Imaging Nonsolar Planets by Nulling Interferometry
Ronald N. Bracewell
A planet resembling Jupiter, circling a Sun-like star that is 10 parsecs away from us, would subtend 0.1 milli-arcseconds, while the star would subtend 1 milliarcsecond. A planet circling the star at a distance of 5 AU (Jupiter's distance from the Sun) would present at most an angular separation of 0.5 arcseconds. Existing telescopes have neither the angular resolution nor sensitivity to detect such a planet directly.
Jupiter-sized planets circling their star rapidly at separations of about 1 AU have revealed their presence indirectly by inducing minute, periodic Doppler shifts in the light of their star, and one looks forward to improvement in sensitivity to cope with the smaller shifts induced by `Jupiters' at 5 AU and 'Earths' at 1 AU.
Meanwhile, a space-based interferometer receiving at infrared wavelengths has received national support. The idea is to adjust an interferometer for a null in its median plane so as to null out the radiation received from the star, and to spin the interferometer about the Earth-star line to gain the signal-to-noise improvement associated with phase-sensitive detection. Angel and Woolf have improved the concept by proposing a four-in-line configuration that widens the central null and broadens the band to include spectral lines of water, ozone and carbon dioxide, all molecules relating to life. Hinz et al. have demonstrated the nulling principle, without going into space, using two elements of the Multiple Mirror Telescope (which has now been dismantled). The thermal radiation from Betelgeuse was attenuated sufficiently to reveal a faint zodiacal light cloud that would be conventionally lost in the glare of Betelgeuse. These factors constitute important inputs to NASA's Terrestrial Planet Finder mission.
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