Just for Earth's own galaxy, the Milky Way, experts have estimated that there might be up to 1 million advanced societies.

This extraterrestrial credo has fueled not only countless books, movies and television shows -- not to mention hosts of Klingons, Wookies and Romulans -- but a long scientific hunt that uses huge dish antennas to scan the sky for faint radio signals from intelligent aliens.

Now, two scientists say the conventional wisdom is wrong. The search for alien life, they say, is likely to fail.

Drawing on findings in astronomy, geology and paleontology, the two argue that humans might be alone in the cosmos. They say science is showing that Earth's composition and stability are extraordinarily rare.

Most everywhere else, the radiation levels are too high, the right chemical elements too rare, the hospitable planets too few and the rain of killer rocks too intense for life to have evolved into advanced communities.

Alien microbes may survive in many places as a kind of cosmic shower scum, they say, but not extraterrestrials civilized enough to be awash in technology.

Their book, Rare Earth, is producing whoops of criticism and praise, with some detractors saying that the authors have made their own simplistic assumptions about the adaptability of life forms while others call it ‘brilliant’ and ‘courageous’.

‘We have finally said out loud what so many have thought for so long -- that complex life, at least, is rare’, said Dr. Peter Ward of the University of Washington, a paleontologist who specializes in mass extinctions and whose previous works include The Call of Distant Mammoths. ‘And to us, complex life may be a flatworm.’

Ward's co-author is Dr. Donald Brownlee of the University of Washington, a noted astronomer, member of the National Academy of Sciences and chief scientist of NASA's $166 million Stardust mission to capture interplanetary and interstellar dust.

‘People say the sun is a typical star’, he said. ‘That's not true. Almost all environments in the universe are terrible for life. It's only garden-of-Eden places like Earth where it can exist.’

Dr. Geoffrey Marcy of the University of California at Berkeley, a leading seeker of planets around other stars, 31 of which have been found so far, hailed Rare Earth as likely to spark a revolution in thinking about extraterrestrial life.

‘It's brilliant’, Marcy said. For instance, he said, it shows how giant planets discovered so far outside the solar system bode ill for the development of complex life.

The notion that alien civilizations are ubiquitous arose in a scientific sense four decades ago.

Dr. Frank Drake, then a young astronomer in West Virginia, in 1960 was the first to scan the skies for alien signals. He laid out his ideas in 1961, in what came to be known as the Drake equation.

The equation made educated guesses for the rate at which stars form, the fraction of stars with planets, the number of those planets on which life arises and so on, including the average lifetime of technological civilizations.

By his logic, the Milky Way had about 10,000 civilizations capable of interstellar communication.

Later, famed astronomer Dr. Carl Sagan raised the estimate to a million alien worlds.

New findings, however, according to the authors of Rare Earth, show Drake's equation is riddled with optimistic assumptions.

Ward was drawn to the topic by his studies of mass extinctions. Increasingly, top culprits are seen as speeding rocks from outer space that hit Earth in huge explosions, with one 65 million years ago killing off the dinosaurs.

New studies, Ward said, suggest that things could be worse. The rate of impacts could be as much as 10,000 times higher but for the presence of Jupiter, the solar system's largest planet, which absorbs many killer rocks and flings others into deep space.

‘We're right on the edge of the abyss’, Ward said, in terms of higher bombardment rates that have probably precluded the development of advanced life.”

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