The Heavens Declare His Glory (part 3)

It's in the Stars
(Part 3 in a 4 part series)

Now that we know where we need to be positioned in this universe, let's look at the kind of local habitat we would need to support life.

The first thing that life needs is a safe and stable home. It needs to be warm enough to facilitate chemical reactions, but not so hot that complex molecules break down. It needs a usable energy source, yet protection from damaging energy sources like cosmic and high-energy radiation. It needs a diverse source of key materials, but the ability to isolate from volatile materials. And it needs this environment to be stable for a long period of time (an enormous period of time if you insist on presupposing evolution).

Now, I don't wish to be accused of begging the question here. I'm not just implying human life or "life as we know it"; I'm talking about any kind of complex, sentient life that would be a candidate for (or result of) God's special attention. We do, after all, have a fairly good grasp of chemistry and the kind of general conditions that must prevail in order to support complex chemical bonds and reactions. And unless God wished to "ensoul" a molecular cloud (which would lead to interesting problems of where one "person" ends and another begins), then we must look to certain constraints.

The obvious solution to the habitat problem is a planet, which, of course, would be dependent upon a star for its energy supply. An alternative might be something like an interstellar dust cloud or nebula. But while these may be candidates for the formation of simple materials upon which life is based, they offer nothing like the kind of stability, protection, and concentration of resources that can be found on a planet.

So a solar system it is then, but first we need a proper star. Unfortunately, not all stars are created equal. We know by example that our own kind of star is a good host, but when astronomers call our sun "average," it is not that the majority of stars are like it; it is more due to the fact that our star falls right in the middle of some important stellar classifications. Indeed, our sun is anything but average. To begin with, 90% of all stars are smaller than our own. If anything could be said to be an average star it would be a red dwarf, the most common type. But for several reasons, these are bad candidates for a parent star.

These smaller stars output far less energy, so a habitable planet that hoped to maintain a reasonable average temperature (and liquid water) would have to hold a close orbit. Unfortunately, close-orbiting bodies have the effect of putting the least massive partner into gravitation (or "tidal") lock. This is where one side of the satellite body always faces what it is orbiting. For instance, the same side of our moon always faces us, and this is true for nearly every other moon in our solar system. Having a planet tidally locked to its sun would be devastating to the climate of both sides of the planet.

Being close to your sun also means that you are susceptible to greater exposure to its miscellaneous undesirable emissions, like solar winds, flares, and general radiation. Just because a star may give off less of the energy you need does not mean that every other emission is proportionately smaller. In fact, these red dwarf stars have been found to have a rather erratic and volatile nature. At worst, these common stars are unsuitable for life. At best, life would be downright nasty under their dominion.

Of course, not all stars are smaller than ours. Stars can be up to 150 times the mass of our sun. Unfortunately, larger stars are even worse candidates as a host star. I'll mention just three reasons why.

First, large stars tend toward the blue end of the spectrum, which means a greater abundance of ultraviolet radiation, among other things. This would not only have an adverse effect on life's biochemistry, but would cause toxic chemical reactions in the very atmosphere of a planet (like ionizing it).

Second, adding mass to a star does not proportionately increase its energy output, it exponentially increases it. This means that even if it were possible to maintain a hospitable planet around one of these infernos, it would have to be positioned in a distant orbit that would nullify the temperature-balancing effects of short seasons (assuming the planet is even tilted on its axis). More on this later.

Third, contrary to what you might expect, the larger the star the shorter its life-span. This is because the increased gravitational pressure causes the star's fusion engine to run more efficiently. (Of course, I am assuming that the reader knows that stars are nothing more than giant fusion reactors, which spend the bulk of their lifetimes fusing hydrogen into helium). In the same way as the energy output increases, and for the same reason, the lifespan exponentially decreases with mass. So a star only 1/4 again the size of our sun may have its lifespan reduced by half. And the largest stars number their years in the millions (or hundreds of thousands), rather than 10 billion like our own.

A shorter star life is not only bad news in the general sense that shorter is worse, but it is bad because a shorter lifespan serves to condense the lifecycle of the star in a way that causes a briefer possible sweet-spot. You see, during a star's main sequence burn phase it grows progressively hotter, brighter, and larger. Where a planet may once have rested comfortably within the habitability zone it could quickly find itself on the inside of it if its star were too large. Our own planet has been slowly "moving" from the outer edge of its zone to the inner edge. But because of the smaller size of our sun this progression has happened slowly enough to give us a long life-window and to permit complex planetary adaptations (e.g., the reduction of carbon dioxide so as not to trap the increasing level of solar energy).

It might be worth mentioning here that, for the reason I just discussed, these planetary habitability zones are not as broad as they may at first appear. In reality, any planet with the hope of long-term habitability must begin at a very narrow position on the outer portion of the zone so that it will stay within it as long as possible while the star progressively increases in luminosity.

But what about the stars with approximately the same mass as our sun? Surely there are plenty of these out there in such a big galaxy. Well, that may be, but there is more to making a star than mere mass. The amount and mix of heavy elements it contains — its "metallicity" — controls its burn rate, energy output, and variability. And its spin rate affects things like its magnetic activity, which drives its flare intensity and frequency.

When looking for twins of our own star (which SETI scientists seem to think very important) there are a lot of sensitive variables that we are only beginning to understand. In fact, even using the criteria that we can understand and measure we have found, to date, precisely zero solar twins, that is, unless you wish to loosen up the criteria for "twin" (which they intentionally do for solar "analogs"). The closest contender of the thousands of candidates reviewed is 46 light years away, but even this one has its problems. Here are just two quotes that point out some of the notable differences.

"HD146233 [also known as '18 Scorpii'] is the only star in the ELODIE library which merits the title of solar twin because it has passed the filter of all methods [of identification]. It is not however a perfect twin and differs from the Sun by its higher [Lithium] content [3 times as much], slightly higher age (6 Gyr against 4.6 Gyr for the Sun) and higher luminosity (Mv = 4.77 against Mv⊙ = 4.82)." (Source)

"18 Scorpii's activity cycle [it has actually been classified as a variable star] may be of greater amplitude than Sol's and that its overall chromospheric activity level is noticeably greater than Sol's. Hence, this otherwise excellent solar photometric twin therefore may be a less perfect spectroscopic twin." (Source)

At 46 light years away, and being a relatively small star, 18 Scorpii is too dim to see with the naked eye. Even assuming that this star were a solar twin, thus being a prime candidate for a habitable system, this means that of all the stars you can observe in the night sky not a one of them is much like our own. Even extending your vision with binoculars and low-powered telescopes, you will only include a couple of possible candidates (the runner up is 126 light years out).

There's just one other very important thing I'll mention by way of requirements for the host star itself: it must be a loner. If it has one or more companion stars, then any planet's hope of maintaining a long-term, regular orbit (i.e., constantly remaining in a habitability zone) is slim to none. This further narrows the field of available parent stars in that more than half of all solar systems are binary, or greater (the percentage being lower for red dwarves and higher for large stars).

So far we've seen that when considering the ideal home for life we not only must consider the galactic neighborhood, the kind of galaxy, and the position within the galaxy, but the very details of the star as well. All factors make a dramatic difference to the stability, longevity, and energy output. It is no mere bias that leads us to consider our own Sun as the benchmark of habitability; it is the perfect star on theoretical grounds, by comparison, and by experience. Either ours is the only possible kind of star able to support complex life like our own, or God simply delights in exceeding requirements.