Chapter 23 We Are Not the First | The Aliens Beat Us to the Punch | The Building Blocks of Life | How Soon Did Life Evolve in the Universe? | How Soon Did Intelligent Life Evovle? | Look to the Trees | Answering Questions on Earth's Uniqueness | The Sun | Planets | The Quest for Liquid Water | Tilt | The Moon | Density and Rotation | Gas Giant Meteor Shield | Life without Gods or Aliens

The Aliens Beat Us to the Punch

There are at least 200 billion stars in our Galaxy.  Is our sun the only one to spawn intelligent life?  Who cares – that's the wrong question.  Here is a better question:  There were at least 200 billion stars in our Galaxy before our sun even existed.  Did one of them spawn intelligent life a long time before us?  And if so, should we not expect such an ancient intelligent species to be much more advanced than we are?  Earth is 4.6 billion years old, but the universe is 13.7 billion years old.  That's a difference of 9 billion years during which intelligent life could have been evolving before earth even existed. 

The Building Blocks of Life

To exist, life needs certain elements.  Among them are hydrogen, carbon, nitrogen, oxygen, phosphorous, and sulfur.  Many complex life forms also require heavier elements such as iron, zinc, copper, manganese, magnesium, calcium, potassium, and sodium. 

The first of these, hydrogen, is easy to come by.  Accounting for nearly three quarters of all atomic matter in the universe, hydrogen is by far the most common element.  Moreover, hydrogen came into existence very soon after the Big Bang.  Chances are good that early life did not suffer from a lack of hydrogen.

All the other elements necessary for life came on the scene somewhat later.  They were the product of nuclear reactions deep inside large stars, which dispersed their payload of life-giving elements across the vast reaches of space by means of massive stellar explosions called supernovae.  These supernovae debuted about 400 million years after the Big Bang.  They were formed from colossal hydrogen gas clouds that had self-compacted under the force of gravity.  The gravity was so intense that it compressed hydrogen atoms together, making new atoms that were larger and heavier than hydrogen.  This is called nuclear fusion.  As Carl Sagan explained it,

"Hydrogen fuses into helium, helium into carbon, carbon into oxygen… all the way to iron." 1 

In extremely dense hot stars, silicon burns to create iron, and in lesser quantities, the elements of intermediate weight between silicon and iron.  Among these are the life-giving elements phosphorus, sulfur, potassium, calcium, and manganese. 2

After only 10 million years or so, the massive stars blew apart in supernovae, scattering their diverse treasures of elements far into space, where they were subsequently incorporated into future solar systems, such as ours, which are rich in these heavier life-giving elements.  In supernovae explosions, large numbers of neutrons are released from the interior of the star.  They use the energy from the blast to fuse with iron atoms, thus forming elements heavier than iron.  Among these are nickel, copper, zinc, silver, tin, iodine, platinum, gold, mercury, lead, and finally uranium.  At uranium, the elements become unstable and radioactive, the nuclear fusion process stops, and radioactive decay begins. 3 4  When this radioactive uranium is subsequently incorporated into planetary bodies, it, together with gravity, causes volcanic and geothermal activity, which is essential to life, because volcanic activity is necessary for the creation of an atmosphere. 5 

Hence, the massive hydrogen supernovae stars of deep antiquity were the first vital step toward the evolution of life, because they seeded the universe with the elements necessary for life.

Stars similar to the earliest stars still exist today, because there are still so many clouds of hydrogen collapsing into balls of burning gas.   Two stars in particular, HE1327-2326 and HE0107-5240, have been extensively analyzed under telescope.  These two stars have created a large amount of carbon, nitrogen, and oxygen. 6 7  Hence, those stars in the first and second generation with a mass twenty times larger than our sun are empirically demonstrated to have seeded the universe with carbon, nitrogen, and oxygen. 8  Sodium, magnesium, and aluminum may also be made inside these stars, 9 together with a large quantity of phosphorus. 10 

How Soon Did Life Evolve in the Universe?

The first supernovae stars began to exist about 400 million years after the Big Bang.  From birth to death, these stars only lasted about 10 million years.  The universe is 13.7 billion years old.  If you could fit 13.7 billion years into a 24 hour day, then 10 million years would be just one minute.  410 million years would be 43 minutes.  So in astronomical terms, if the lifetime of the universe is compared to a single day, all the elements necessary for life were spawned within the first hour of that day. 

Moreover, heavy organic elements tend to sink toward the center of the Galaxy, so we should expect life to emerge first toward the central core of the Galaxy – perhaps not directly in the central core, because of the frequency of collisions, but rather just outside the central core.  Yet earth is more than half-way to the outer rim, which means we are comparatively lacking in organic elements.  Since the earliest possible date for life is dependent upon the calculation of the earliest date that heavy organic elements were available in sufficient quantities, it is very likely that regions more toward the center of the Galaxy produced life at an earlier date than our earth did.  This fact significantly reduces earth's chances of being the first planet to produce intelligent life in our Galaxy.

The pertinent question is this:  At what time in the history of the universe did the life-giving elements become common enough for life to first evolve?  Measuring the occurrence of iron is a good benchmark by which to answer the question.  The iron content of stars is comparatively easy to establish because it exists throughout the universe in sufficient quantities to measure.  Also, iron is one of the heaviest, and therefore one of the last of the organic elements to fuse.  Where iron is present, the other life-giving elements are also likely present. 

Iron rich stars emerged early in the history of the universe.  A sample of 462 single F and G class stars (that is – stars similar to our sun) studied by Nordstrom suggest that iron rich stars appear to have existed very early in the history of the universe, and have remained more or less consistently present in the universe at a surprisingly stable distribution.  The first iron rich stars appeared at approximately 13 billion years ago, and a substantial number apparently existed about 11 billion years ago.  Nordstrom remarks,

"There is clearly no significant rise of overall iron abundance ([Fe/H]) with time." 11 

The rapid creation of iron, which occurred during the youth of the universe, may be a function of the fact that the early supernovae had relatively short life spans compared to our sun, many just 10 million years or less.  Consequently, the first few generations of stars came in rapid succession, seeding the universe with large amounts of iron and other life-giving elements even while the universe was still quite young.  Even the proponents of the Rare Earth Hypothesis admit that the universe was seeded with a sufficient quantity of life-giving elements just 2 billion years after the Big Bang. 12

Therefore, stars capable of producing life appear to have been just about as plentiful 11 billion years ago as they are today. Insofar as our sun is about 5 billion years old, and life first evolved no later than 1.5 billion years afterward, we may reasonably postulate that simple life forms first evolved 1.5 billion years after iron-rich stars became plentiful.  In other words, simple life forms similar to bacteria probably first evolved about 9.5 billion years ago, at the latest. 

How Soon Did Intelligent Life Evolve?

But who cares about bacteria?  We want to know about intelligent life.  In earth's history, it has taken 3.5 billion years for us to evolve from bacteria into humans.  If evolution on other planets happens similarly, we may postulate that the first intelligent species in the universe evolved roughly 6 billion years ago (3.5 subtracted from 9.5 billion years ago).  Hence, the most intelligent species in the universe is 6 billion years ahead of us in its evolutionary improvements.  No wonder they seem like gods to us.

Actually, the evolution of intelligent species probably took even less time on other planets.  This is because the early earth was comparatively too hot to handle, a fact which may have significantly retarded the evolution of higher life forms.  Earth's geothermal activity, that is volcanoes and earthquakes, is still going strong even after 4.6 billion years.  Geothermal activity is caused by two things – gravity and radioactivity – which in turn are caused by high levels of dense materials such as iron and even denser radioactive materials such as uranium.  On lighter planetary bodies, like the Moon, geothermal activity died out a long time ago, because they have less gravity and because their uranium became depleted.  But earth is still pumping out steam and lava because it is heavier and contains more radioactive material deep within its core.  As planetary geologist Ellen Stofan says,

"The larger a planet is, the more heat will be generated in the interior, and thus the more active the surface will be." 13

Geothermal activity also pumps out carbon dioxide, which is a greenhouse gas, and thus causes global warming.  If geothermal activity causes too much global warming, then water is too hot and life dies.  Such was the case in the earth's early oceans.  Earth's oceans first appeared 4 billion years ago, but were extremely warm, almost to the point of boiling.  Although simple prokaryotic cells like bacteria could survive under near-boiling conditions, more complex eukaryotic cells could not, because the best of them can only withstand temperatures up to 60 degrees Celsius, or 140 degrees Fahrenheit. 14  Eukaryotic cells absolutely must be the building blocks of all complex and intelligent life, because prokaryotic cells do not have the organelles necessary to sustain an organism beyond just a single cell.  Prokaryotic cells seem to have evolved very quickly after the first oceans, the first evidence of them standing at 3.85 or 3.5 billion years ago.  In contrast, eukaryotic cells emerged much later.  This is not a function of the time it takes for eukaryotes to evolve, but rather resulted from the fact that early earth was simply too hot for eukaryotic cells to even exist at all, regardless of their evolutionary potential.  On a less radioactive planet, eukaryotic life may have evolved much more quickly.  If this is true, then the first intelligent species is even older than 6 billion years.

Also, early earth had too much iron on its surface.  There was so much iron that it soaked up great quantities of oxygen for well over a billion years after life first appeared on earth.  This is recorded as bands of gray and red in archaic rocks.  Animal life cannot exist without free oxygen in the air.  Therefore, surface iron severely delayed the emergence of even the most primitive worms and creepy-crawlies.  In another world, where surface iron was less plentiful in earth's early years, intelligent beings might have evolved much more quickly. 

For these reasons, the earth is too iron-rich and too radioactive to make a likely candidate for the first intelligent life.  This possibly speaks for the universe at large being past its prime.  Supernovae stars have perhaps over-seeded the universe with heavy elements, such that it can no longer produce life with the same degree of efficiency as it used to.  Thus the universe is facing its midlife crisis.  In its younger days, the universe may have spawned intelligent species much more efficiently than it does now.

In another event which may have significantly retarded the development of life on earth, we may note that 700 million years ago, the earth was covered in ice.  This situation persisted for millions upon millions of years.  If other planets did not suffer from this, it follows that they developed intelligent life many millions of years before us.      

Look to the Trees

250 million years ago, mammal-like reptiles dominated the earth.  Some of them even climbed trees, and if it weren't for the pointless evolutionary diversion of the dinosaurs who temporarily replaced them, the tree dwellers may have even developed opposable thumbs, which is a prerequisite to having the ability to use tools.  With tools, a species can become technological, and thereby become an intelligent advanced civilization like humans.  Animals that live on the ground need all four feet to run fast, so they have little chance of developing hands that can grip tools.  Animals that live in the water need fins or flippers, so the same is true for them.  Animals of the air exchange hands for wings.  So when we look for intelligent life, we should look to the trees, for it is in the trees that hands evolve.

Although dolphins and whales are intelligent, they will never be technological, because they lack the ability to grip and use tools.  You can't write Moby Dick with a flipper.  You need hands, and hands evolve from swinging in trees, and tree-swingers cannot coexist in a world filled with very tall carnivorous dinosaurs whose heads are above the treetops.  The long-term dominance of giant carnivores like Tyrannosaurus and Allosaurus may have significantly retarded the development of technological species on this planet.  Such dinosaurs dominated earth for well over 100 million years, until finally the mammals made a comeback, thanks to the luck of a fallen star slamming into the Yucatan.  If the dinosaurs never existed, then intelligent life may have evolved on earth over 100 million years earlier.

If the evolution of interstellar intelligent species is a race between planets, and if earth was distracted in the middle of the race by so many complications, how is it possible for us to have won the race?  If earth was running around in a reptile zoo instead of running the race track, while other planets were sprinting toward the finish line, then how can we possibly believe that we are the first intelligent species to evolve? 

It is not difficult to imagine a tree-covered planet filled with monkeys where dinosaur-sized predators never existed.  Such a planet would have more gravity than earth, which would get rid of the dinosaurs and carnivorous birds and pterosaurs, since large terrestrial animals and flying creatures take a long time to evolve mechanisms to cope with gravity.  But the extra gravity would not deter trees, for the physical properties of water cohesion pull water up the trunks of trees with a force of 130 megapascals, which is several times the cohesion strength needed to overcome gravity, even for tall sequoia redwoods. 15 

But what of the extra geothermal pressures resulting from more intense gravity?  If the planet were high in iron and silicon, but low in uranium, geothermal activity would increase in proportion to iron and silicon, but decrease in proportion to uranium, and thus a favorable level of geothermal activity would be maintained despite the higher level of gravity.  As an added benefit, such a planet would also enjoy more constant levels of geothermal activity over time, instead of being skewed in favor of the early years of the planet – a fact which would make such a planet cooler than earth in its early life and thus likely to evolve eukaryotes at an earlier date than earth.  Hence, the ideal planet for intelligent life, I think, would be heavier than earth, but with less uranium.  Such a planet could probably produce intelligent life much more quickly than earth did.

Answering Theories on Earth's Uniqueness

Some argue that earth is unique in so many ways that life is unlikely to exist elsewhere.  Among the points that make earth supposedly unique are our sun, our distance from the sun, a nearly circular orbit, liquid water, density, rotation, volcanism, the moon, and the presence of a gas giant to act as a shield from meteors.  Each is answered in turn below.

The Sun

A conservative estimate for the number of stars in the Galaxy is 200 billion.  Of these, 5.6% are estimated to be G-type stars like our sun. 16  Stars larger than the G-type generally have too much ultraviolet light, and they burn through their fuel too quickly for life to evolve.  Smaller stars, such as brown dwarfs, don't have enough gravity to produce energy by nuclear fusion, and so they don't give much heat.  By the time a planet gets close enough to receive adequate heat from a brown dwarf, it is believed that its proximity to the star will place it in tidal lock, which means that the same side of the planet faces the star at all times; the "dark side" of the planet gets very cold and freezes the whole atmosphere, including all water as it evaporates and moves across the dark side by the wind, where it permanently freezes; or, if the planet is large, then wind speeds will constantly be of ultra-hurricane strength, in an effort to redistribute heat to the dark side.  Other stars include pulsars, which kill everything nearby; neutron stars, which are dead x-supernovae; red giants; and white dwarfs – all of which are entirely unsuitable for intelligent life.  Hence the probabilities for intelligent life should be factored by the number of G-type stars. 

Yet as a side note, the red giants and white dwarfs were at one time stars similar to our sun, and thus give testimony that potentially life-giving stars like our sun have existed even in extremely ancient times.  Our own sun will become a red giant and then a white dwarf within about 5 billion years.  Hence, it is quite possible that a few of the red giants and white dwarfs we see in our telescopes today had at some time in the distant past given rise to intelligent life before us.

Two-thirds of stars in our neighborhood are in systems with multiple stars, and this number is expected to rise in areas with a higher density of stars, such as clusters and the Galactic center.  Insofar as systems with multiple stars are likely to produce radical effects on orbit, and therefore on climate, these might not normally be capable of producing complex life. 17  Also, the possibility of being struck with excessive radiation from pulsars, supernovae, neutron stars, and gamma rays, is greater in clusters and in the Galactic center.  Hence, the odds that any given star would produce intelligent life may be confined to something like 0.5-2%.  Still, this is 1 to 4 billion stars in our Galaxy. 

Some believe that there exists a "Galactic Habitable Zone" or "GHZ" outside which life-giving stars cannot exist.  This theoretical zone excludes the Galactic center, star clusters, and the presumably metal-poor areas of the outer rim.  However, the idea is controversial, and scientists cast doubt on it.  Prantzos states,

"We conclude that, at the present state of our knowledge, the GHZ (Galactic Habitable Zone) may extend to the entire MW (Milky Way) disk… Even if 100% lethality is assumed for all land animals after a nearby SN (Supernova) explosion, marine life will certainly survive to a large extent, since UV is absorbed from a couple of meters of water.  In the case of Earth, it took just a few hundred million years for marine life to spread on the land and evolve to dinosaurs and, ultimately, to humans; this is less than 4% of the lifetime of a G-type star.  Even if land life on a planet is destroyed from a nearby SN explosion, it may well reappear again after a few 108 (100 million) yrs or so… the probability for surviving SN explosions, which is null in the inner disk at early times, becomes quite substantial in late times." 18

If it were common for fledgling life forms in the universe to be wiped out by such radiation from deep space, then shouldn't we see at least a few extinction events in the fossil record that have no explanation save radiation?  As it is, all major extinction events known to science in the fossil record are clearly tied to other events besides interstellar radiation.  The Permian was tied to geothermal activity, the Ordovician and Pleistocene to ice ages, the terminal Cretaceous and Frasnian-Famennian to extraterrestrial impacts, the Miocene to climate change, and the Ediacaran to higher life forms.  If gamma rays and supernova bursts have completely wiped out other planets, they should have at least partially wiped out ours, but such is apparently not the case.  Therefore, the danger from interstellar radiation is probably next to nothing.

Planets

270 planets have been found outside our solar system, most of them around stars like our sun.  Most of these planets are giants like Jupiter and Saturn, because they are the easiest, and until only very recently, the only planets that could be detected.  About 7% of stars are believed to have such giants.  Based on the observation of "super-Earth" planets, 33% of stars like our sun are believed to have planets between the size of Earth and Neptune orbiting close to the star.  Udry states,

"It is most probable that there are many other planets present:  Not only super-Earth and Neptune-like planets with longer periods, but also Earth-like planets that we cannot detect yet." 19

Unfortunately, as of this writing, planets the size and distance of earth cannot be detected.  Planets are detected by measuring their gravitational impact on their star, which necessarily means that more massive planets that are closer to their star are easier to detect.  The realization that so many stars have very large planets orbiting their stars at a distance only a fraction of earth's distance to the sun is disconcerting, because it means that these planets probably formed far away from their stars as gas giants, and later lost their distance – a phenomenon that would most likely strip a solar system of any planets in the habitable zone, for as the orbit of the gas giant deteriorates, it brings the smaller inner planets closer to the sun with it. 

However, this might be a problem only for very heavy solar systems.  It is demonstrated that stars with a greater metal content than our sun are the same which harbor "hot Jupiters" and "super-earths."  This makes sense because more metal means more gravity, which in turn causes planets to loose their orbit.  In contrast, stars with a lower metal content are believed to still have enough metal in their proto-planetary disks to form earth-like planets, even though they might not be able to produce hot Jupiters, and thus, earth-like planets should, according to current data, be rather common. 20 

The Quest for Liquid Water

Liquid water is necessary for life to exist.  Thankfully, liquid water is very common in the universe.  It exists on comets, Jupiter's moons, and probably even once existed on Mars.  Water in ice form exists on Uranus and Neptune.  Outside our solar system, liquid water might exist just 41 light years away, on a planet of a star that is already known to have five planets orbiting around it.  According to Marcy, the star 55 Cancri has a mysterious gap between its fourth and fifth planets, in which it is believed there are smaller planetary bodies that could be much like earth.  Telescopes and gravity measurements are not strong enough yet to see earth-sized planets.  What they can see is a gas giant beyond it, which likely serves like Jupiter, blocking meteors from the smaller life-giving planets. 21  In another case, a red dwarf star only 20 light years away was found to have two planets believed to be near the habitable zone, Gliese 581c and 581d.  Upon studying them, it was found that 581c is too close to the star and 581d is in tidal lock with the star.  Hence, neither is very promising for complex life, although 581d may have microbial life. 22  Water in steam and solid form is also known to exist on a planet orbiting the star GJ 436, which is 30 light years away. 23 

Although none of these planetary discoveries really hits the mark, they do provide indisputable evidence that planets are common.  The fact that no truly earth-like planet has been found is merely a function of earth's small size and long distance to the sun.  In less than a decade, astronomers have gone from seeing "hot jupiters" close to their stars, to now seeing "super-earths" smaller than Neptune.  Technology is in the works to eventually see planetary systems in higher resolution, and thus find earth-like planets.

So how do planets get liquid water?  Answer:  from volcanoes.  Volcanoes bring carbon dioxide and hydrogen to the surface of planets.  The chemical reaction of carbon dioxide (CO2) with hydrogen (H) leads to the production of steamy water vapor (H2O), and methane (CH4). 24  As the steam rises, it cools, then turns to water and falls as rain.  Sometimes planets acquire additional water vapor and methane from their moons. 

Water must be in liquid form for life to exist – not steam or ice.  If a planetary body is too hot, all its water will be steam.  If too cold, it will all be ice.  We are 93 million miles from the sun.  Some people assume that if we were a little further we would freeze like Mars, and that if we were a little closer we would be scorched like Venus.  But this is not correct.  Believe it or not, Venus, Mars, and the Moon are all close enough to the sun to sustain life.  What killed them was not proximity to the sun, but rather an imbalance of carbon dioxide.  In Venus' case, a collision was the likely culprit.  In Mars' and the moon's case, lack of size was responsible.

Venus has too much carbon dioxide because its slow rotation cycle caused excessive vulcanization.  Its slow rotation was perhaps caused by a collision with another object.  Hence, our solar system is actually unlucky, for if we had not suffered the untimely death of our twin, Venus, we would have two life-giving planets in our solar system. 

The problem with Mars is too little carbon dioxide.  Mars cannot retain heat without it, and without heat, all its water freezes and life cannot exist.  Planets get carbon dioxide from volcanoes, which pump it out with their lava.  As stated above, volcanoes are a form of geothermal activity which is driven by gravity and radioactivity.  Mars is deficient because its small size and lack of density translate into low gravity, and therefore fewer volcanoes.  Although Mars does show signs of being currently volcanically active, 25 it lacks the density and the mass needed to produce and retain enough carbon dioxide to compensate for its distance from the sun.

The moon was quite volcanically active about 3 billion years ago, 26 but with the depletion of its uranium, it has become even more hopeless than Mars.  Small bodies, especially moons, often loose what little carbon dioxide they have because their gravity is not strong enough to retain it.

Volcanoes are to planets what blood is to humans.  They are the circulatory system, transporting heavy elements and molecules through arteries of liquid rock to the surface.  Without volcanoes, the surface would not receive the elements necessary for life.  Luckily, volcanoes are quite common.  Recent volcanic activity is affirmed on Venus and on Mars – and also on several of the moons of Jupiter and Neptune, including Io, Triton, and Europa.  Europa appears to be especially active. 27  If volcanoes are as universal as numerous witnesses in our solar system testify, then the lifeblood of planets is also universal, and thus life must also be universal.

Carbon dioxide is to planets what clothing is to humans.  If things get too cold, you can put on more clothing.  Conversely, if things get steamy, you can take off your clothes.  Here's how it works:  Carbon dioxide is pumped into the atmosphere by volcanoes, animals, and anything that burns as fuel.  But it is taken out of the atmosphere by the rocks and the ocean.  Rocks are made of silicon, which, when eroded by weather, combine with carbon dioxide to produce limestone.  When temperatures are warm, the cycle of evaporation and rainfall becomes more intense, which causes more erosion, which in turn breaks down more silicon rocks, so that carbon dioxide can combine with it.  When this happens, carbon dioxide is taken out of the atmosphere, and temperatures fall.  Falling temperatures cause less rain, which causes less erosion, and so the earth is self-stabilizing like a thermostat. 28  The ocean and the atmosphere also play a balancing game.  If the ocean has more carbon dioxide relative to the atmosphere, it yields carbon dioxide back into the atmosphere.  Conversely, if the atmosphere gets too much carbon dioxide, the ocean absorbs it. 29 

Of course, if, in a single century, we burn all the fossil fuels that have ever been produced, then atmospheric carbon dioxide might rise faster than natural processes can suck it up, which could lead to severe environmental consequences in the short term.  But in the long term, the earth will heal itself, as it always has, despite numerous cataclysms which have befallen it over the aeons.  Even though carbon dioxide might cause a short term global warming catastrophe, in the long run, it is our eternal friend. Mother Earth is a tough old bitch.  Don't underestimate her resilience.  For example, she was completely covered in ice 700 million years ago, but the volcanoes just kept belching out more carbon dioxide until she warmed up.  Because everything was ice, there was no rain, and therefore no erosion, and therefore no rocks were broken up to absorb the carbon dioxide.  So the carbon dioxide just kept building up until earth got warm again. 

The realization that earth has a carbon thermostat gives us more hope for finding life on other planets.  It expands the distance a planet can be from its star and still have a suitable temperature.  Just as the thermostat on your wall allows you, a tropical ape, to build a house in Alaska and survive; so too, nature's thermostat might allow extraterrestrial intelligence to abound in places we might not expect it.

Hence, a planet's ability to sustain liquid water, and ultimately its life-giving potential is not as dependent upon the distance to its star as one might think.  Depending on atmospheric content regulators, carbon dioxide can bring an otherwise frigid planet within a suitable temperature range, and keep it there, thanks to the thermostat.  A planet more distant from its star than earth may still yield life if it has more greenhouse gases.  One planet's pollution is another planet's lifeblood. 

Since the frequency of carbon dioxide in the universe is high, thanks to the ubiquitous presence of volcanoes, expectations for finding life elsewhere in the universe should also be high.

Tilt

Earth spins on an axis that runs from the North Pole to the South Pole.  This axis is tilted 23 degrees relative to the sun.  Axial tilt is "the reason for the season," as they say at Christmas.  About Christmastime, the most intense sunbeams hit earth at the Tropic of Capricorn, which runs across northern Australia.  On June 21, they hit the Tropic of Cancer, just south of Florida.  What would happen if instead of 23 degrees, the axial tilt were 45 degrees?  At Christmastime, the South Pole would theoretically be as warm as the equator, but Christmas in Los Angeles would feel like Siberia!  The more the tilt, the more extreme the winters.  This is a problem, because it constricts life to the tropics, and increases the likelihood that the planet will slip into a downward spiral of glaciation, whereby heat is reflected by the white snow back into space, the planet cools, and becomes one big snowball.  What would happen if instead of 23 degrees there was no tilt?  We need only to look at Mercury, which spins at zero degrees relative to the sun.  Mercury's equator is seething hot, but its poles are frozen.  Without tilt, the poles never receive direct sunlight, and so they freeze.  Worse, they freeze all water vapor that the wind blows across them, and this increases the chances of atmospheric freeze out, similar to the planets in tidal lock believed to orbit brown dwarfs.  In time, the oceans would all evaporate only to fall as snow on the poles, never to melt.  Too little tilt, and irreversible ice ages result.  The same is true of too much tilt. 

Therefore, a planet with life should have moderate tilt.  Only then can the growth of polar ice caps be checked.  Earth's 23 degree tilt is a deciding factor, because it distributes warmth across the planet evenly, thus reducing the chances of atmospheric and oceanic freeze out. 

The next question is, how common is a favorable tilt?  Do many planets in the universe have this tilt, or are we lucky?  Looking at our own solar system gives us a good idea.  Of nine planets, four are within an acceptable range.  Mars spins at 25 degrees, Saturn at 27 degrees, Neptune at 28 degrees, and Earth at 23 degrees.  Of course, Mars, Saturn, and Neptune cannot have intelligent life for other reasons, but the point here is that tilt is not the cause of their lifelessness.  Thus, a tilt that is favorable to life is normal, not unique.  All the other planets can be explained as abnormal.  Mercury is gravitationally tied to the sun, so its tilt is zero.  Jupiter is nearly zero also, probably because it is so large that nothing was big enough to knock it off kilter as it was being formed.  Uranus spins at 98 degrees, which may be caused by interactions with its larger neighbors.  Venus and Pluto are said to spin "backwards," at 177 and 122 degrees respectively, probably because in their early years they got hit with other planets so hard that they got knocked upside down; in Pluto's case, it has a large moon, Charon, to account for the damage.  Creationists love to point to Venus and Pluto and say, "Why are they spinning backwards?  If the solar system formed as a spinning disk of gas and debris, everything should be spinning the same direction.  Nothing should be spinning backwards, unless God did it."  This is nonsense.  They are not spinning "backwards."  They only appear to be, because they got knocked upside down by large planetary bodies as they were forming from that cloud of spinning gas and debris.

The Moon

Earth has a large moon for its size.  The large size of the moon keeps the winds low, stabilizes our tilt, and enriches the ocean with nutrients because of the tides.  This is all good for complex life. 30 

There is no particular reason to believe that the moon is unique.  While it is true that the moon is large relative to the earth's size, we must ask, relative to what?  The moons of the gas giants?  Granted, the moons of the gas giants, such as those of Jupiter and Saturn, are proportionately smaller compared to their planets.  But why is that?  Is it because we got lucky with a large moon?  Or is it because the gas giants are made of gas? 

While the gas giants were forming, the planetismals that formed in their proto-planetary disk smashed into each other, thereby expelling their lighter-than-air helium and hydrogen, such that the larger of the colliding objects inherited most all their gas.  Hence, huge planets formed, hogging most of the gas.  The leftovers of the planet-forming process were small, gas-deprived, rock moons.           

In contrast, the planetismals from the proto-planetary disks of inner planets formed from more solid material, and so when their planetismals collided, some of the smaller planetismals survived as large moons, since their structure was more solid and less gas, and therefore less likely to be stolen by the gravity of the larger planet.  Hence, it is not surprising that our moon is so large.

Even in our own solar system, our moon is not alone.  Pluto's moon, Charon, is even more disproportionately large compared to its planet.  Also Venus has a slow rotation possibly because it collided with a moon it once had.  It is possible that Mars might have grown larger, and have larger moons, if the mass of giant Jupiter had not sucked away a great deal of loose material from it, and from the asteroid belt.  In fact, we might even have had three earth-like planets in this solar system if it weren't for Jupiter sucking up their material – Earth, Mars, and the failed planet represented by the asteroid belt. 31  A large moon might or might not be essential, but in any case, there is no particular reason to suppose it is rare.  At a minimum, Pluto and earth have one.  At a maximum, all the terrestrial planets except Mercury could have had one.  Shouldn't we consider the odds elsewhere to be likewise rather favorable?

Density and Rotation

Earth has a density of 5.5 grams per cubic centimeter.  Hardly unique, this density is common for inner planets.  Venus' density is almost the same at 5.2 g/cm3 and Mercury's density is even closer at 5.4 g/cm3. 32  Therefore, as a prerequisite for life, density requirements are most likely frequently met throughout the universe. 

High densities such as these indicate an iron core, which together with the speed of rotation give the planet a magnetic field.  The magnetic field gives it the ability to fend off the devastating effects of certain kinds of radiation. 33  Earth is not unique in rotation speed.  Although lacking in other respects, Mercury also meets this prerequisite.  Venus might have done so too, if not for the collision earlier in its history.  Mars spins fast enough but doesn't have enough iron. 

So, of the planets in the inner ring of our solar system, three out of four have a favorable rotation speed, three out of four have a favorable density, and two out of four, Mercury and earth, have both.  In a Galaxy with 200 billion stars, two out of four is damn good odds.

Gas Giant Meteor Shield

It is sometimes argued that a gas giant such as Jupiter is a prerequisite for life because it absorbs meteors and comets that would otherwise strike us.  Driving this argument is the assumption that extraterrestrial collisions are detrimental to evolutionary progress.  Is this assumption correct?  Earth has suffered many collisions throughout its history, but only one has been so devastating that it significantly impacted evolutionary progress – and this impact was favorable – namely, the collision that killed the dinosaurs.  The dinosaur extinction stands apart from other extinctions because of its abruptness.  Other major extinctions, such as the Permian-Triassic extinction, which was actually even more devastating, happened over longer periods of time, and although comparatively fast in geological terms, were by no means immediate, and thus could not have been caused by a collision.  Truly devastating collisions are extremely infrequent. 

Moreover, it only took 10 million years for life to substantially re-diversify after the dinosaur extinction, and this turned out to be a good thing for intelligent life, because humans would not exist otherwise.  Hence, it seems that meteor and comet strikes do not annihilate all life from a planet, but rather just make room for different life, which in terms of evolutionary progress, is probably a favorable event, not unfavorable.  Meteor impacts are like hitting the reset button – you don't want to do it too often, but every once in a while it is necessary to hit it when evolutionary progress freezes.  Such was the case with the dinosaurs, because they had made life in the trees impossible for anything with opposable thumbs. 

Gas giants are likely to exist in other solar systems, because the same laws of physics apply to other solar systems as to our own.  Heavy material sinks toward a source of gravity, and that is why stars close to the sun like Mercury, Venus, and earth contain a lot of heavy material like iron and silicon.  Lighter material stays afloat, which is why the outer planets are comprised of hydrogen and helium and other light elements.  The outer planets are gas "giants" because their gas is not compacted into a small space like the solid rocks of earth, but is swirling around in large clouds.  Since these planets result from normal physical processes which we should expect to see elsewhere, we should not consider the presence of a gas giant such as Jupiter to be unique.  Rather, we should expect to see billions of similar gas giants in solar systems throughout the universe.  Indeed, as mentioned above, large planets are already observed in other solar systems.

Life without Gods or Aliens

The discerning soul may question my logic, asking if perverted space alien gods caused evolution on earth, then who or what caused the evolution of the alien gods?  First, I do not suppose that evolution is dependent upon divine or extraterrestrial intervention.  To be sure, the alien gods have impacted the course of evolution on earth, but this is not the same as saying evolution is impossible without them.  They have not systematically engineered evolutionary advancements.  They have merely interfered because of their lustful curiosities and bizarre fetishes, and perhaps occasionally because of a desire to experiment with some kind of new form. 

On the first planet to host intelligent life, there were no gods to interfere, because the gods had not evolved yet.  This could mean that there was no Cambrian Explosion on such a planet, or that new forms appeared more gradually than they did on earth.  However, this does not mean that no evolution took place.  Darwin's theory of natural selection is a proven fact, and although it cannot explain the sudden origins of some forms in earth's fossil record, this does not negate the force of Darwin's theory when applying it to life on earlier planets, nor, for that matter, on earth. 

Before the Cambrian Explosion, highly complex eukaryotic cells had already evolved on earth, and there is no particular evidence to support that space alien gods had anything to do with it.  Also, certain sponges and Pre-Cambrian species of a quasi-complex nature predate the Cambrian Explosion.  Therefore, if undisturbed evolution left in its natural state mirrors that of the Pre-Cambrian, and if such life progressed on a planet without divine or extraterrestrial interference, then there is no particular reason to believe that such a planet would have been incapable of producing intelligent life.  We might say that there would have been less diversity among life forms, for there was no extraterrestrial tampering, and therefore we may suppose there were only a couple of animal phyla on the planet, not 20 phyla as were produced during the Cambrian Explosion on earth.  Nonetheless, a lack of diversity is not the same as lack of intelligence, and there is no reason to suppose that given enough time, another planet could not have developed intelligent life despite lacking the diversity of forms found on earth.