Then early in this century -- as a result of problems with the nebular theory of the creation of the Solar System -- the pendulum swung back to the belief that the Solar System was a rarity, produced by the freak close approach of another star to the Sun.

In the Forties, when the nebular theory was successfully revised, opinion swung once again to the belief that not only were solar systems common, but so was life -- and probably intelligence.

But now -- just as we have finally discovered the first planets around other stars --an increasing number of scientists are once again questioning whether planets actually capable of sustaining life (or at least life beyond the microbe level) may not be very rare after all.

Partly this is due to the so-called "Fermi Paradox": Unless alien civilizations are very rare, shouldn't we have heard their radio broadcasts by now? But there are also several astronomical reasons for the new doubts -- and some of them, ironically, are due to the very nature of the new solar systems we have discovered.

In this and my next two reports, I'll examine the reasons for the new debate -- and how the evidence now looks.

First, there is the question of the "Habitable Zone" --- the range of distances around a star in which a planet is neither too hot nor too cold to have liquid water (which virtually everyone thinks is necessary for life) on its surface.

In the late Seventies, astronomers -- led by Michael H. Hart -- began to believe that it might be much narrower than had been thought, because of destructive self-amplifying "feedback" effects of a planet's atmosphere on its surface temperature.

Hart proposed that, if Earth hads been only slightly closer to the Sun, it would have met the same fate as Venus. There would have been much more water vapor in its air -- and since water vapor is itself a greenhouse gas, this would in turn have raised the planet's temperature still higher.

In the case of Earth, this process soon leveled off -- but in the case of Venus, it amplified itself until the planet rose above the boiling point and all its water was in its air.

Over the next few hundred million years, this water vapor -- lofted by its warmth into the planet's upper atmosphere -- would have been broken down and destroyed by the Sun's ultraviolet light.

But the planet's supply of carbon dioxide -- also belched out in huge amounts by its volcanoes -- wopuld have remained in the atmosphere, instead of being dissolved in the planet's liquid water supply and then reacting with its rocks to form carbonate minerals, as has actually happened to most of earth's CO2.

This huge mass of CO2 -- which is 100 times more massive on Venus than Earth's entire atmosphere is today -- would have produced enough greenhouse effect itself to make Earth's surface an inferno, just as Venus' temperature is still 460 deg C.

On the other hand, Hart suggested that if Earth had been just slightly farther from the Sun than it actually is, the opposite would have occurred: "runaway glaciation".

The planet would have been largely covered in dazzling white ice, which would have reflected back more sunlight instead of absorbing it to warm the planet -- and this would have lowered the temperature further and produced still more ice, until Earth's entire surface water supply was frozen solid.

Again, we escaped this fate becaause we happened to be in just the right distance range from the Sun for this process to level off instead of endlessly amplifying itself.

To make matters still worse -- there is the fact that after the Solar System formed 4.6 billion years ago, the Sun's energy output was only 70 percent of what it is today.

Since then, it has been steadily rising -- and unless the initial high level of carbon dioxide in Earth's air had, by sheer chance, dropped at just the right rate to match this, Earth would have tipped over at some point before now to either the runaway greenhouse or the runaway icebox.

In short, Hart proposed that Earth had been very lucky, and that life-bearing planets are rare. His calculations indicated that, if Earth had been just four percent (six million kilometers) closer to the Sun, it would have turned into a runaway-greenhouse inferno at some point in its past -- and that, as the Sun continues to get brighter, it will become one only a few hundred million years from now.

If the Earth had been a mere one percent -- 1.5 million km -- farther from the Sun than it is, it would have been a frozen "runaway glaciation" iceball that would only now be starting to thaw out and become capable of developing life!

Other stars would have habitable zones of varying thicknesses for their planets, depending on how massive the star was (bigger stars age, and thus increase in temperature, faster) -- and on how fast the planet's own initial high level of carbon dioxide, and thus its greenhouse effect, naturally dropped -- but in all cases, the zone in which the planet would have been habitable long enough for complex life to evolve would have been narrow.

A gloomy picture. But in the early Eighties, several planetologists -- led by James F. Kasting -- called it into question, by pointing out that there was an important natural process that Hart had neglected: a natural thermostat that natually raises and lowers the level of CO2 in an Earthlike planet's atmosphere to compensate for the level of energy poured into it by its sun's light, and thus naturally regulates its temperature.

As I say, the initial high level of CO2 belched out by the early Earth's volcanoes was gradually removed from its air by the fact that it dissolved in Earth's liquid water and then reacted with its surface rocks.

But this process is mostly due to rain falling on Earth's dry land and then running off over it in streams and rivers to the sea -- and the warmer a planet gets, the more rainfall it has. On the other hand, the countervailing process -- in which Earth's crustal tectonics (its "continental drift") gradually pulls parts of its crust down into its molten interior, breaking down the carbonate minerals so that Earth's volcanoes belch the CO2 back into the air -- runs at a steady rate regardless of the planet's surface temperature.

So, if Earth had been closer to the sun and thus started to become warmer, more of its CO2 would have been pulled out of its air and trapped in its crustal minerals, reducing its greenhouse effect and lowering the planet's temperature back down.

If Earth had been farther from the sun and thus started to become colder, its rainfall would have been less, and CO2 would have built up in its air, warming it.

There are limits to this natural thermostat, of course. If Earth had been much closer to the Sun than it is, the greenhouse effect of the added humidity in its air would have outpaced the decrease in its CO2 level and the runaway greenhouse would have occurred.

At a somewhat greater distance from the Sun, while the greenhouse would have leveled off at a temperature low enough to allow liquid-water oceans to exist, its stratosphere would have been above the freezing point -- allowing water vapor to drift into its upper atmosphere and be destroyed by the sun's UV light, so that the planet's entire water supply would have hemorrhaged away that way in just a few hundred million years.

Kasting's calculations indicated that Earth would have met the second fate by now at 143 million km from the Sun -- so in the end Hart was right after all about the inner limit of the Sun's habitable zone.

On the other hand, if Earth had been much farther from the Sun, its stratosphere would have been cold enough for clouds of frozen carbon dioxide ("dry ice") to form -- and Kasting thought that, while this dazzling white cloud layer would have reflected back much of the Sun's light before it could warm the planet's ground, it would have been transparent to infrared radiation and thus allowed the planet's heat to escape into space, so that it would have had a cooling effect.

If Earth had been far enough from the Sun, then no matter how thick a CO2 atmosphere it developed, the cooling effect of the resultant thicker CO2 cloud layer would have overpowered the CO2's warming.

But while he was a lot less certain about the distance of the habitable zone's outer boundary, he was certain that it was much farther than Hart had thought -- somewhere between 210 and 245 million km from the Sun today -- and thus that the total habitable zone was much thicker than Hart had thought, both for our own Sun and for other stars. And soon, most astronomers were convinced -- and still are -- that he was correct.

Moreover, there was one remaining surprise. The one problem with Kasting's theory was that, during the Solar System's early days when the Sun was dimmer, Mars was well outside the outer bounds of the habitable zone -- so that, even though we are virtually certain that Mars' volcanoes belched up a thick CO2 atmosphere in its early days (maybe several times as dense as Earth's whole atmosphere is today), Mars could never have been anywhere near warm enough for liquid water to exist.

Yet the evidence -- in the form of ancient dried-up riverbeds -- that Mars did have water on its surface during its first billion years or so is very strong.

In 1997, however, this problem was solved, when studies by two atmospheric scientists indicated that Kasting had been wrong about dry ice clouds -- they were nowhere near being transparent to IR radiation, and so would have had nothing like the cooling effect he had thought.

This meant that, even in the Sun's dimmer early days, Mars' thick early air could very well have warmed it enough for liquid water to exist there. In fact, the revised theory suggests that Earth would be warm enough for water oceans today even it was as much as 200 million km from the Sun -- all the way out in the inner fringes of the asteroid belt!

Mars became desolate not because it is too far from the Sun, but because it is too small. Its weaker gravity and its lack of a magnetic field mean that the solar wind has been gradually sweeping its upper atmosphere into space for eons; and since it's harder for heat to build up in its interior and drive the churning process of crustal tectonics, as its CO2 was converted into carbonate minerals in its early days it was lost permanently, instead of the carbonates being dragged underground and heated to re-release the CO2 as happened on Earth.

Thus most of its air vanished within its first billion years -- and not only did its warming greenhouse effect vanish, but its air pressure finally dropped to the point that liquid water couldn't exist on its surfce even if the planet was warm enough.

If Mars was as big as Earth, we would almost certainly have another planet friendly to life in our Solar System -- although it would be largely shrouded in dry-ice clouds, and so plants would have to get along with far less sunlight than on Earth.

Even if it had been farhter from the Sun, so that it still was frozen today, there is a good chance that it would thaw out and become capable of sustaining life as the Sun slowly becomes brighter over the next six billion years.

In addition, it appears that -- for the first billion or two years after its formation, when the Sun was dimmer -- Venus may have been cool enough to possess oceans and microscopic life. But, given the vast difficulty of exploring it and the fact that since then its volcanoes have repaved virtually all its surface with lava, it's very unlikely that we could ever find the fossils to prove it.

The smaller (and slower-evolving) a star, the wider its long-term habitable zone is. The Sun is a "G-class" star; somewhat dimmer "K" types -- which are three times commoner -- may be even more likely to have habitable worlds.

The commonest stars by far are the dim "M-class" red dwarfs, and their long-term habitable zones (close to the star) are the widest of all -- but they have another problem: Any habitable planet must be so close to one of them that tidal forces would quickly stop its rotation so that it keeps one face permanently toward the star as Mercury was once thought to do, and utill recently it was thought that this would mean that its atmosphere would actually freeze out solid on its cold nightside.

But recent studies suggest that even a modest CO2 atmosphere would transfer enough heat to the planet's nightside that this wouldn't happen, and that such stars too may have many habitable planets (although plants can only grow on their dayside).

In short, it appears that Hart's problem hs been resolved (although many habitable planets may be shrouded in such thick layers of dry-ice clouds that green planets can't get enough light to survive there, and so the only life they possess is primitive microbes).

But there are other problems, and in my next installment I'll look at one of them -- the possibility that a habitable planet may need a large moon like ours, and that such moons may be rare.