It would be made using an optical or infrared interferometer. This is a system that has two or more optical collectors that look like mirrors, whose light is combined using an 'optical bench' consisting of many folding, refocussing mirrors and optical delay lines. The light is combined exactly in-phase, and buy measuring the intensity of the combined light signal at many different orientations, you can resolve and perhaps form a crude image, of the light from a faint planet orbiting a bright star. The 'nulls' of the diffraction rings from the star, would project into rings of space surrounding the star where the sensitivity of the instrument was at its highest, and any planets reflecting enough light in these zones would be detectable. Because the light from the star overwhelms that of planets, you would probably want to work in the infrared where the contrast between a planet glowing by its heat radiation and the star, is at a maximum. Also, careful attention must be paid to tracking the array and avoiding scattered light from the star. This suggests a space-based instrument of the kind now being designed by NASA scientists.
The instrument would operate in a 'spectrometer' mode too isolate specific lines such as that of free oxygen, water and ozone. Only biogenic processes produce large atmospheric quantities of free oxygen. Also, by knowing how far the planet is from its star, its surface temperature can be easily determined, and this will tell you if liquid water can be found on its surface.
For more about this, visit NASA/JPL's Terrestrial Planet Finder project page.
The NASA/Ames center has also submitted a proposal for the Kepler Mission to detect Earth-sized planets. Transits by Earth-size planets produce a fractional change in stellar brightness of 5 x 10-5 to 40 x 10-5 lasting for 4 to 16 hours. The orbit and size of the planets can be calculated from the period and depth of the transit. The Kepler observatory is a one-meter aperture photometer with a 84 deg2 field of view. It continuously and simultaneously monitors brightnesses of 100,000 main-sequence stars brighter than 14th magnitude. The experiment is not biased by preselection of stellar type. They expect to detect over 170 earth-sized planets and over 400 larger planets by this method.
Copyright 1997 Dr. Sten Odenwald
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