Although no direct evidence has been uncovered to show that life exists elsewhere, scientists present reasonable arguments to postulate what may be. These arguments, for the most part, are based on the Principle of Mediocrity, which states that what happened in our solar system and on Earth is typical of the pre‐biotic and biotic evolution anywhere else in the universe. In other words, Earth is not special.
For a variety of reasons, we can expect that other life is probably based on organic molecules, carbon‐nitrogen‐oxygen‐hydrogen compounds. These elements are among the most abundant in the universe. Additionally, water (H 2O) provides an excellent solvent necessary for introducing compounds into and removing waste chemicals from living systems. Furthermore, carbon can build up long molecules, a requirement in the complexity of living systems. Carbon‐based compounds can also be oxidized to yield the energy necessary to maintain life. Observational and laboratory study of a vast variety of environments suggests that all are locales where complex organic molecules can form through nonbiological processes. Such environments include pre‐planetary nebula (as evidenced by organic molecules in carbonaceous chondrites), the primordial Earth atmosphere (replicated in laboratory experiments), comets, the atmosphere of Jupiter and the surfaces of its moons, interstellar clouds, and so on. In this circumstance, a major part of the path leading to the advent of life may be completed before a habitable environment on a planetary surface becomes available. On Earth, the primordial soup out of which life evolved may have been filled with all the necessary chemical requirements coming from the capture of comets.
Chemically, the only reasonable alternative to carbon is the element silicon. However, silicon is less common than carbon in the universe and has no effective solvent. Silicon can build long molecules, but it prefers the strong bonds of SiO 2 (sand). Also, silicon‐based reactions are less energetic than carbon reactions. In other words, if carbon is present, silicon is most unlikely to be the basis for life. Exobiologists point out, however, that these considerations do not mean that other carbon‐based life would look or act like terrestrial creatures.
Is there any evidence for life anywhere else in the universe? At present, the answer has to be a resounding no. And how would someone discover otherwise? This question is analogous to a situation in which two people live on an otherwise uninhabited island. One person's discovery of the other person's existence could happen in several ways. Scientists have classified these methods of discovery as type Ia, Ib, II, and III encounters, based on proximity. A type Ia encounter would involve remotely sensing some inadvertent sign of the other, such as the smoke from a cooking fire or detecting the broadcast signal for an I Love Lucy program from the 1950s. A type Ib encounter would involve remotely sensing a deliberate effort of the other to make his or her presence known—for example, the radio distress signal that was sent out by the Titanic shortly before it sank in 1912. Finding an artifact left by the other person (a beer can, or the Pioneer 10 and Voyager spacecraft, which have left the solar system) represents a type II encounter. Lastly, a type III encounter is an actual meeting ( Close Encounters of the Third Kind, in which friendly aliens arrange a meeting with us at Devil's Tower in Wyoming).