Habitat for Humanity
(Part 4 in a 4 part series)
Now that we know the ideal kind of star to support complex life — a star very much, or perhaps exactly, like our own — let us turn to the planetary requirements.
As I discussed in part 3, a habitable planet must be in a narrow zone of distance from its star. If it is to have any hope of maintaining a temperate climate without runaway heating, like Venus, or runaway cooling, like Mars, then it must begin with its proximity to the sun. Unfortunately, there are things that stand in the way of even a chance at this. Looking at our own solar system you would think that it is natural for rocky, earth-like planets to lie in the warmer, inner region of star systems and to have the gas giants lie in the outer region. But the lesson learned from our extra-solar planetary observation is that this arrangement may, in fact, be the exception to the rule. It is true that at this point we are having difficulty detecting earth-sized planets, but of the over 200 large planets thus far detected, the position we find them in seems to be either in a tight orbit around their star or in a wider elliptical orbit.
So, why is this a problem? Well, first of all, because of the crushing gravitational pressure, the internal heat, and the toxic soup of which their atmospheres tend to consist these gas giant planets are not prime candidates for life. So what you need most of all is for them to be out of the way of the habitable zone. When these gravitational nuisances are either too close, or in a lopsided elliptical orbit, they prohibit other planets from achieving stable, circular orbits. Our own solar system is an amazing arrangement of planets in a delicate balance of near circular orbits around its star. This is partially thanks to the fact that Jupiter and the other gas giants are in stable orbits well outside of the domain of the inner planets.
The other problem relates to the issue of why we have found so many gas giants so close to their suns. You see, planetary origin theory has the larger planets forming far away from the star. This is both for reasons of gravitational competition and because the lighter gases of which these planets primarily consist are more prone to drift outward on the solar winds than the heavy elements that formed the rocky, inner planets. Astrophysicists are not now questioning their planetary models as much as they are attempting to explain how these planets managed to migrate to their present positions. And a migration of a giant like these (or any sized planet, really) means that anything between it and the sun either gets swallowed by or ejected from the system. In any event, our own solar system is no longer said to be "typical." Indeed, the present line of inquiry is determining why our own solar system has so many gas giants so far away from its star.
On having the galactic neighborhood, the right star, and the right position around that star we only begin the task of describing a suitable planet. There are so very many factors which play in to making a friendly and sustainable habitat that I'm afraid I must stick to the highlights here. Of course, the obvious ones are a suitable atmosphere and an abundant supply of H2O, but what is not always pointed out is that the exact amount and mix of these things makes a profound difference to the environment. For instance, having too much CO2 or other "greenhouse" gasses can lead to overheating, as happened with Venus, but not having enough will lead to a snowball planet.
But even having the right mix and quantity of atmospheric gasses is not enough. It must be maintained over long periods of time in conjunction with the energy output of the star. Many things may affect atmospheric composition — volcanic outgassing, biological life, solar wind and radiation — and as I mentioned previously, even stars that have a steady burn will increase in size and intensity over their lifetimes. Losing the delicate war of atmospheric balance even once can send the planet into a temperature tailspin from which it may never recover.
Beyond the obvious life-sustaining properties of water (SETI and NASA certainly seem to think it critical), it adds value in numerous other less apparent ways. For one, it acts as a lubricant for tectonic activity, which is essential for recycling materials, like carbon, that would otherwise become locked up in the crust. For another, it acts as a temperature buffer, absorbing heat at certain times and places and redistributing it elsewhere. For this reason, even the shape and volume of the ocean is important in order to insure good weather and temperature balance.
Even assuming water was common to find in abundance on rocky planets, it is no guarantee that the water will long remain on the planet. You see, one effect of the sun's ultraviolet rays is that they tend to break apart water molecules into their constituent atoms — hydrogen and oxygen. Once hydrogen (the lightest element) is loose in the environment, it moves toward the upper atmosphere where it can be whisked away by solar winds. Conversely, free oxygen has a tendency to bind with just about anything it can find (oxidation) and will not be recycled unless plant life happens to already exist. A long enough cycle of this kind will eventually exhaust the water on a planet.
This kind of H2O breakdown appears to be exactly what happened to Venus, accompanied by a build up of CO2. The reason that this has not also happened to Earth is because we have the benefit of a strong magnetic field, which shields us from the majority of the solar winds and ultraviolet rays. This is due to our unusually large iron core (it has to be big enough to have a solid central core and a metallic liquid outer shell) in conjunction with our high spin rate. Venus, our "twin" planet, suffers in both these areas.
In fact, it is something of a mystery why we have such a very large iron core. The answer to this question may be bound up in the reason why our moon has virtually no metallic core. The recently developed theory for the origin of our moon is that the Moon is the result of a collision between the Earth and a Mars-sized body. This theory has fast risen to dominance because it answers so many unsolved questions, like why we have less atmosphere than expected, why the Moon is composed of material similar to Earth's crust, and why the other theories have dead-ended. But most pointedly, it answers our question of why the Earth's metallic core is so large and the Moon has almost none: the colliding planet's core was transferred to the Earth upon impact. This "big crunch" theory implies that the long-term preservation of Earth's atmospheric and hydrological stability is dependent upon a "chance" collision with just the right kind of object (I won't even go in to the delicate requirements of speed, trajectory, and size of that colliding object).
But more than contributing to the formation of our magnetic field, having a resulting moon of the size and position we have further works toward making our planet hospitable for life. For one, its tidal forces serve to mix our oceans to maintain a good temperature and nutrient distribution. And these same forces keep the tectonic activities running more smoothly. Another very important thing that the Moon does is to keep the axial tilt of the Earth locked into place (in tandem with the tilted plane of the Moon's orbit). This is important; as a wandering axis will eventually lead to a devastating tilt that can produce atrocious weather at best and sterilizing temperature imbalances at worst. The absolute worst case would be a 90-degree tilt, which would alternatively bake and freeze the entire earth from one pole to the other over the course of each year. Even the equator would undergo a scorching summer and month-long sunset twice a year. Our own modest 23-degree tilt permits us the benefit of some temperature relief and distribution across the globe, but leaves the bitter weather isolated at the poles. Which is coincidentally where the ozone layer and magnetic fields are most vulnerable.
There are many more requirements for making a suitable world that I could mention, and I suppose I should, since the force of my case is somewhat dependent upon the number of points I pile upon the heap of improbability. But in the interest of space, let me just summarize a few of the most compelling ones.
Surface gravity (escape velocity) — If this were stronger then the atmosphere
would retain too much of certain toxic elements, like ammonia and methane. If it were weaker, the atmosphere
would lose too much water.
Rotation period — If this were slower, then the temperature differences between day and night would become more extreme, and the magnetic field
would be weaker. If it were faster, then weather
would become more extreme (e.g., higher winds, hurricanes, etc).
Thickness of crust — If this were thicker, then volcanic activity would be minimized thus reducing the amount of materials, like carbon
and sulfur, that are returned to the ecosystem. If thinner, then volcanic and tectonic activities would be more widespread and perhaps even toxic to life.
— This has to do with the amount of light that strikes the earth vs. that which is reflected back by clouds, water, land formations, and ice. If it were greater at any time, runaway glaciation would develop. If less, then a runaway greenhouse effect would occur.
Water vapor level in atmosphere — This is affected by such things as wind velocities, atmospheric density and mix, temperature, and ocean volume and distribution. If this reached too high a level, then a runaway greenhouse effect
would develop. If this were less, then the continents could be barren, and erosion (which releases nutrients and minerals into the ecosystem) would be minimal.
Atmospheric electric discharge rate — Lightning is actually beneficial
in that it produces ozone and breaks apart atmospheric nitrogen
(a strong N2 bond) so that it is available to form other molecules necessary to life. Therefore, less lightning is disadvantageous, but more would cause too many fires and deaths.
Mass and distance from the sun of Jupiter and other gas giants — If too large or close to us, then gravitational disturbances would result. If too small or far away, then there would be less protection
from incoming cometary and Kuiper Belt materials.
Atmospheric pressure — If too small, then liquid water would evaporate too easily and condense too infrequently. If too large, then liquid water would not evaporate easily enough to maintain a hydrological cycle, insufficient sunlight would reach the planetary surface, and insufficient UV radiation would reach the planetary surface for photosynthesis.
Radioactive material in core — Radioactive potassium, uranium, and thorium are the primary contributors to the continued internal heating
of Earth. If there were too little in the core, then we would not have the required molten iron to generate the protective magnetic field, and tectonic activity would not exist. If there were too much, then the surface temperature would be affected, and tectonic activity would be too severe.
Could life survive in a system where all these factors I mention are less than ideal? Some limited kind of life, conceivably, but it is noteworthy that our planet just happens to have all these things in its favor. And it has gratuitous items thrown in as well, like abundant mineral resources, a transparent atmosphere (so we can see out), and a large assortment of planetary neighbors for exploration.
The skeptic might reply that we just happen to be on one of those lucky planets positioned in just the right place. Even granted the odds that there could be such a very lucky place as this, it only affirms my original thesis that we are indeed in a very special place. The skeptic's "argument from insignificance" is a failure, and in proving that failure we reveal the very uniqueness of our home and, by association, life itself.