No little green men from Mars - as yet

Astronomers in Texas tomorrow will ponder whether life exists in other solar systems. Peter Bond reports

All fans of science fiction believe the universe is populated with innumerable planets inhabited by a bewildering variety of living creatures. Tomorrow, astronomers at the American Astronomical Society's annual meeting in Texas will reveal the latest science facts.

Most scientists have gone along with the science fiction aficionados, reasoning that with 100 million stars in our galaxy there is statistically some likelihood of other Earthlike worlds orbiting at least some of them. But the first real evidence that our solar system was not unique came in 1983. An American-Dutch-UK infrared astronomy satellite known as IRAS found some 50 stars that seem to be surrounded by discs of gas and dust. Astronomers believe that the Sun and planets condensed from just such a rotating disc of interstellar material some 5 billion years ago. Like a giant gramophone record with the Sun at its hub, the rotating solar disk probably extended far beyond the present orbit of the outermost planet, Pluto.

While supersonic solar winds from mature stars such as the Sun soon blow away most of the surrounding material, newcomers to the galaxy that have only recently begun their nuclear reactions still retain their dense, dusty shrouds, which may actually be the swaddling clothes of new planets. Unfortunately, such cold clouds do not glow visibly for observation in optical telescopes, so astronomers turn to telescopes operating in the infrared region, such as the UK Infrared Telescope in Hawaii and the instruments on board IRAS. Many more of these cool circumstellar clouds may be revealed in the next few years by the European Space Agency's newly launched Infrared Space Observatory (ISO), which is much more sensitive than IRAS.

Further breakthroughs are being provided by the refurbished Hubble Space Telescope. A recent survey of the famous Orion Nebula revealed more than 150 circumstellar disks tilted at different angles to Earth. The dark discs were silhouetted against the brightly illuminated background of the hot nebula.

Finding huge discs of material billions of miles across is one thing. Trying to prove the existence of individual planets is quite another. Even the most sensitive detectors attached to the largest telescopes cannot yet detect Jupiter-size planets around relatively nearby stars.

Yet progress is being made. Within the past year, several faint objects known as brown dwarfs have been identified. One of these, Gliese 229B, is at least 250,000 times dimmer than the Sun and is the faintest object ever seen orbiting another star. With an estimated mass 20 to 50 times that of Jupiter, the brown dwarf is too large and hot to be considered a planet but too small and cool to shine like a star.

One of the most fascinating revelations about this unusual object is the discovery that, like Jupiter, it is rich in methane. Familiar to us as natural gas, methane is a common constituent of large gas planets but is not found in ordinary stars. It seems that Gliese 229B shines dimly because it is gradually heating up as it shrinks in size through gravitational collapse. Jupiter has a similar, though much weaker, internal heat source. Although it probably originated as the smaller member of a binary star system, some astronomers believe that it may be the first known example of a "superplanet".

Recent discoveries suggest that, despite their wide differences, the nine planets of our solar system may not be fully representative examples of the planetary population in the universe as a whole. In 1991, two American radio astronomers declared that they had discovered regular variations in the radio waves emitted by a strange, dense star known at a pulsar. These variations are now accepted as firm evidence for the existence of at least two planets orbiting the star.

Unfortunately, this pulsar is not a normal star like our Sun. It is an ancient remnant of a star that exploded more than a billion years ago. Such a violent event would have completely blown away any planets, so the orbiting companions must have evolved in more recent times. According to Professor Andrew King of Leicester University, the planets must have formed from the remains of a small companion star that was completely disrupted by the pulsar. "The material goes into a large, disclike nebula, like the rings of Saturn, around the pulsar," he said. "This nebula would be an ideal place to form planets."

Last November, the discovery of an equally strange planet was announced by two Swiss astronomers, who noticed a regular wobble in a nearby star known as 51 Pegasi. They deduced that a large object was pulling on the star. Although 51 Pegasi is similar to the Sun, nothing like its new-found companion exists in the solar system. If the planet exists, it must be about the same size as Jupiter, but its orbital distance is only 8 million kilometres so that it sweeps around the star once every four days compared with 12 years for Jupiter.

What are the chances of finding life on smaller, Earthlike worlds? Sadly, not very high. Neither the Hubble Space Telescope nor modern ground- based instruments are sufficiently sensitive to detect such small bodies over cosmic distances. As for the existence of alien life, no one knows. A dedicated team of American radio astronomers persists in scanning the radio waves for any sign of an alien intelligence, but our galactic neighbours remain stubbornly silent.

The arguments for and against are summarised in a recently published book entitled Extraterrestrials. Where Are They? in which one of the contributors, Dr Michael Hart, concludes: "The universe ... contains an infinite number of inhabited planets, but the chance that any specific galaxy will contain life is extremely small. Most intelligent races should see no other civilisations in their galaxy; indeed, they should see no others in the entire portion of the universe which they are able to observe with their telescopes." Such is current scientific orthodoxy.

'Extraterrestrials. Where Are They?' is edited by Ben Zuckerman and Michael Hart and published by Cambridge University Press.

Peter Bond is the space science adviser for the Royal Astronomical Society.