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Not All Habitable Zones Are Created Equal
by Bruce Moomaw
 Cameron Park - August 9, 1999 - In this series of reports, I'm examining the current tendency of some astronomers to believe that planets habitable to life -- such as Earth -- may be relative rarities.

In Part One, I examined the question of the "habitable zone" -- the range of distances around a star in which a planet has a climate that allows the existence of liquid water on its surface -- and noted that the evidence accumulated over the past few years is that the zone is comfortably wide after all.

But orbiting in the Habitable Zone is by no means a guarantee by itself that a planet can allow the development of life - and in particular complex life - on its surface.

One other serious possible problem is the planet's "obliquity" -- which is simply another name for the tilt of its spin axis. Right now, most of the Solar System's planets have nicely moderate axial tilts. Three of them -- Mercury, Venus and Jupiter -- have tilts of only a few degrees; they are almost perfectly side-on toward the Sun and have no seasons.

Earth, Mars, Saturn and Neptune all have reasonable tilts of 23-29 degrees, so that they have moderate seasons as one pole or the other tilts modestly toward the Sun and receives somewhat more sunlight. Only two planets -- Uranus and tiny Pluto -- "lie on their sides", so that during part of their orbits around the Sun one of their poles or the other faces almost directly toward the Sun.

Keep in mind that a rotating planet is like a gyroscope -- it continues to tilt in the same direction as it revolves around the Sun during one of its orbits. But this situation may not always have been the case.

Smashing Planets
It is now completely accepted that, during the formation of the Solar System, there was a long period during which large "proto planets" or "planetesimals" several hundred to several thousand kilometers across were sailing all over the Solar System and crashing into each other and into the gradually forming planets.

It seems very likely that both our own Moon and possibly Pluto's moon Charon were created when such objects struck Earth and Pluto with a major blow, splashing a plume of debris into orbit around each planet that later coalesced into a moon. There is a good chance that Uranus' strange tilt was also caused when a protoplanet that may have been ten times as massive as Earth crashed into one of its poles, tipping the planet on its side.

Furthermore, recent research in the past few years indicates that the initial tilts of the four small inner metal rich planets may have had completely different tilts from their current angle of tilt.

Any planet that rotates has a very slight bulge around its equator because of centrifugal force; and if its axis is tilted, the tidal tuggings of the Sun at this bulge cause the planet's spin axis to very slowly "precess" -- that is, its spin axis slowly slews around like a wobbling top.

Earth precesses every 26,000 years -- a process which means, for instance, that thousands of years ago the Pole Star was Vega rather than Polaris. And, mostly as a result of the gravitational tuggings of the giant planets Jupiter and Saturn, the tilt of the inner planets' actual orbits' around the Sun also wobble slightly, in a complex set of rhythms.

These shifts in the tilt of the planets' orbits around the Sun are very tiny -- but for over 25 years, it has been recognized that they can interact with a planet's precession wobble to produce unexpectedly dramatic results.

A Resonance of Wobbles
What happens is that, if a planet's precessive wobble is slow enough, it can get into rhythm with some of the periodic rocking motions of its orbit around the Sun -- and it turns out that, when this happens, the Sun's rhythmic tidal tuggings at the planet's bulge can slowly cause its spin axis to tilt over more and more extremely, and then tilt slowly back again to lesser levels.

This is a so-called "resonance" effect, like pushing rhythmically at a child on a swing to increase his swing to greater and greater levels. Such amplifying resonance effects are common in the Solar System.

Mars' precessive period is 157,000 years long, and periodically gets into rhythm with the slow rhythmic tilts of its solar orbit. Since 1973, it's been known that this causes its axial tilt to slowly rock back and forth between 15 and 35 degrees over a period of several hundred thousand years.

But in 1993, astronomer Jacques Laskar carried out more detailed computer analyses which showed that the effect was more extreme than had been thought, and that -- over cycles of several tens of millions of years -- Mars' axial tilt swings back and forth between 0 and 60 degrees!

It is now accepted that, much of the time, Mars joins Uranus and Pluto as the third planet in the Solar System that lies on its side. It's only by chance that its tilt right now is at a reasonable 25 degrees.

Because Mercury and Venus are closer to the Sun, long ago the Sun's larger tidal tuggings greatly slowed their rotation down until their days became months long, and then pulled their equatorial bulges "into line" with the Sun so that they have almost no axial tilt -- but before that happened, their spin axes rocked back and forth even more dramatically. In fact, Venus may have turned completely topsy-turvy at one point, which could explain why today it rotates slowly backwards.

Earth Survives
Why didn't this also happen to Earth? For one reason: the existence of our Moon. The Moon's orbit is modestly tilted relative to Earth's equator, so that its own tidal tuggings at Earth's equatorial bulge greatly increase the speed with which Earth's spin axis precesses.

Thanks to the Moon, Earth precesses every 26,000 years, much more rapidly than any of the rhythms in which its orbit wobbles -- and so it doesn't undergo any of the rhythmic resonance effects that would drastically change the amount of its spin-axis tilt.

Its tilt does change back and forth over a 2.5-degree arc every 41,000 years -- and even that slight change is probably enough to cause the Ice Ages.

Laskar calculated that, without the existence of the Moon, Earth's obliquity would slew even more wildly than Mars' -- its axial tilt would slew back and forth between 0 degrees and 85 degrees every few tens of millions of years!

It is now thought that there was about a one-in-three chance that Earth, during the Solar System's formation, would undergo a collision that would form a large moon.

But if that Moon-creating collision had happened at another time, when Earth's axis was keeled over more extremely, then the Moon's tuggings would have permanently stabilized the Earth at that more extreme tilt. So there is only about a one-in-twelve chance that Earth would permanently have a tilt as mild as it actually does.

Axial Heat
What would have happened if Earth had an extreme spin-axis tilt relative to the Sun? Very strange -- and very bad -- things. Twice during each orbit, it would have been side-on to the Sun just as it now is -- but at other points during each year, either the North or South Poles would have pointed straight toward the Sun.

It's been known for a long time that this would do utterly grotesque things to its climate. In 1997, climatologist James Kasting carried out detailed computer analyses and discovered just how bad it could get.

If Earth were tilted 85 degrees today, each of its hemispheres would be permanently shrouded in night for six months at a time -- but the other pole would undergo a six-month-long day, during most of which the Sun would be blazing down on it from as high an angle as it blazes down on our own tropics for the few hours around noon each day.

The natural result would be that the temperature at that pole would climb to very high levels. The temperature at the North Pole might climb as high as 50 deg C (over 120 deg Fahrenheit). And because the South Pole is located in the middle of Antarctica, away from the temperature-moderating effects of the ocean, its temperature could climb as high as 80 deg C (176 deg F).

If Earth's continents were all lumped together into a single big continent (as may very well be the case on many other Earth-type worlds), and that continent were centered around one of the poles, its temperatures inland would peak at even more savage levels -- possibly approaching the boiling point of water. And the only known living things that can endure temperatures over about 60 deg C are some kinds of bacteria.

The strangest thing of all would be the fate of Earth's equatorial region. Instead of being the steaming tropics that we know, it would be below freezing -- and covered in ice -- all year round.

For two points during each year, when Earth was "side-on" to the Sun, it would receive as much heat from the Sun each day as our own Equator does -- but during most of each year, the Sun would be shining down on it from much shallower angles, as the Sun shines down on our own high-latitude and polar regions.

In fact, even during the six-month-long periods when one of Earth's poles was shrouded in permanent night, that pole wouldn't get quite as cold as the equator would always be, because it would still be cooling down from the huge amount of solar heat it soaked up during its long broiling day -- and it might not ever get below freezing.

As a matter of plain geometry, if a planet is keeled over more than 54 degrees, the total amount of sunlight energy that a point at its north or south pole soaks up over a year is actually greater than the total amount of sunlight that a point on its equator soaks up over the same year. Instead of having polar ice caps, Earth would have a permanent equatorial "ice belt"!

At moderate latitudes, of course, Earth's temperature extremes would be much less. Nova Scotia would oscillate from freezing to about 13 deg C -- 55 deg F -- each year.

And, of course, if Earth's axial tilt was less than 85 degrees, the extremes would be less - and Kasting did ignore the effects of clouds, which might somewhat reduce the hot temperatures. But it is clear that, if Earth had an axial tilt even modestly greater than the one it has today, it would be a much nastier place.

Kasting calculated that if Earth had a tilt of only 35 degrees but had a single huge continent centered around one pole, daytime temperatures at that pole would still hit up to 91 deg C -- 196 deg F!

In such an environment, life could almost certainly still appear, but it would have much more difficulty evolving into forms that could survive such grotesque temperature extremes -- which would greatly slow down its evolution into more complex forms, maybe by billions of years.

This interference would happen to a large extent even if Earth had no Moon so that its axial tilt swung back and forth from a destructive extreme to a more moderate level over cycles of tens of millions of years. And, as I said earlier, such crazily tilted worlds are probably more common in the universe than modestly tilted worlds like our own.

Habitable Zones
At this point, however, the cavalry comes riding in, just as it did during our earlier discussion of the width of Habitable Zones -- and, oddly, it's the same cavalry.

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 and More Real Estate

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