"Innumerable suns exist; innumerable earths revolve about these suns in a manner similar to the way the seven planets revolve around our sun. Living beings inhabit these worlds." The Italian philosopher-astronomer Giordano Bruno was burned at the stake for these words in the year 1600.

Not surprisingly, then, even four hundred years later, Darren Williams is a bit more circumspect. "We are starting to gain confidence that planetary systems are common, Jupiter-sized planets are common -- maybe Earths are common too," says the Penn State graduate student in astronomy.

Speculating about the existence of life on these planets -- life beyond Earth -- has long ceased to be a capital offense. Recently, however, such speculation has taken a large step forward in terms of scientific respectability. In early August, NASA caused a media flurry by announcing (prematurely, some scientists suggest) the discovery of what may be fossil evidence of primitive life on Mars: tiny one-celled tubules encased in meteor fragments. Only sightly less dramatic are a series of recent astronomical observations. For the first time we have clear evidence of planets outside the solar system.

Penn State astronomer Alexander Wolszczan discovered one such set of planets in 1994. Analyzing radiowaves emitted by a pulsar, the burned-out remnant of a neutron star, Wolszczan detected the presence of three bodies in orbit around it, spaced similarly to the way Mercury, Venus, and Earth are placed in relation to the sun. Within the last 15 months, there have been several more "sightings." In October 1995, Michel Mayor and Didier Queloz of the Geneva Observatory announced the detection of a large planet orbiting the star 51 Pegasi, some 40 light years distant. In January 1996, Geoffrey Marcy and R. Paul Butler of San Francisco State University and the University of California at Berkeley reported the existence of still larger planets circling 47 Ursae Majoris (in the Big Dipper) and 70 Virginis, about 35 light-years away. In June, George Gatewood of the University of Pittsburgh made public his discovery of two potential planets orbiting Lalande 21185, only 8.1 light-years away.

These planets are massive, Jupiter-sized (or larger) bodies in fast orbits -- the ones easiest to detect. And in fact, none of them has been seen directly. Their presence has been deduced by gravitational effects on their parent stars. But if we can "see" these few, the thinking goes, then there must be many more planets out there. And if so, some of them are bound to look, and act, like Earth.

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For James Kasting, the search for extraterrestrial life breaks down into two very down-to-Earth deductive steps: Determining habitability, i.e., where in the universe life could possibly be, given what we know; and recognizing habitation, i.e., once we've determined where to look, knowing what to look for. Kasting, professor of geosciences at Penn State, has been influential in the development of the concept of the habitable zone, that region around any star that can support liquid water on a planet's surface.

In order to host life, Kasting writes, "a planet must satisfy a number of conditions. . . . It must have water, carbon dioxide (for photosynthesis), and other volatile compounds (ones containing N, P, and S) available at its surface. It must have sufficient mass to hold onto an atmosphere and it must be in an orbit that is stable over long periods of time. It also needs to have a stable climate that is, at the very minimum, conducive to the continued presence of liquid water."

The basic determinant of climate on a planetary scale is distance from the sun. The habitable zone, then, is the range of distances that is neither too close nor too far, i.e., too hot or too cold.

On Earth, or any other planet with water, this range is extended by a negative feedback system: the carbonate-silicate cycle. Carbon dioxide in the atmosphere dissolves in rainwater, creating a weak acid which wears away silicate rocks on land. Byproducts of this weathering are carried by rivers and streams into the ocean, where they are made into calcium carbonate shells by snails and other mollusks. When the snails die, their shells are buried in the crustal sediment. Then, as tectonic plates collide, the crust gets subducted: parts of it are driven downward, and at high sub-surface temperatures the geochemical process is reversed: calcium carbonate is broken down into silicates, which are released as CO ₂ by volcanoes.

Within the habitable zone, this system regulates itself according to temperature. "Weathering requires liquid water," Kasting explains. If the Earth were farther from the sun, temperatures at the surface would drop. Weathering would slow, and carbon dioxide would begin to accumulate in the atmosphere. The resulting increase in the greenhouse effect would push temperatures back up again. If Earth were closer, on the other hand, warmer temperatures would cause increased weathering, CO ₂ levels would fall, and the climate would begin to cool.

Beyond a certain point in either direction, however, and the system shuts down. Too far, and carbon dioxide starts to condense, forming clouds that block out the sun's rays, and everything freezes. Too close, and the oceans boil away.

Even so, the habitable zone is fairly roomy -- big enough to harbor plenty of planets circling plenty of stars. But are such planets really likely to contain life? Not according to French astronomer Jacques Laskar.

In 1993, Laskar published the results of a study suggesting that conditions for life around the universe might not be rosy. Laskar's focus was not solar distances but planetary dynamics, and in particular the way planets wobble in their orbits. The technical terms for these motions are precession and nutation.

The obliquity of a planet is the angle of its spin axis -- that imaginary rod that skewers a planet through the poles -- in relation to the plane of its orbit. In Earth's case, this angle is a modest -- and reasonably steady -- 23.5 degrees, enough of a tilt to account for our seasons. Because of Earth's obliquity, in the northern hemisphere the winter sun stays low in the sky, casting its rays obliquely across the landscape. In summer it climbs to more nearly overhead, an angle much better for warming.

We living organisms depend, maybe more than we know, on the stability of Earth's obliquity. A shift in tilt as small as 1.3 degrees, scientists reckon, could trigger an Ice Age. At higher obliquities, greater than say 54 degrees, the poles would actually receive most of the sun's radiation, and the equator only a small fraction -- the reverse of today. At 90 degrees, here in Pennsylvania, the sun would not set between mid-May and mid-August, and not rise from late November to the middle of February. The equator would grow a permanent ice cap.

Obliquities this high would present a couple of basic problems for life as we know it. One is surface temperature. Our northern hemisphere, in this scenario, would be a very different place, with summer temperatures averaging a hellish 120 to 140 F at Pennsylvania latitude, and 180 to 200 F at the North pole. "We have to consult the biologists," Williams says, "but I think DNA breaks down at about 200 F. So it's doubtful that life as we know it -- water-dependent, carbon-based life -- could exist in such an environment." Even if it could, the winters would pose a different challenge. "A microbial life form might be able to survive high temperatures," Wiliams speculates, "but six months later it would have to fend off temperatures as low as -50 F. It would have to be very migratory to survive."

Yet Laskar's results suggest that all of the interior planets of the solar system, including Earth, have experienced high obliquities at one time or another. Over the course of their evolutions, in fact, the obliquities of Venus, Earth, and Mars have fluctuated widely -- and unpredictably. These planets have wobbled all over the place.

Mars, Laskar showed, still does, its obliquity varying chaotically between 0 and 60 degrees. The only thing that saved Earth from a fate worse than Mars', he says, was hooking up with the moon.

A little orbital mechanics may be in order here. Earth is a body in complex motion. It rotates, of course, once every 24 hours. At the same time, on a different time scale, it revolves around the sun. But there are other, subtler, motions which must also be accounted for. Earth's imperfection as a sphere, for one thing, adds a motion called precession, akin to the wobbling of a spinning top. Earth wobbles, as noted above, only slightly and in very slow motion, its obliquity oscillating from 22 to 24 degrees every 40,000 years.

Meanwhile, however, the gravitational pull of the other planets in the solar system, especially the giants Jupiter and Saturn, causes a different kind of precession: a wobble in the plane of Earth's orbit. Imagine the orbital plane as a solid object, a spinning frisbee or a dinner plate, with the sun a dollop of mashed potatoes in its center, and Earth a wad of chewing gum stuck to its outer rim. The insistent tug of these outer, larger planets against the sun's stronger pull causes the plate to wobble as it spins.

The real action, in terms of shifting obliquity, comes when these two types of wobble -- the planetary and the planar -- stumble onto the same frequency. Then you get what's known as a spin-orbit resonance: in synch, the two motions combine their energy, creating a much larger force. It's not unlike the kind of timing it takes to keep a hula-hoop spinning around your hips, or to successfully push a child's swing. For a planet, however, resonance means chaos, as two small competing wobbles become one huge concerted one.

What saves Earth from falling into a spin-orbit resonance, Laskar says, is the moon. Because of its size and proximity, the moon exerts a strong gravitational pull of its own on our home planet -- a pull which turns out to be a stabilizing influence. The lunar effect acts to accelerate Earth's global precession, maintaining it at a steady frequency well higher than the torpid wobbling of the orbital plane. Take the moon away, Laskar says, and keep things otherwise the same -- give Earth the same mass, orbital position, rotation rate, etc. -- and Earth's obliquity would fluctuate between 0 and 85 degrees.

Laskar's finding, Darren Williams says, has broader implications for the probability of life on other planets. "We think the moon is the result of an accident of accretion, that it was formed by a chance collision between Earth and a Mars-sized object," he explains. If this is so, "then statistically, many other Earths should be moonless." On these "Earths," Laskar's result suggests, because of high and chaotically fluctuating obliquities, the presence of life would not be likely.

As a first-year astronomy graduate student in 1994, Williams took Kasting's seminar on the origins of Earth and Moon. When Williams subsequently came to Kasting looking for a project for his thesis, Kasting thought of Laskar's work. He, Kasting, had been interested in testing Laskar's conclusion in the light of his own research. Williams' astrophysics background made him an ideal candidate for the job.

As a first attempt, Williams and Kasting determined surface temperatures on a hypothetical high-obliquity Earth, a world whose angle toward the sun was 90 degrees. "We found enormous swings in temperature, especially on high-latitude continents," Willims remembers. This finding seemed to confirm Laskar's pessimism. "But this was really just a detailed back-of-the-envelope calculation," Williams notes. Then, in 1995, he won a three-year NASA graduate fellowship, enough funding to build a climate model that would enable him to go deeper into the problem. Immediately, things started to look more complicated. The harsh surface effects of high obliquity, Williams soon found, would likely be negated -- or at least substantially softened -- by certain factors not accounted for in Laskar's theory.

The placement of continents on a planet, for one thing, has a big impact. Surface temperature varies greatly depending on whether that surface is land or water. A land mass heats and cools quickly. Ocean, on the other hand, has a much higher heat capacity. It warms very slowly, absorbing great quantities of radiation that would otherwise escape through the atmosphere. Over ocean, there's less seasonal variation. "That's why the temperature in Maine doesn't swing as wildly as that in Oklahoma," Williams says.

On our present Earth we have a continent at the South Pole, and an ocean (currently an ice cap) at the North, and continents scattered through the mid and higher latitudes between. But if all the Earth's land mass were gathered at one Pole, and the rest of the planet was ocean, the effects of high obliquity at the surface would largely depend on where on the globe you were measuring. At the land-mass end, temperatures would swing by 200 degrees F. At the ocean end, however, the temperature variation would be significantly lessened.

Even more important, Williams found, is the atmospheric density of carbon dioxide. Earth's is very low: 3x10^-4 bar. There's relatively little to insulate us from the sun's scorching rays. But on a planet with a thicker CO₂ blanket, the heat from those rays would be evenly distributed. "In a dense CO₂ atmosphere," Williams says, "you have very efficient heat transport between latitudes." On Venus, which has a 90-bar atmosphere, the difference in surface temperature between the equator and the poles is a mere 3 F. Seasonal extremes are minimal.

What kinds of planets are likely to have denser atmospheres? Those that are farther from their stars than Earth from Sun -- but not too far. The carbonate-silicate weathering cycle, remember, is temperature-driven. As surface temperature drops with increased distance from the sun, weathering decreases, and carbon dioxide begins to accumulate. Up to a point, anyway, temperatures remain stable.

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Williams thinks the number of Earth-like planets with thick atmospheres will turn out to be pretty substantial. So, evidently, does NASA. In the past few years the space agency has made a major priority of finding out what lies beyond our galaxy. Informally, the mission is known as "Planet Finder."

Seeing distant planets, especially small planets like Earth, is beyond the capacity of current telescopes. The meager light reflected from these dark objects is completely overwhelmed by the radiance of their parent stars. In the infrared range, however, Williams says, this "swamping" effect is significantly lessened. "With an infrared telescope you could possibly resolve the light from an Earth-like planet, and separate that light from the parent star." Even in infrared, however, the task would require a very big telescope deployed in space, beyond the muddiness of Earth's atmosphere. A gigantic telescope -- with a mirror 60 meters or more in diameter. Were such a telescope even technically feasible, its cost would be prohibitive.

The answer is a sophisticated imaging trick known as interferometry. Two small telescopes deployed at some distance from one another are trained on the same star, aligned so that each one receives the exact same pattern of light waves. The image from one of these telescopes is then inverted so that, laid atop one another, the reverse images completely cancel out the star's light. What's left is only the light of any planets that may be nearby. In effect, the combined instruments act as a single giant telescope.

NASA hopes to deploy such a space-based system within a decade. Once in orbit, proponents say, the interferometer should be able not only to make out small Earth-like planets in distant corners of the galaxy , but to check them for signs of life. Kasting recently gave a talk to Planet Finder brass on detecting the tell-tale presence of ozone in a planet's surrounding atmosphere.

But before they go looking for life, astronomers will need to know which planets are the likeliest places to look.

"What we have determined so far," Williams says, "is that high obliquity would not necessarily rule out a climate suitable for life." Only a small subset of planets would be rendered inhospitable by the tendency to wobble wildly, he adds: those with thin atmospheres and without large moons. One of his tasks over the next two years will be to narrow things down even further. He will do so by posing what-ifs: setting up various hypothetical scenarios on his model and observing the results. What if the Earth's moon were larger? Smaller? Farther away? (When formed, Williams notes, the moon was two to three Earth radii from Earth; now it is at 60 Earth radii and fading.) Then he'll take Earth and moon together and place them in a different kind of solar system, just to see what happens.

"In the future," Williams says, "we will probably discover other solar systems. Then we will have to check these out. Is the system likely to be stable? What planets within it might be habitable?" This kind of modeling, he hopes, will help lay the crucial groundwork for the Planet Finder mission.

"We hope to be able to tell them where to point their telescope."

Darren M. Williams is a Ph.D. student in the department of astronomy and astrophysics, Eberly College of Science, 406 Davey Laboratory, University Park, PA 16802; 814-863-7947. His adviser, James F. Kasting, Ph.D., is professor of geosciences in the College of Earth and Mineral Sciences, 211 Deike Building, 865-3207. Williams' research is funded by a graduate fellowship from the National Aeronautics and Space Administration.