EXTRASOLAR PLANETS
1. Stars like our Sun are common in our galaxy.
2. In our galaxy, extrasolar planetary systems are common around stars similar to our Sun.
3. Among extrasolar planetary systems, those similar to our own solar system are also commonplace.
4. A high percentage of extrasolar planetary systems are likely to have at least one Earth-like terrestrial planet located in the host star's habitable zone, meaning there probably are literally billions of Earth-like planets in our galaxy.
EXTRATERRESTRIAL LIFE
1. The basic ingredients of life are common in our galaxy.
2. Simple, single-cell life is probably common in our galaxy. Wherever such life can gain a foothold, it will. Primitive microbial life has been found in the most inhospitable environments on Earth, and may exist in locales in our solar system wherever liquid water may exist, such as Mars, Europa, Titan and Enceladus.
3. Complex, multi-cellular life is probably common on Earth-like extrasolar terrestrial planets located in habitable zones, given that the planetary system in question is of sufficient age.
4. Given sufficient timescales -- most likely billions of years -- intelligent, technological species will probably evolve on Earth-like terrestrial planets.
ARTIFICIAL INTELLIGENCE AND ROBOTICS
1. An advanced, technological species -- such as humans -- will quickly develop the ability to greatly extend its intellectual capabilities and its longevity through the exploitation of computer technology and robotics. This development will take place relatively quickly after achieving the ability to communicate by radio and travel in space, on a timescale measured in centuries if not decades.
2. Given the age of the known universe -- approximately 13.7 billion years -- any technologically advanced, extraterrestrial civilization in existence right now is statistically likely to be far older than we are and, consequently, far more technologically advanced.
SPACE TRAVEL
1. An advanced, technological species will rapidly exploit space travel. Within a relatively short time of achieving primitive space travel, such a species will attempt interplanetary travel within its own solar system and will actively explore the possibility of achieving interstellar travel.
2. An advanced technological species will first achieve interstellar travel by means of relatively small, autonomous probes that are under computer as opposed to biological control. Only when a species is capable of achieving speeds of at least a significant fraction of the speed of light will "manned" interstellar missions be attempted, unless the large timescales for such missions can be managed via the use of biological-robotic hybrids or through the exploitation of other techniques to make such a voyage tolerable, such as suspended animation.
THE BAD NEWS . . .
As advanced technological civilizations develop, the threat of self-destruction may increase greatly. Only civilizations that have learned to effectively manage any tendency for self-destructive behavior are likely to achieve interstellar travel on any significant scale. Many technological species may never achieve this level of development, falling victim to their own technology through war, disease or catastrophic climate change.
The threat of self-destruction, however, is not the only thing an advanced, technological species needs to worry about. Asteroid or cometary collisions, supernova, or other cosmic disasters may spell the end of intelligent species in our galaxy.
THE GOOD NEWS . . .
An advanced technological civilization may achieve a form of societal immortality if it survives long enough to achieve interstellar travel and the ability colonize other planets. It is highly probable any such civilizations in existence now surpassed our current level of technology thousands or even millions of years ago.
What would they do?
