Extraterrestrial life is difficult to define, but could range from single-celled organisms to intelligent beings. Evidence for life on Mars is inconclusive, but the search for extraterrestrial life continues through programs like SETI. The search is limited by human perspective and the potential dangers of encountering unknown organisms. Despite this, the possibility of life on other planets remains a tantalizing prospect.
Extraterrestrial is defined as things located or occurring outside the earth or not within the earth’s atmosphere, while the term life is much more difficult to define. For the purposes of defining extraterrestrial life, it might be simpler to say that life would be anything living, from single-celled organisms to complex multicellular organisms (such as plants or animals), that didn’t come from earth. Depending on a person’s imagination and theories, extraterrestrial life can conjure up images of aliens with semi-humanoid characteristics and with sufficient intelligence to eventually contact humans. Alternatively, some people might view life forms on other planets as predominantly microbial in nature and suggest that earth is the only planet where life in more complex forms has evolved.
Despite the fact that scientists have yet to find true evidence of extraterrestrial life, there are some theories that we accidentally found and killed tiny microbes on Mars during Viking 1 and 2 explorations in the 1970s. Evidence in the late 2000s for the presence of methane gas in Mars’ atmosphere could also suggest some primitive life forms, but this gas can also be created by geochemical reactions. There is certainly much support in the scientific community for the idea that simple life forms exist on at least other planets, but less total support for the idea that complex forms of intelligent life exist, although this still fuels the imaginations of writers science fiction and the general public.
There are many precedents for believing in extraterrestrial life. Several ancient Greek astronomers speculated that there must be inhabitants on other planets. The expression that God created life on other worlds appears in the Jewish Talmud, the Koran and many Hindu texts. As astronomy became more advanced, the ability to see the many existing planets and many stars through things like telescopes made it seem more likely that life must exist on other planets, that the earth could not be as unique as the only life-bearing planet in the galaxy or discussed. This has been argued from religious and scientific perspectives, although it has also been argued from some religious perspectives that earth is the only planet on which life exists.
One ambitious project that has searched for signs of extraterrestrial life is the SETI or Search for Extraterrestrial Intelligence program, which among other things has sent signals via radio wave transmission and continues to analyze radio wave signals as potential evidence of life beyond out of the earth. The program hasn’t found definitive evidence of life elsewhere, but for the many who believe other planets have intelligent life, this may just be evidence that these life forms use other modes of transmitting signals. There is also some concern about what is called active SETI (sending messages), as it certainly cannot be assumed that extraterrestrial life would welcome Earthlings with open arms. Life elsewhere may be hostile, and Earthlings may be better off not alerting other life forms to their presence in the universe.
Carl Sagan said, “The universe need not be in perfect harmony with human ambition.” This is perhaps one of the difficulties of the search for extraterrestrial life. Humans tend to see things through the lens and experience of being humans, even with highly evolved scientific ideas, and people may not be able to imagine how inhabitants of other planets would see things, evolve technologies or communicate . People remain limited from being people, and it may be impossible to conceptualize how to find intelligent life, how to search for it, and whether it’s a good idea to search. Even the simplest single-celled organism poses a potential threat to humans, since it did not evolve within the same framework as human, plant, and bacterial evolution on earth, and it is difficult to know how such an organism could potentially interact with terrestrial beings to maintain life; life tends to be aggressive in trying to stay alive.
However, the many planets of the universe, and even within this galaxy, remain a grave temptation. They are an open invitation to research to find out if humans and other life forms on earth are truly alone in the universe. Is it all hollow rock and barren nothingness, or are these planets teeming with life and representing diversity in species and evolution as earth does?
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