1. TIME
Big Bang Theory: age of universe somewhere between 8 and 18 billion years ago
Universe formed and made up of 75% Hydrogen and 25% Helium, plus "contaminates"
Time needed for stars to form
2. MATERIALS
Planets need the elements of Carbon, Oxygen, Iron, Calcium, etc. to support life
Formed inside massive stars
Must have enough material to form terrestrial planets
3. LOCATION
Terrestrial planets that are formed close to the star (Our Sun)
Formation of the Solar System
Evolution of Venus, Earth, Mars
Conditions between 4.0 - 3.6 billion years ago
MARS
(Discovered by Galileo in 1609)
VENUS
(Discovered in 1610 by Galileo)
TITAN
(Discovered in 1665 by Christiaan Huygens)
EUROPA
(Discovered in 1610 by Galileo)
HABITABLE ZONE
Areas where conditions might potentially support life is called the "Habitable Zone." In this area planets nee to be close enough to the Sun for solar energy to drive the chemistry of life. A common assumption about extraterrestrial life is that water is a requirement: water in its liquid state. Recently, however, astronomers have discovered that there are other factors that need to be taken into consideration, such as the atmosphere of the planet/moon, the amount of heat coming from the planet core, and what geological process, if any, are taking place on the surface of the planet and/or moon. All of these can affect a planet's ability to nurture and support life.
For a quick tour of our solar system, look at the video.
Atmosphere is looked at as an indication of biotic activity when it contains large amounts of oxygen. That is usually a signal that something is constantly replenishing the atmospheric supply, since oxygen is highly reactive and will not remain in an atmosphere for any length of time.
Astrobiologists have not overlooked the fact that life elsewhere may be completely alien in the chemistry and therefore have different requirements. Finding such organisms may prove difficult, simply because they might not be recognized as living beings.