whose apparent separation is 0.5 arcsec. For the Earth/Sun system, at 10 par- 

 sees, the contrast ratio is 2X10 -10 and the separation on the sky is only 

 0.1 arcsec. Contrast ratios in the infrared may be less severe for the giant planets, 

 and in general smaller, fainter stars make the contrasts more favorable. The 

 primary limitation for ground-based systems is that the atmosphere spreads out 

 the image of a star over a circle that is at best 0.5 arcsec in diameter, inhibiting 

 the discovery of any close-in orbiting planets. Planets in larger orbits will reflect 

 less starlight and thus be intrinsically fainter and more difficult to detect with 

 any imaging device. 



There are also three indirect methods for deducing the presence of planetary 

 mass companions in orbit about nearby stars. The first measures the reflex 

 motion of the star resulting from the fact that both stars and planets orbit about 

 a common center of mass. This orbital motion introduces a tiny "wobble" into 

 the space motion of a nearby star when measured against the more distant fixed 

 stars. Astrometry is the attempt to accurately measure the relative position of a 

 star over decades in order to uncover this small periodic displacement. To infer 

 the presence of J upiter in orbit about a solar mass star at a distance of 1 parsecs 

 requires the position of the star to be measured with an accuracy of 0.3 milliarc- 

 sec, while an accuracy of 1 microarcsec is required to detect the influence of 

 Earth. With extreme care and modern instrumentation, ground-based observator- 

 ies can achieve an accuracy of 1 milliarcsec by combining many observations 

 over a period of months. The atmosphere imposes an absolute limit of 0.1 milli- 

 arcsec, but long-term systematic changes in the telescope itself may prevent this 

 limit from being achieved. Unlike direct imaging, astrometric measurements 

 must be continued over time scales comparable to the orbital periods of the 

 planets in order to verify that the detected wobble has the expected periodic 

 shape. 



If the orbital plane of the planets is nearly perpendicular to the plane of the 

 sky, two other indirect detection techniques are possible. The orbital motion 

 about a common center of mass can be detected because of the periodic Doppler 

 shift it introduces into the spectrum of the star. An emission or absorption line 

 from the atmosphere of the star will be blue-shifted as the star approaches the 

 observer and red-shifted as the star recedes along its orbit. Searches based on 

 these shifts in the wavelength of the stellar lines are called spectroscopic, and 

 this is the method of detection that is least disturbed by atmospheric attenua- 

 tion and turbulence. Great precision in measuring the change in the wavelength 

 is required over periods comparable to planetary orbit time scales. Great preci- 

 sion is currently being achieved from the ground. This method is nearly indepen- 

 dent of the distance to the star, provided that the star is sufficiently bright. The 

 effects of a Jupiter-sized planet in orbit about a large number of relatively 

 nearby stars should be discernible from the ground, but the searches have been 

 under way for only a few years and there are not yet enough data. Terrestrial 

 atmospheric effects will preclude the detection of the smaller wavelength shifts 



54 



