Okay, some interesting stuff again:
http://en.wikipedia.org/wiki/Extraterrestrial_life"Due to their relative abundance and usefulness in sustaining life, many have hypothesized that lifeforms elsewhere in the universe would also utilize these basic materials. However, other elements and solvents could also provide a basis for life. Silicon is most often deemed to be the probable alternative to carbon. Silicon lifeforms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as on planets which are very close to their star. Life forms based in ammonia (rather than water) have also been suggested, though this solution appears less optimal than water.[4]
Technically, life is basically a self-replicating reaction, but one which could arise under a great many conditions and with various possible ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive. Suggestions have even been made that self-replicating reactions of some sort could occur within the plasma of a star, though it would be highly unconventional."
Drake equationIn 1961, University of California, Santa Cruz astronomer and astrophysicist Dr. Frank Drake devised the Drake equation. This controversial equation multiplied estimates of the following terms together:
- The rate of formation of suitable stars.
- The fraction of those stars which contain planets.
- The number of Earth-like worlds per planetary system.
- The fraction of planets where intelligent life develops.
- The fraction of possible communicative planets.
- The “lifetime” of possible communicative civilizations.
Drake used the equation to estimate that there are approximately 10,000 planets containing intelligent life, with the possible capability of communicating with Earth in the Milky Way galaxy.[27]
Based on observations from the Hubble Space Telescope, there are at least 125 billion galaxies in the universe. It is estimated that at least ten percent of all sun-like stars have a system of planets[28], thus if a thousandth of a percent of all stars are sun-like, and there are roughly (estimates may vary) 500 billion stars, on average, in each galaxy[citation needed], there are 6.25*1018 stars with planets orbiting them in the universe.
If even a billionth of these stars have planets supporting life, there are some 6.25 billion life-supporting solar systems in the universe.