The Italian philosopher Giordano Bruno argued that space is filled with infinite numbers of planetary systems, inhabited by a multitude of creatures. For this and other indiscretions, Bruno was burned at the stake in 1600. Yet the question remains: are there other planetary systems in the universe? In the 1980s, for the first time, some evidence was found for disks of material surrounding other stars. However, as yet, no definitive observations have been made of a planet in orbit around another star. Some of the instruments proposed for the 1990s have the capability for detecting the Jupiter-sized planets of other stars.
Direct imaging of a distant planet is not easy, because the light from the parent star is billions of times brighter than the planet and because the star and its orbiting planet appear so close together as to be almost inseparable. The problem is analogous to trying to find from a distance of 100 miles a firefly glowing next to a brilliant searchlight. Fortunately, there are other ways to find planets. The position of the central star of a planetary system should wobble slightly in response to the changing gravitational tugs of its orbiting planets. At the distance of Alpha Centauri this shift in position amounts to a shift in angle of only a few thousandths of an arcsecond for a planet the size of Jupiter. Yet optical and infrared telescopes linked together as ground- or space-based interferometers will be capable of measuring such small angles and of surveying hundreds of stars within 500 light-years for the presence of distant planets like our own Jupiter. The orbital wobble of parent stars should also produce small velocity shifts that should be detectable with sensitive instruments on the large ground-based telescopes to be built in the 1990s.
Our sun and its solar system were formed about 4.5 billion years ago, as deduced from the proportions of uranium and lead measured in meteorites. In the intervening eons, life formed, and, most recently, humans began to explore the universe. What molecules were present to make the amino acids, proteins, and DNA that formed the first living creatures? What was the origin of life?
Weather, volcanoes, and bombardment by asteroids have erased from the planets almost all traces of the initial conditions in the solar system. Comets, however, spend most of their time far from the sun and may show us the pristine material of our world. Astronomers were surprised to find that the carbon in Halley's Comet (Figure 2.1) exists in the form of “tar balls” of complex organic molecules, rather than in simpler methane and carbon monoxide gas. The biological significance of this discovery is still hotly debated.
Comets will be closely studied in the coming decade. In addition to the close-range exploration planned for NASA's Comet Rendezvous Asteroid