Darwin is a European Space Agency (ESA) program designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. The launch date will not be before 2016. The current design envisions three free-flying space telescopes, each at least 3 meters in diameter, flying in formation as an astronomical interferometer. These telescopes will redirect the light to the main spacecraft which will contain the beam combiner, spectrographs and cameras for the interferometer array, and which will also act as a communications hub.
The space telescopes will observe in the infrared part of the electromagnetic spectrum. As well as studying extrasolar planets, the telescopes will probably have a general purpose imaging mode which will produce very high resolution (i.e. milliarcsecond) infrared images, allowing detailed study of a variety of astrophysical processes. The infrared region was chosen because in that part of the spectrum an Earth-like planet is only outshone by its star by a factor of a million; in the visible spectrum an Earth-like planet is outshone by its star by a factor of a billion.
The planet search would use a nulling interferometer configuration. In this system, phase shifts would be introduced into the three beams, so that light from the central star would suffer destructive interference and cancel itself out. However, light from any orbiting planets would not cancel out, as the planets are offset slightly from the position of the star. This would allow planets to be detected, despite the much brighter signal from the star.
For planet detection, the telescopes would operate in an imaging mode. To detect an Earth-like planet would require about 10 hours of observation in total, spread out over several months. Once a planet is detected, a more detailed study of its atmosphere would be made by taking an infrared spectrum of the planet. By analysing this spectrum, the chemistry of the atmosphere could be determined, and this could provide evidence for life on the planet. The presence of oxygen and water vapor in the atmosphere could be evidence for life. Oxygen is very reactive, and if, nevertheless, large amounts of oxygen exist in the atmosphere, it must be continually reproduced, probably, by biological processes, i.e., photosynthesis.
However, the presence of oxygen alone cannot be seen as conclusive evidence for life. Numerical simulations have shown that under proper conditions it is possible to build up an oxygen atmosphere via photolysis of carbon dioxide. Photolysis of water vapor and carbon dioxide produce hydroxyl and atomic oxygen, respectively, that, in turn, produce oxygen in small concentrations. This process and, possibly, lightning produced oxygen for the early atmosphere before photosynthesis became dominant.
H2O -> H + OH produces a hydroxyl radical (OH) and
CO2 -> CO+ O produces an atomic oxygen (O).
The OH is very reactive and combines with the O
O + OH -> O2 + H
The hydrogen atoms formed in these reactions are light and some small fraction escape to space allowing the O2 to build to a very low concentration, probably only about 1% of the oxygen available today.
When O2 is produced by H2O photolysis at high altitude, hydrogenous compounds like H, OH and H2O are produced which attack very efficiently O3 and prevent its accumulation. The only known way to have a significant amount of O3 in the atmosphere is that O2 be produced at low altitude, e.g. by biological photosynthesis, and that little H2O gets to high altitudes where UV are present. Consequently, for terrestrial planets, the simultaneous presence of O3 , H2O and CO2 in the atmosphere appears to be a reliable biosignature.
Planet Gliese 581 d, discovered in 2007, is considered a good candidate for the Darwin project. It orbits within the theoretical habitable zone of its star, and scientists surmise that conditions on the planet may be conducive to supporting life.
Currently awaiting funding approval, the interferometric version of NASA's Terrestrial Planet Finder mission is similar in concept to Darwin and also has very similar scientific aims. According to NASA's 2007 budget documentation, released on February 6, 2006, the project was deferred indefinitely. Antoine Labeyrie has proposed a much larger space-based astronomical interferometer similar to Darwin, but with the individual telescopes positioned in a spherical arrangement and with an emphasis on interferometric imaging. The spherical geometry reduces the amount of pathlength compensation required in re-pointing the interferometer array. This "Hypertelescope" project would be much more expensive and complex than the Darwin and TPF missions, involving many large free-flying spacecraft.
Published - July 2009
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