes wider, so ammonia-based life need not be “cold” life.
While ammonia is analogous to water, as they’re both polar molecules, non-polar liquids like carbon dioxide and methane have been discussed as homes for some kind of life. Super-critical carbon dioxide – i.e. warmer than 31 C – has also been discussed as a medium of interesting chemistry relevant to life, but our ignorance of the limits of chemistry hobble our imaginations.
Stephen Baxter, the British SF writer, made the interesting suggestion of metallic life arising from even more exotic liquid environments – oceans made of iron carbonyl, which decomposes at relatively low temperatures into iron metal and carbon monoxide. His “robotic” aliens, the Gaijin (“alien” in Japanese), are initially believed to be artificial, but instead evolved on such an exotic world, in his novel “Manifold: Space”.