ASTRONOMICAL SEARCHES FOR EARTH-LIKE PLANETS AND SIGNS OF LIFE

If Earth-like planets orbit nearby stars, they could be detectable wit h specially designed telescopes. Direct observations would be very rev ealing, particularly low resolution infrared spectra, which could esta blish habitability on the basis of temperature and atmospheric water. Abundant, primitive life based on organized molecular structure might reveal itself, as on Earth, by an atmospheric composition modified in ways unlikely to be from inorganic processes. The technical challenge is to detect and obtain spectra of an object with M-bol similar to 28 that is very close to a star and some 5 x 10(9) times less luminous. I ndirect methods, used to detect Jupiter-mass planets, do not seem to o ffer an easy intermediate step to finding Earth-like planets. However, the direct detection techniques needed for spectroscopy also offer a viable method for discovery by imaging. Thermal infrared wavelengths, in which a planet emits most energy, are :he most favorable. A robust search for planets of similar to 100 nearby solar-type stars, with spe ctroscopic follow-up of Earth-like candidates, could be made with an i nterferometer similar to 75 m in length. In visible light, the Next Ge neration Space Telescope (NGST) could, with the addition of a high res olution correction instrument, see Earth-like planets around a dozen o r so of the nearest stars. Both infrared and optical instruments are p ossible within the range of current space agency plans.