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.[1]  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.[2]  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.[3]

 

     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.[4]

 

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.[5] 

 

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.[6]  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.[7]  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.[8] 

     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).[9]  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,[10] 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,[11] 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.[12]  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.[13]  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.[14] 

     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.[15] 

     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.[16]  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.[17]  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.[18]  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.

Read more about the evidence for intelligent life before humans.

The creationist narrative in Genesis 1 is contradicted by many ancient Christian texts.  Instead of an Almighty Creator God, ancient Christian texts espouse that the universe is born from blind arrogance and stupidity.  The angels caused evolution to occur from species to species.  There are many gods, (or aliens?), and the Christian God is just one among them.  Satan the Devil writes scripture, and thus the Bible was polluted with Genesis 1.  Archaeology and modern scholarship demonstrate that Genesis is indeed corrupted.  Cavemen walk with Adam and Eve.  Esoteric prophecies reveal the coming of Christ, and also reveal the dark forces that govern the cosmos.  Such are the ancient Christian writings.

Science vindicates the truth of these ideas.  Evolution often happens too fast for Darwin’s theory.  Gaps in the fossil record indicate that some kind of unnatural force acts together with natural selection.  Astrobiology reveals that intelligent life probably evolved long before us.  The fossil record reveals strange clues that aliens abducted species and transported them across oceans, and that DNA from diverse lineages was combined to spawn hybrid species.  Evidently, aliens influence evolution, and they are the gods of the world’s religions. 

This is not fiction.  All these facts are thoroughly documented in the links above.

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