Astrobiologists outline four requirements for life to exist anywhere in the universe: an energy source, a type of atom such as carbon that enables complex molecular structures to exist; a liquid solvent such as water that supports the chemical reactions required for biological processes; and finally, sufficient time for life to evolve.
On Earth, life has been found in the most hostile environments including deep-sea hydrothermal vents, methane seep ecosystems, ocean floor brine pools, sub-freezing polar ice caps, and the inside of rocks. Microbes, in particular, are able to survive in these hostile environments as they excel at exploiting different sources of energy. For this reason and others, many people believe that microbes are the most likely sort of life to be found on other worlds.
Astrobiology (also called exobiology) is a multidisciplinary field utilizing physics, biology, geology, and philosophy to study the origin, distribution, and evolution of life throughout the universe. Assuming that the same laws of physics apply throughout the cosmos, astrobiology searches for stars with planetary systems, life-supporting environments within those systems, and evidence of water to support carbon-based life.
In the quest to find signs of life in the universe, scientists search for planets and moons that share the same characteristics as Earth. Since 1995, astronomers have located about 250 planets outside of our Solar System. Several are larger than Jupiter and easier to spot. Smaller planets and moons, however, usually avoid detection due to the bright glare of stars.
Ironically, astronomers try to work around this glare by looking directly at the star. A periodic wobble in a star's position may reflect a planet’s gravitational influence. A dimming of a star's light may be due to a planet passing directly in front of the astronomer's line of sight.