Claims have been made on behalf of techniques known as ‘adaptive optics’ as a means of detecting planets (Wilson, 1995). This technique was developed by the US military as a means of spotting Soviet satellites in space. Adaptive optics involves techniques designed to correct the distortions created by the Earth’s atmosphere. The objective is to create a mask to absorb most of  the  light from the central star. This involves the employment of four large deformable mirrors at least six metres across which can suppress the halo around a star’s image so that a telescope could produce a clear crisp picture of the observed object and could pick out giant planets after only a few hours of watching (J. Davies, 1995: 24). Although this technology is in its infancy, future developments suggest that the increased sensitivity it offers to ground-based observations may make it possible to observe planets the size of Jupiter around stars in the Milky Way. One discovery relating to this technique is a companion to the star, Gliese 229.

At present it is difficult to maintain a distinction between planets and brown dwarfs. This is not merely a problem relating to observational techniques; it is bound up with theories concerning the origin and nature of both planets and brown dwarfs. Brown dwarfs are not as bright and as  large as stars, and very often cannot be distinguished from gas giants. It is not clear  whether they undergo the same formation process as giant planets, and they may arrive at similar ends via different processes. But if many of the alleged planets turn out to be brown dwarfs, then expectations of large numbers of ex-solar planets will have to be reduced. It is also likely that as more planets are detected with counter-claims that they are brown dwarfs, then the pressure to distinguish between them will itself generate both theories and observations which will reveal diversity with regard to their formation. It is safe to predict that several theories regarding the origins of planetary systems will be called into question during the next few years.

Already it is becoming apparent that very few of the observed ex-solar planets exhibit features resembling our solar system. Early in 2000 astronomers at the University of Oxford and the University of Hertfordshire in England discovered thirteen planets with masses less than Jupiter. What was significant about these planets is that they are free floating; they drift through space by themselves and do not orbit any star. Another feature of recently observed planets is that many of the giant planets are much closer to their stars than Mercury is to the Sun. Yet according to the prevailing model of planetary formation, small rocky planets, like Mercury, Venus, Earth and Mars, should occupy the hot inner regions, while the gas giants in the outer regions are able to grow large by virtue of their longer orbits which enable them to collect more material. The fact that so many of the gas giants recently observed have extremely close orbits around their stars may require further explanation. It appears that the detection of ex-solar planetary systems presents a significant challenge to the standard model of planet formation.

Among the searches for ex-solar planets is NASA’s Astronomical Studies of Extrasolar Planetary Systems (ASEPS), while the European Space Agency (ESA)

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has identified planet detection as a priority and is considering several rival planetary detection methods. A proposal called DARWIN, by ESA in 1993, was for a search of signs of planetary life using a space-based interferometer consisting of several infrared telescopes which could detect a planet with oxygen in its atmosphere. Its rival was a programme called GAIA using astrometric techniques. There is also a proposed search for the Frequency of Earth Sized Planets (FRESIP) by the NASA Ames Center in California. The objective of FRESIP is to monitor the brightness of stars to search for changes caused by planets crossing them. This may be a small effect, but it can last for several hours and happens once a year. Such a regular feature would rule out random factors and allow predictions to be made regarding the planet’s transit. The programme requires round-the-clock monitoring and will have to be conducted from a space station, where there is no interference from daylight or bad  weather. Bill Borucki of NASA’s FRESIP proposes a search of 5,000 Sun-like stars. If planetary systems are common, FRESIP should detect about fifty transits each year (J. Davies, 1995). Certainty in the search for ex-solar planets will be enhanced by means of the Terrestrial Planet Finder, due to be launched in 2011, which will combine light from four giant telescopes to create sharp images of planetary systems.

All of the above-mentioned developments in the methods of detecting and observing ex-solar planets, and their prospect of success indicate a shift from merely listening for ET signals to an active search for potentially habitable sites. Although the detection of potentially habitable planets is not in itself supportive of ETI hypotheses, common sense suggests that these sites are the most promising places to concentrate future searches.

Pulsars and planets

Several claims have been made regarding the detection of planets which, so it is claimed, orbit pulsars. Pulsars are the result of the explosion of massive stars. When the core of a star collapses, the atomic particles may be squeezed so hard that they coalesce to form neutrons. A star composed almost entirely of neutrons is called a neutron star. Many of these are pulsars, emitting powerful beams of radio energy from their poles in regular time bursts as they complete a full rotation in a fraction of a second. They offer a unique opportunity to determine if planets orbit them, as they emit energy in regular pulsating bursts. Each radio pulse is 10 to 20 milliseconds long every 1.33730113 seconds. Hence an interruption by a regular orbiting body can be easily detected. Thus when the beam passes through the Earth it is detected as a radio pulse. As these pulses are extremely regular, any variation in the timing of pulses can be accurately detected. Reflex motion, caused by gravitational attraction of an orbiting mass, will affect the timing of the pulse.

In 1991 Aleksander Wolszczan, of Arecibo Radio Observatory in Puerto Rico, and Dail A. Frail of the National Radio Observatory in New Mexico,

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