In my last installment I noted that in the 1980s Kasting -- who is generally recognized as the leading expert in the matter of habitable climates for planets -- discovered that the width of the habitable zone for a planet around a star is greatly widened by the existence of a natural "carbon dioxide thermostat" that tends to regulate the temperatures of Earth-type planets, keeping them from getting too hot or too cold for liquid water to exist.

Such a planet's volcanoes belch out CO2 at a steady rate but - within those broad limits - as a planet gets warmer, its rainfall increases. This helps to wash more CO2 out of its atmosphere to react with its surface rocks, so that the greenhouse effect from the CO2 decreases and the planet cools back down. Exactly the opposite happens as a planet gets cooler. All of this happens over periods of several hundred thousand years, so it has no relevance to our possible current problem of man-made 'global warming'.

It turns out that Earth is actually quite close to the inner edge of our own Sun's habitable zone -- only 10 million km closer, and despite the thermostat effect, Earth's liquid water supply would all evaporate into the upper atmosphere and be destroyed by the Sun's ultraviolet light within a few hundred million years, turning Earth bone-dry and allowing CO2 to build up in its atmosphere until it turned into a sweltering greenhouse inferno like Venus.

But there is a lot of room for additional habitable planets in the opposite direction, further from the Sun. The effect that the thermostat effect has on the level of CO2 in a planet's atmosphere is astonishing -- if Earth were as far from the Sun as Mars, it would have 12,000 times as much CO2 in its air as it now does, and four times as much CO2 as the current total pressure of its atmosphere!. The greenhouse effect from this thick blanket of carbon dioxide would keep it warm enough for liquid water and life. Mars is desolate not because it's too far from the Sun, but because it's too small and consequently has lost most of its air.

Well, it turns out that such a thick CO2 atmosphere has a very nice fringe benefit -- it is extremely efficient at spreading heat evenly around from one part of a planet to another. Venus, with its superdense CO2 air, is only two degrees C cooler at its poles than at its equator.

In the same 1997 paper, in which Kasting and Darren Williams calculated the disastrous climatic effect of Earth having a large axial tilt, they measured the effects that such an extreme tilt would have on Earth if it were 210 million km (130 million miles) from the Sun.

It turned out that, even if Earth were keeled over 90 degrees so that its poles periodically pointed straight at the Sun, its climate would be positively balmy -- the equator would be 11 deg C (52 deg F), and the poles would never rise above 46 deg C (115 deg F) or fall below 3 deg C (37 deg F). Earth would have no ice anywhere on its surface, except on some of its highest mountains.

Carbon Broadens Life Zone
Since then, other meteorologists have found that the clouds of "dry ice" -- frozen CO2 -- that would form in the upper atmosphere of such a distant high-CO2 world would actually have a further warming effect, rather than a cooling effect as Kasting had thought, so that it might be possible for Earth to be comfortable for life even if it were fully twice as far from the Sun as it is -- as far out as the inner fringe of the Asteroid Belt.

In short, while most Earth-type planets in the universe probably do indeed spend all or much of their time tilted much more than Earth, for most such planets if they are in their sun's outer habitable zone, such tilts may not be disastrous after all as they may have thick enough carbon dioxide atmospheres to compensate - even without a large moon.

But they may have another problem, - lack of light. At Mars' distance from the Sun, sunlight is less than half as bright as on Earth -- and as an Earth-type planet gets farther from its sun, the dry ice clouds that will shroud it will block out a great deal of the remaining sunlight.

Of course, if a planet has a large axial tilt, its higher latitudes will be completely in darkness for months at a time. In such an environment, it would obviously be a lot harder for green plants to survive. They could, of course, leave behind seeds or tubers capable of surviving a months-long night -- but such a long night would make it harder for green plants to evolve in the first place.

It is only the oxygen dumped into Earth's air by photosynthetic plants that allows animals to breathe -- for the first few billion years there were no animals.

Even if green plants were able to evolve on a darker world, it would probably take much longer for them to raise the level of oxygen in the air to the point that animals could appear, leaving less time for those animals to evolve intelligence before their planet's sun finally consumed all of its hydrogen and helium and expanded into a life-destroying red giant.

It may be that most of the intelligent races that do exist in the universe, having evolved on such darker planets, have big bug eyes or reflective eyes like cats. If plants do not evolve, life on such worlds would be forever limited to anaerobic bacteria. It would be ironic if such an unexpected problem does turn out to considerably limit the number of worlds in the universe where complex life exists.

Even so, Kasting and Williams concluded that dry ice clouds wouldn't form in large amounts on Earth unless it was more than 210 million km from the Sun -- so the light problem, while it certainly must be taken into account, does not by itself prove that planets capable of producing complex (or intelligent) life are rare.

But there is still another problem to contend with: the fact -- which we have discovered only since 1995, as we have actually started to detect planets in orbit around other stars -- that our nice orderly Solar System may be the exception rather than the rule, and that giant planets like Jupiter may barge in far closer to their suns in most solar systems -- close enough to disastrously disrupt the orbits of habitable inner planets. In the third part of my series, I will examine this problem.

The Search For Life at SpaceDaily