The possibility that life, primitive or advanced, might exist in other places of the Universe has occupied the thoughts of scientists and laymen for thousands of years. It is only in the last 25 years, however, that we have entered the experimental phase of the search for extraterrestrial life, where considerable progress has already been made.

On this silver anniversary of the first published proposal for a search of the skies to seek communications over interstellar distances, I present a rather personal account of the origins of the proposal, and an appraisal of the new and hopeful status of the search worldwide.

If asked to assess the evolution of SETI in the quarter of a century which has elapsed since its proposal, I would say that it parallels that of most novel and challenging human endeavours; a mixture of technical and cultural progresses and set-backs.

This Welcoming Address on behalf of Boston University, the host institution for IAU Symposium 112, was given at the Symposium Banquet, which was held at the new hall of flags of Boston University in the evening of June 19, 1984.

This invocation was offered at the Banquet of IAU Symposium 112 in the evening of June 19, 1984.

Planetary systems appear to be one of the crucial links in the chain leading from simple molecules to living systems, particularly complex (intelligent?) living systems. Although there is currently no observational proof of the existence of any planetary system other than our own, techniques are now being developed which will permit a comprehensive search for other planetary systems. The scientific rationale for and methods used in such a search effort are reviewed here.

A statistical analysis of IRAS observations indicates that about 20% of the nearby main sequence stars, and probably a similar fraction of main sequence stars not detectable by IRAS, shows evidence of significant infrared excess beyond 12 microns. Spectral type A dwarfs are predominant, but not exclusive in showing large 60 micron excesses. Our solar system, when viewed from sufficient distance, appears to have, in contrast to this, less than 1% of cool infrared excess. These observational facts are consistent with the working hypothesis that cold infrared excess in stable main sequence stars is due to proto-planetary material, observable during the accretion phase in the evolution of a solar system.

The discoveries of small particulate clouds surrounding several young stellar objects may be the first direct indications of ongoing planet formation in the Galaxy. If these clouds are indeed the initial stages of new planetary systems around the stars, then planetary systems are probably not rare occurrences around stars and may even be quite common. The evidence for these clouds and their interpretation as young planetary systems is reviewed and discussed in the context of estimating the probability for planet formation around solar-mass stars.

Detection of planets by virtue of their gravitational perturbations of nearby visible stars generally requires relative astrometric accuracies of ~ 3 × 10−4 arc sec for Jovian planets and ~ 3 × 10−7 arc sec for terrestrial planets. Two-color optical astrometric interferometers on earth should be capable of ~ 10−4 arc sec rms accuracy for stars brighter than magnitude 7–10, while small one-color space systems should achieve at least ~ 10−5 arc sec. Two-color systems permit correction every few milliseconds for the random wavefront tilt due to atmospheric turbulence; the correction is based on the observed offset between the red and blue fringes, and the known atmospheric dispersion. A 3.4-meter baseline interferometer on Mount Wilson has demonstrated ~ 0.02 arc sec rms fluctuations for 1-second samples.

As a follow-up to the development of astrometric detectors of sufficient accuracy to detect extrasolar planetary systems, the Allegheny Observatory is modernizing their 30-inch Thaw Refractor. Changes include a new objective lens and the transition of the observational bandpass from blue light to red. The new system will have sufficient accuracy to detect jovian planets orbiting any of several dozen of the Sun's nearest stellar neighbors.

The Hubble Space Telescope (ST) will be launched with five dedicated scientific instruments and a capability to do astrometric measurements with the fine guidance sensors. Four of these – the Faint Object Camera, the Wide Field Camera, the Fine Guidance Sensors and the High Speed Photometer – can be used in the search for extrasolar planetary systems. The Faint Object Camera will be able to directly detect planets around a few of the nearby bright stars. The Wide Field Camera and the Fine Guidance Sensors can be used astrometrically, both with an accuracy of about 2 mas per observation. The High Speed Photometer possibly can detect planets during occultation of stars by the moon and minor planets. The ST is expected to be launched in mid-1986 and these observations are the among the first planned with the instruments.