Exobiological exploration beyond this solar system is founded on the postulate that signatures of life (biosignatures) from a habitable and inhabited planet will be recognizable by remote sensing techniques that are conducted across interstellar distances. A biosignature is an object, substance, or pattern whose origin specifically requires a biological agent creating it. To be scientifically useful, a biosignature should also have a low probability of any nonbiological processes causing it.
The search for habitable planets around nearby stars is based on the further assumption that even the most basic life-forms on a suitable extrasolar planet will be global in extent. Exobiologists also assume that characteristic biosignatures, which are the evidence for life, from the habitable planet's surface or atmosphere will be readily recognizable in the spectrum of the planet's infrared light.
NASA's Terrestrial Planet Finder mission will use direct imaging detection and spectroscopic characterization techniques to detect the large-scale effects and signs of life on Earth-like planets around nearby stars. By analyzing the "colors" (wavelengths) of infrared radiation detected by TPF, astronomers and exobiologists hope to search for atmospheric gases such as carbon dioxide (CO2), water vapor (H2O), and ozone (O3). Together with the surface temperature and size (diameter) of detected extrasolar planets, scientists can establish which of these extrasolar planets is habitable and perhaps even whether some are inhabited by basic forms of life.
Using this solar system and life on Earth as a reference, exobiologists suggest that the best candidates that merit closer investigation will be any found planets located in the continuously habitable zone (CHZ) around their parent star. Planets outside the CHZ are either too hot or too cold to support life as understood here on Earth. If a planet is too hot, all liquid water on the planet's surface eventually becomes vapor and then slowly escapes into space from the atmosphere. If the planet is too cold, all liquid water on the surface freezes. In the extremes, the former condition is called a runaway greenhouse effect, and the latter condition is called an ice catastrophe. Either of these environmental extremes would make even an Earth-sized extrasolar planet very inhospitable for life.
As a point of comparison, in humans' solar system, the continuously habitable zone starts beyond Venus (which is too hot) and ends before Mars (which is too cold and can no longer retain liquid water on its surface).
Scientists suggest that just existence of large amounts of oxygen (O2) in a planet's atmosphere could be regarded as a strong biosignature. For example, in Earth's atmosphere, oxygen is a byproduct of photosynthe-sis—the fundamental process by which green plants and certain other organisms convert carbon dioxide and water into carbohydrates, using sunlight as the energy source. However, oxygen molecules do not linger long in a planet's atmosphere. Atmospheric oxygen experiences a process called oxidation, recombining with other types of molecules, so if they detected an extrasolar planet in the continuously habitable zone, scientists would be tempted to conclude that some type of life is keeping the supply of oxygen replenished on the planet.
However, scientists also recognize that there are nonbiological processes that could result in an oxygen-rich atmosphere on an abiotic planet, so conservative exobiologists do not regard the presence of oxygen in an extrasolar planet's atmosphere as an unambiguous biosignature. These cautious scientists would prefer to also detect ozone coexisting with nitrous oxide or methane before deciding whether an extrasolar planet is habitable. Once the Terrestrial Planet Finder mission provides evidence that a planet has oxygen, ozone, and methane in its atmosphere, for example, then the exobiologists consider such observational data convincing evidence not only that the candidate planet is habitable but also that it is inhabited.
NASA planners envision taking the results provided by the Terrestrial Planet Finder mission to shape the development of an even more sophisticated mission called Life Finder. Between 2020 and 2025, NASA would fly an array of large telescopes into space. These telescopes would combine infrared light from candidate extrasolar planets to create high-resolution spectra of their atmospheres. Exobiologists would then carefully examine these data for unambiguous indicators of biological activity, such as seasonal variations in the levels of methane and periodic changes in atmospheric chemistry. Of course, the scientists must remain ever mindful that life on Earth is their only known reference and that suspected biosignatures from candidate alien worlds may not appear exactly as the characteristic "life signs" produced by planet Earth.
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