Concept
The space telescopes were to observe in the infrared part of the electromagnetic spectrum. As well as studying extrasolar planets, the telescopes would probably have been useful for general purpose imaging, producing very high resolution (i.e. milliarcsecond) infrared images, allowing detailed study of a variety of astrophysical processes.
The infrared region was chosen because in the visible spectrum an Earth-like planet is outshone by its star by a factor of a billion. However, in the infrared, the difference is less by a few orders of magnitude. According to a 2000 ESA bulletin, all spacecraft components in the optical path would have to be passively cooled to 40 kelvins to allow infrared observations to take place.
The planet search would have used 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 star's position. 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. The detection of an Earth-like planet would require about 10 hours of observation in total, spread out over several months. A 2002 design which would have used 1.5 metre mirrors was expected to take about 100 hours to get a spectrum of a possibly Earth-like planet.
After the Darwin spacecraft detected a planet, a more detailed study of its atmosphere would have been made by taking an infrared spectrum of the planet. By analyzing 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 vapour in the atmosphere could be evidence for life. Oxygen is very reactive so if large amounts of oxygen exist in a planet's atmosphere some process such as photosynthesis must be continuously producing it.
The presence of oxygen alone, however, is not conclusive evidence for life. Jupiter's moon Europa, for example, has a tenuous oxygen atmosphere thought to be produced by radiolysis of water molecules. 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 produces hydroxyl ions and atomic oxygen, respectively, and these in turn produce oxygen in small concentrations, with hydrogen escaping into space. 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 is present. For terrestrial planets, the simultaneous presence of O3, H2O and CO2 in the atmosphere appears to be a reliable biosignature, and the Darwin spacecraft would have been capable of detecting these atmospheric components.
Read more about this topic: Darwin (spacecraft)
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