3.8 The Detection of Other Planetary Systems 



One major goal for exobiology is to determine the distribution of potential 

 sites for the emergence of life in the universe. Since life as we know it is a plane- 

 tary phenomenon, this means detecting and surveying extrasolar planetary sys- 

 tems as well as systems where planets are about to form or where planets have 

 failed to form. It is of particular interest to discover whether the general charac- 

 teristics of our solar system, with its trio of early water-bearing planets, bom- 

 barded by organic-rich meteorites and comets, are repeated frequently in other 

 systems. We may be able to make great progress in understanding why Venus 

 and Mars apparently contain no extant life by comparative planetology studies 

 within our own solar system alone. However, to the extent that the limitations 

 on life involved the cosmic environment of these two planets, other planetary 

 systems must be studied to assess how probable the sets of conditions favorable 

 to the formation of life really are in the universe. 



There is currently no unambiguous evidence for the existence of any plane- 

 tary system outside the solar system. Recent advances in ground-based instru- 

 mentation have resulted in the detection of large disks of dust and gas surround- 

 ing nearby stars. Nonetheless, evidence for extrasolar planetary systems is still 

 lacking. Star-formation theory predicts that planets will abound, but none has 

 been found, only the intriguing clues. This is perhaps not too surprising; until 

 recently, the measuring tools available were woefully inadequate for the discov- 

 ery of all but the most massive planets around the nearest stars. On more than 

 one occasion, the classical ground-based observational programs have resulted in 

 exciting discovery claims of one or more planets orbiting familiar stars, such as 

 Barnard's star. However, more recent measurements with modern photoelectric 

 detectors negate these earlier discoveries and illustrate how difficult it is to 

 fully account for all the systematic biases that may develop as equipment ages in 

 a dynamic environment under the influence of 1 Earth gravity. The opportuni- 

 ties for stably orbiting long-lived instrumentation above the Earth's atmosphere 

 in the near future promise to revolutionize this situation. 



There are four different ways to search for extrasolar planetary systems; all 

 are complementary and all will probably be necessary to detect and study all 

 the members of another system. Of these four methods, three will derive great 

 benefit from orbiting instrumentation which will remove the performance limita- 

 tions imposed by the atmosphere. Definitive results can be expected within the 

 decade, and spectacular successes are possible if dedicated systems can be 

 afforded. 



How do you detect another planetary system? Given sufficient angular reso- 

 lution and the ability to detect a very faint object in contrast to a very bright 

 object, one might hope to directly image extrasolar planets at optical or infrared 

 wavelengths. The task is extremely difficult. To detect Jupiter in orbit about 

 the Sun, if the system were at a distance of 10 parsecs (33 light-years), requires 

 the ability to distinguish two objects whose relative brightness is 2X1 CT and 



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