Searching for Extrasolar Life with Space Telescopes

Abstract

Telescopes placed above the earth’s atmosphere could obtain resolved images of planets around nearby stars. Given sufficiently large and accurate mirrors, even planets as small as the earth and as close to the star as the earth is to the sun could be distinguished. With a resolved image, the planets atmosphere could be studied by spectroscopic analysis. If, as on the earth, there are photosynthesizing organisms, then oxygen features would be expected in the spectrum along with those of water and carbon dioxide.

Spectra of either the reflected light or the thermal emission would show features of oxygen if it is present. O₂ absorption is found at 0.76 microns wavelength in the optical spectru, O₃ at 10 microns in the infrared. The minimum size telescope needed to resolve planets close enough to a number of good candidate stars is 1.5m diameter at optical wavelengths, 16–20m at 10 microns wavelength. In practice the optical signal is so weak that, even with a number of 1.5m telescopes or a single larger telescope, several days of integration would be needed to obtain a useful absorption spectrum. The accuracy of the mirror surface needed in the optical far exceeds that of the Hubble space telescope. The problems of working in the infrared are the large size of the mirror, and the need to operate it at a temperature of 80K. If these can be accommodated, the accuracy requirements are not nearly so severe, and the telescope in short integrations would yield high quality spectra.