Dumb or brainy, fair or hideous, extraterrestrial life forms are often pictured by scientists and writers of science fiction as inhabiting worlds just the right distance from stars -- neither too hot nor too cold. Rays of starlight in such temperate zones are seen as warming planetary surfaces and alien races, providing a ready source of energy and, most important, the right amount of heat to keep life-giving water from boiling away or turning into ice.
But a quiet revolution is now challenging this view and shaking the foundations of exobiology, the study of the possibility of life elsewhere in the cosmos.
Alien life, the new thinking goes, might not actually need the warming rays of a nearby star. It might thrive inside dim moons and planets. The dark ecosystems would be warmed by inner heat, bathed in melted ice and powered by chemicals.
Lightless realms on Earth have been found to teem with interesting creatures. Now scientists wonder whether similar environments elsewhere in the universe are home to alien microbial hordes and, in some cases, to large beasts and beings higher up the extraterrestrial food chain.
This change in thinking drives the excitement over Mars and Europa, a large moon of Jupiter, both of which have recently yielded tantalizing clues of conditions favorable to subsurface life. Scientists also speculate that the interiors of up to 10 bodies in the solar system may harbor extraterrestrial forms of life. So might the dim netherworlds around distant stars, where scientists have been finding more and more evidence of planets.
If these theories are right, alien creatures may be far more numerous throughout the cosmos than previously thought -- and much closer to home. That idea is rapidly changing plans for space probes as well as for research programs that study dark ecosystems on Earth, which are increasingly seen as a good way to get to know the extraterrestrial odds.
"We're in a paradigm shift," Dr. Frank D. Drake, a pioneer in the scientific hunt for extraterrestrials, said in an interview. "We're realizing that biology is very opportunistic and can adapt to a much greater variety of conditions than we imagined.
"The number of planets capable of supporting life is probably much greater than we thought in the past."
Dr. Wesley T. Huntress Jr., associate administrator for space science at the National Aeronautics and Space Administration, said discoveries on Earth were largely behind the changed view of the possibility of life elsewhere.
In the past, exobiologists always focused on surface life, he said. "Now we've found that life on Earth doesn't need light and can exist under extreme conditions we never expected," Huntress added. "Those aren't so different from what exists on other planets. So the probability that life may have arisen somewhere else in this solar system has gone up."
Dr. Bruce C. Murray, a scientist at the California Institute of Technology and president of the Planetary Society, a private group in Pasadena that backs space exploration, said the exobiology fervor was being fed by a rush of recent findings about Mars, Europa and extrasolar planets. "The confluence of new discoveries is incredible," Dr. Murray said in an interview. "They're driving the paradigm shift."
Dr. Thomas S. Kuhn, one of the century's leading historians of science, coined the term "paradigm shift" in the 1960s to describe how plodding science sometimes breaks into revolutionary periods in which old frameworks are quickly torn down and new ones erected. By all accounts, the field of exobiology is now in such ferment.
The tumult is changing not only mind-sets but also exploratory plans. The National Science Foundation, the government's main source of financing for basic science, recently started a program called Life in Extreme Environments, which aims to study terrestrial darks, hells and ices for clues to the existence of otherworldly life.
And NASA is revamping its whole approach to alien hunts. Biologists are being hired to help shape the agenda as other agency experts revisit and revise plans for existing probes of Mars, Jupiter and Saturn as a first step toward the development of new missions.
"Subsurface environments," Huntress said, have rather suddenly "become as important as those on the surface," and sometimes more so.
The first systematic hunt for space aliens was led in 1960 by Drake, who is now an astronomer at the University of California at Santa Cruz. He scanned the heavens with a huge dish-shaped antenna, listening for signals from advanced civilizations. The assumption was that many thousands of them existed among the 400 billion or so stars of the Milky Way galaxy, living on the surface of planets around Sun-like stars, warmed by light.
But in the 1970s, Dr. Michael H. Hart, a physicist at the National Center for Atmospheric Research in Boulder, began to ridicule the alien hypothesis. He argued that the habitable zones around stars were in fact quite narrow, with planets at their edges either boiling hot or freezing cold. If Earth's orbit were just a few percent smaller or larger, he held, no life on Earth could exist. The fact that it does is a lucky accident, he said.
In time, Hart's critique was viewed as cogent by some scientists, even though experts still debate the likely size of light-based "Circumstellar Habitable Zones," as a 1996 book edited by Laurance R. Doyle put the topic.
As the debate heated up, however, a radically new approach began to emerge, based on the work of Earth scientists probing the blackness of the deep sea. Diving in deep waters off the Galapagos Islands in 1977, they stumbled on a lush ecosystem whose otherworldly fauna included giant clams and fields of tube worms topped by bright red plumes.
The eerie world turned out to be powered by a volcanic rift in the seabed. The Earth's inner heat released chemicals that fed tiny microbes, which in turn became the founding link of large food chains, with the microbes playing a role analogous to that of plants in sunlit realms. Today, more than a hundred dark oases have been found in the sea's depths, along the volcanic rifts that gird the global seabed.
Meditating on such discoveries, as well as on the detection of microbes miles beneath land, Dr. Thomas Gold of Cornell University proposed in 1992 that bacteria might be ubiquitous throughout the upper few miles of Earth's crust, inhabiting the fluid-filled pores, cracks and interstices of rocks while living off Earth's inner heat and chemicals.
He calculated that the total mass of this hidden biosphere might rival or exceed that of all surface life.
"Such life may be widely disseminated in the universe," Gold said in his 1992 paper, suggesting that the solar system alone might harbor as many as 10 alien biospheres.
His theorizing embraced the possibility of life on the Moon; Mars; the large asteroids Ceres, Pallas and Vesta; Europa and Ganymede, two moons of Jupiter; Titan, a moon of Saturn; Triton, a moon of Neptune, and Pluto, the solar system's outermost planet.
All these rocky bodies, he speculated, might have enough interior heat by virtue of radioactive decay and other forces to melt ice into water and sustain a jungle of inner life.
Gold's provocative thesis, at first ignored or derided, is increasingly seen as worthy of investigation, especially as a series of recent discoveries have driven home the possibility of deep extraterrestrial biospheres.
Last August, NASA scientists said they believed a meteorite carved from the Martian depths and found in Antarctica harbored compelling signs of primitive life. They said they had identified organic molecules, minerals and cracks filled with carbonate globules that are all associated with microbes. The scientific jury, however, is still out on what that really shows.
Last month, NASA released the most detailed pictures yet of Europa, fueling a new burst of speculation.
Scientists said they were more confident than ever that a global sea of liquid water or slush lay just beneath the Jovian moon's thin crust of cracked ice. That, they added excitedly, might be the perfect place to look for extraterrestrial life.
"This is a very convincing set of pictures with respect to the presence of a liquid ocean," Dr. Michael H. Carr, a planetary geologist with the United States Geological Survey, said at a news conference.
NASA recently decided to extend the length of the Galileo mission so the spacecraft, now orbiting Jupiter, can focus on Europa in its final days. The mission was scheduled to end in December 1997, but it will now go through 1999 so the probe can swing past the icy moon for a series of close-ups.
Huntress, of NASA headquarters, said these visits were a prerequisite for a more ambitious mission that would use radar to peer through the Europan ice or would drop a pod that would melt through to the inner sea, if it exists.
"We've got a very systematic plan," he said in an interview.
Another target is Titan, an icy moon of Saturn that is already scheduled to be explored by NASA's Cassini spacecraft, which is in Florida being readied for launching this fall.
While orbiting and studying Saturn, Cassini is to deploy a probe to explore Titan, itself the size of a small planet. The probe will ride a parachute through the moon's dense atmosphere, which is thought to be similar to that of the early Earth.
Huntress said the new ideas about alien biospheres "could affect the way we do the observations" of Saturn and Titan. "That's all yet to be determined," he said.
Back on Earth, the new ideas about alien habitats are fueling interest in a huge lake, 140 miles long and 30 miles wide, that lies some two miles beneath the ice near Russia's Vostok Station in Antarctica.
Russian scientists and their associates, including Americans, have drilled deep into this ice but have halted the dig a few hundred feet above the liquid water out of fears that penetration would foul Lake Vostok with surface microbes.
The team hopes to find a sure way to prevent contamination and, perhaps as soon as the Southern Hemisphere summer, pierce the last layer of ice to look for primeval forms of life, which, in theory, might resemble extraterrestrials in this solar system or elsewhere.
Scientists say the quiet revolution could well end in the discovery that subsurface ecosystems are ubiquitous throughout the cosmos.
"Planets that go through a volcanic phase may routinely spawn life," Dr. John R. Delaney, an oceanographer at the University of Washington in Seattle who has championed the idea that Europa might harbor life, said in an interview.
"It's the evolution to the higher forms," he added, "that may be the unusual event."
