Astrobiology seeks to understand the origin, evolution, distribution, and future of life in the universe and thus to integrate biology with planetary science, astronomy, cosmology, and the other physical sciences. The discipline emerged in the late 20th century, partly in response to the development of space exploration programs in the United States, Russia, and elsewhere. Many astrobiologists are now involved in the search for life on Mars, Europa, Enceladus, and beyond. However, research in astrobiology does not presume the existence of extraterrestrial life, for which there is no compelling evidence; indeed, it includes the study of life on Earth in its astronomical and cosmic context. Moreover, the absence of observed life from all other planetary bodies requires a scientific explanation, and suggests several hypotheses amenable to further observational, theoretical, and experimental investigation under the aegis of astrobiology. Despite the apparent uniqueness of Earth’s biosphere— the “n = 1 problem”—astrobiology is increasingly driven by large quantities of data. Such data have been provided by the robotic exploration of the Solar System, the first observations of extrasolar planets, laboratory experiments into prebiotic chemistry, spectroscopic measurements of organic molecules in extraterrestrial environments, analytical advances in the biogeochemistry and paleobiology of very ancient rocks, surveys of Earth’s microbial diversity and ecology, and experiments to delimit the capacity of organisms to survive and thrive in extreme conditions.
Stanley Miller demonstrated in 1953 that it was possible to form amino acids from methane, ammonia, and hydrogen in water, thus launching the ambitious hope that chemists would be able to shed light on the origins of life by recreating a simple life form in a test tube. However, it must be acknowledged that the dream has not yet been accomplished, despite the great volume of effort and innovation put forward by the scientific community. A minima, primitive life can be defined as an open chemical system, fed with matter and energy, capable of self-reproduction (i.e., making more of itself by itself), and also capable of evolving. The concept of evolution implies that chemical systems would transfer their information fairly faithfully but make some random errors. If we compared the components of primitive life to parts of a chemical automaton, we could conceive that, by chance, some parts self-assembled to generate an automaton capable of assembling other parts to produce a true copy. Sometimes, minor errors in the building generated a more efficient automaton, which then became the dominant species. Quite different scenarios and routes have been followed and tested in the laboratory to explain the origin of life. There are two schools of thought in proposing the prebiotic supply of organics. The proponents of a metabolism-first call for the spontaneous formation of simple molecules from carbon dioxide and water to rapidly generate life. In a second hypothesis, the primeval soup scenario, it is proposed that rather complex organic molecules accumulated in a warm little pond prior to the emergence of life. The proponents of the primeval soup or replication first approach are by far the more active. They succeeded in reconstructing small-scale versions of proteins, membranes, and RNA. Quite different scenarios have been proposed for the inception of life: the RNA world, an origin within droplets, self-organization counteracting entropy, or a stochastic approach merging chemistry and geology. Understanding the emergence of a critical feature of life, its one-handedness, is a shared preoccupation in all these approaches.
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article. Astrobiologists are engaged in the search for signs of extraterrestrial life in all forms, known as biosignatures, as well as specific signs of extraterrestrial technology, known as technosignatures. The search for technosignatures and biosignatures attempts to identify characteristic evidence of life on other planets that could be detected using astronomical methods. The first scientific searches for technosignatures took place in the 1960s and used radio telescopes to examine nearby star systems for evidence of narrow-band transmissions used for communication. The search for extraterrestrial intelligence and for anomalous radio and optical signals that would indicate intentional or unintentional extraterrestrial communication continues. Advances in ground- and space-based spectroscopy have enabled searches for technosignatures in exoplanetary systems, such as atmospheric pollution, city lights, large-scale surface structures, and orbiting satellites. Some technosignature searches also attempt to search for nonterrestrial artifacts within the solar system on planetary bodies or in stable orbits. Researchers use known technology on Earth as a starting point for thinking about what might be plausible and detectable in extraterrestrial systems. Technology is a relatively recent phenomenon in the history of life on Earth, so the search for technosignatures also employs methods of future projection to explore numerous trajectories for extensions of known technology. The range of possibilities considered by technosignature science can include any known or plausible technology that could be remotely detected and would not violate any known physical laws. Megastructures are examples of theoretical large-scale planetary engineering or astroengineering projects that could be detectable in exoplanetary systems through infrared excesses or gravitational effects. Many other technosignatures are possible, even if they do not draw upon Earth projections, but most astrobiological studies of technosignatures focus on predictions that could be tested with current or near-future missions. The discovery of extraterrestrial technology could be of great significance to humanity, but technosignature searches that yield negative results still provide value by placing qualitative upper limits on the prevalence of certain types of extraterrestrial technology.