introduced by an Earth-sized planet and some, as yet unknown, but quasi- 

 periodic fluctuation in stellar atmospheres themselves may impose a more 

 stringent limitation on this technique. 



The last detection method, called "photometric," is statistical. If the orbital 

 plane is favorably inclined, then the luminosity of the star may be observed to 

 decrease slightly every time the star is occulted by one of the orbiting planets. 

 The event is short-lived and unpredictable because of the unknown inclination of 

 the orbital plane and the phases of the planets when the measurements are 

 begun. Thus many stars must be monitored continuously with precision suffi- 

 cient to detect a change in stellar luminosity of 1CT 6 in each of two broad-band 

 colors, so that fluctuations in the stellar emission can be distinguished from the 

 real eclipse events. Ground-based studies of new instrumentation for this tech- 

 nique are just beginning, and it is not yet clear whether the required technology 

 can be achieved. 



The HST is unlikely to achieve the earlier advertised contrast ratio of 

 2X1CT 7 at a separation of 1 arcsec at visible wavelengths, and its usefulness for 

 direct imaging of planetary systems is dubious. It is more likely to be of greater 

 use in the study of the larger and more extended precursors to planetary systems 

 (see section 3.7). However, if time can be obtained, this instrument may be 

 able to search a handful of nearby stars or follow up on other indications of 

 planetary systems; a systematic survey is unlikely given the enormous demand 

 for observing time. The second generation of HST instrumentation will include 

 one of two infrared instruments currently being developed competitively, either 

 a near-infrared camera (NICMOS) or an imaging Michelson spectrometer (IMS). 

 Neither is ideal for planetary detection, but they do open up the wavelength 

 region with the most favorable contrast ratios for the giant planets. Evaluation 

 studies of the HST optics and the imaging performance of these two instruments 

 should be closely watched by the exobiology community to attempt to ensure 

 that the superior planet detector is selected. ISO and SIRTF will operate in the 

 infrared, and have been discussed as possible opportunities for imaging nearby 

 planetary systems. Although their sensitivity will be extremely good, both tele- 

 scopes lack the required spatial resolution. The proposed "superresolution" 

 capability requires signal-to-noise ratios that are probably greater than those pro- 

 vided by a faint planet. The prospects for direct imaging of a large number of 

 planets in the near term do not look hopeful. 



The HST will also have two instrument systems that can make astrometric 

 measurements. Neither will improve on the current ground-based accuracy and 

 neither will be available for the extensive observing time needed for multiple 

 observations over the decades required to discern the systematic stellar motion. 

 The single best near-term candidate instrumentation for a systematic search for 

 extrasolar planetary systems is a dedicated imaging astrometric telescope 

 mounted on the upper boom of the Space Station. NASA's Task Force on the 

 Scientific Uses of the Space Station has recently concluded that it should be 



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