Life on Earth and Elsewhere

Nonfiction - by J.M. Tanenbaum

Until 1988, we had no reliable information about the existence of any planets orbiting a star other than our own. A few nearby stars appeared to exhibit a slight wobble that might have been caused by orbiting planets, but the data was imbedded in the “noise floor” of the observations. Nevertheless, astronomers made the “reasonable assumption” that since our Sun was similar to many other stars, it was likely that some of them also had planets. Astronomers could see disks of dust and gas orbiting around young stars, from which planets might form. 

Now, orbiting telescopes and vastly-improved ground instruments have given us a flood of new data about the existence of over 5,700 new planets orbiting more than 4,100 stars, more than 22% of them having multiple planets. There are also approximately 10,000 more “likely candidates” whose observations have not been fully analyzed yet. We have even managed to directly image a few exoplanets, a feat long thought impossible. (To be fair, they’re just small blurry spots of light – for now.) And also captured surface features on large stars: blotches that indicate their convections cells are much larger in proportion to those of our own Sun.

Based on the most recent observations, it appears that almost every star in our galaxy has at least one planet orbiting it.

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Exoplanet studies to date, however, have not found any planetary systems comparable to our own solar system. This fact, in and of itself, is not surprising, as most of the current detection methods are biased in favor of large and/or closely-orbiting planets. However, the vast preponderance of massive gas giants discovered very close to their stars is disturbing, not only because massive planets in close orbits can increase the intensity and frequency of stellar flares and CMEs (Coronal Mass Ejections), with nasty effects on any habitable planets farther away, but also, in the absence of any other examples, we had thought that our own solar system was fairly typical, with its gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune) located in its outer reaches. 

The newest theory to account for the close-in giant exoplanets still has them forming in the outer reaches of their stellar nebula, but then migrating inward as collisions with the nebular gas, dust, and larger objects rob them of angular momentum. Or by some other, still undiscovered mechanism. But as these gas/ice giants pass any inner, potentially-habitable rocky planets, they may disrupt their orbits, making them highly elliptical or ejecting these planets from the system altogether.

There is also a class of exoplanets called “super-earths”, about one-and-a-half to ten times as massive as Earth. We have found quite a number of them in other planetary systems, while our own solar system has none, leaving a planetary-mass gap between Earth and Uranus (about fifteen times Earth’s mass). Computer simulations have shown that the presence of a super-earth in our own system, even orbiting as far away as between Mars and Jupiter, could disturb the orbit of the Earth (and many of the other planets) and even cause them to spiral into the Sun or be ejected from the system altogether.

Also, the more massive a rocky planet is, the stronger its surface gravity will be, making it more difficult for animals to move and plants to support a large surface area of solar-energy-absorbing structures.

And there are other differences; more differences than similarities. Many of the exoplanets have highly elliptical orbits, or orbits that are not in the plane of their star’s equator, or with those of other planets in the system. A considerable number of the orbits are retrograde (orbiting in the opposite direction to that the star rotates). These “oddball” orbits may be unstable over astronomical timescales.

At present, there are no acceptable explanations for all these differences.

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The Laws of Statistics state that a sample picked at random will most likely represent the average value in a Gaussian (bell-curve) distribution. Since astronomers of a few decades ago thought that there was no reason to believe we were “special,” they made what seemed to be another “reasonable assumption”: that our system was a typical example of the average stellar planetary system.

They theorized that, when the solar dust and gas nebula surrounding our sun condensed and accreted to create planets, the higher temperatures near the center, combined with the stronger stellar “wind” from the young star, had already driven away the lighter, more-volatile elements, so rocky planets formed near the sun and gas/ice giants and other icy bodies formed farther out. And all of our planets have near-circular orbits in the plane of the Sun’s equator. (Pluto doesn’t, but now we know it isn’t a planet, just one of many similar bodies with similar odd orbits.)

The spacing of the planets’ orbits varies widely, but seems to approximately follow a simple mathematical relation, called Bode’s Law, given by: semi-major axis, a = (2n x 3) + 4. But so far, most of the stellar systems with many rocky planets have most of the planets of about the same size, and much different orbital spacing.

We developed these theories to fit the facts of our own system, and it seemed to make sense. But now they have turned out not to describe any of the new planetary systems we have found so far.

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In addition to conventional planets, it has also been suggested that a gas/ice giant orbiting at an appropriate distance might have Earth-sized rocky satellites with liquid water and an atmosphere conducive to the development of life. The moon Pandora, as shown in James Cameron’s Avatar movies, is a good example. While this is certainly possible, there are several complicating factors.

Gas/ice giant planets with their large metallic-hydrogen (chemical symbol “H”) cores have intense magnetic fields that could funnel trapped radiation onto a satellite. If the satellite is not tidally-locked to the planet, it may experience tremendous oceanic tides. If the planet is tidally-locked, there will be only minimal solar tides. If there are other large satellites, gravitational interactions could produce considerable tidal heating, with resultant severe tectonic activity (volcanos and moonquakes) like Jupiter’s moon Io.

Finally, the massive planet’s strong gravity may continually attract large numbers of cometary and asteroidal impactors, and some of them may regularly bombard the satellite.

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Perhaps the Earth and our solar system, with their many unique features, are not as common as we thought. All our gas/ice giants, including huge Jupiter, still orbit in the outer reaches of our system. This enabled Jupiter (and to a lesser extent, Saturn, Uranus, and Neptune) to disrupt the icy bodies in the distant Kuiper Belt and the much more distant Oort Cloud early on, and bring a very large amount of water (H2O) into the inner system, specifically to Earth. Then, having depleted the easily-perturbed objects, it protected early life here from being “bombed back to the stone age” (actually a lot further back than that) by the remaining comets every few million years or so. Even today, Jupiter continues to deflect most of the occasional intruders. (Some astronomers claim it actually attracts more impactors.)

Another recently-discovered discrepancy: in our solar system, there is an inverse relationship between the mass of the four gas/ice giants and the proportion of heavier elements such as carbon (C) and oxygen (O) in their atmospheres. In the few giant exoplanets where the composition of their atmospheres has been determined, most of them have either more or less heavier elements than would be proportional to their mass, based on the ratios measured for our planets.

The Earth also has an abundance of phosphorus (P), more than is seen in most parts of the universe. This element is one of many that are created in supernova explosions, and subtle differences in the star’s original composition have large effect on the proportions of the various heavier elements they produce in their death throes. For Terrestrial life, phosphorus is a necessity – if this is true of alien life as well, the rarity and uneven distribution of phosphorus adds to our planet’s uniqueness (more on this later).

Earth’s huge oceans of extra-terrestrial water served many functions conducive to the development of life, but the primary one was the absorption of the massive amounts of carbon-dioxide (CO2) released into the atmosphere by primordial volcanos. This produced a solution of weak carbonic acid that in turn reacted with silicate minerals in the seabed to produce carbonate minerals (Urey Reaction), effectively sequestering the carbon-dioxide and preventing a runaway “greenhouse effect” such as seems to have happened on Venus with its 90-times-thicker carbon-dioxide atmosphere and surface temperatures hot enough to melt lead (above 850°F).

Planetary “plate tectonics,” too, works to the advantage of life. “Subduction Zones” in the Earth’s mantle (the thick layer of molten rock between the core and the crust) bury large amounts of carbon-dioxide-containing surface minerals and sequester them for hundreds of millions of years, further helping to prevent a runaway greenhouse effect. And Earth’s circulating mantle is extremely active because of the additional heat from the leftover energy of the early moon-forming planetesimal collision (described below) and the decay of the additional radioactive elements it provided. Also, some of the mantle material was permanently ejected in the impact, leaving a thinner mantle that takes less energy to move.

This conveyor-belt action also helps to bring up certain elements from the depths that may not have been present on the surface after the molten Earth solidified and denser elements had sunk toward the center. Having a wide selection of elements readily available increases the chance of some of the randomly-assembled compounds being necessary precursors for the creation of the first “living” organism.    

Additionally, the occasional breaking up and combining of continents isolated various groups of plants and animals and then introduced new ones, driving genetic diversity as life continued to evolve.

However, the Earth does have a natural greenhouse effect – our atmospheric water-vapor and carbon-dioxide together increase the surface temperature about 104°F over an airless planet at the same distance from the Sun. (Water vapor produces about 66-85% of the effect; carbon-dioxide about 9-26%.) Cloud cover (water-vapor that has condensed into tiny droplets of liquid water or frozen ice crystals) exerts a regulating (thermostatic) effect – as the temperature rises, more liquid water evaporates from the surface to form more clouds, which in turn reflect away more sunlight. Then the Earth cools a little, and less clouds form, allowing the temperature to rise again.

But this thermoregulation works only within a narrow, critical range. Too hot, and all the liquid water gradually evaporates, never to condense into clouds or form oceans again. Too cold, and more and more of it freezes into white ice, increasingly reflecting the Sun’s light away, until the Earth is a solidly-frozen wasteland.

Our atmosphere also blocks a considerable amount of incoming dangerous solar and cosmic ionizing radiation. Additionally, its high oxygen concentration, about 21%, allows for the formation of a high-altitude ozone (O3) layer, which shields our lifeforms from severely-damaging levels of the Sun’s ultraviolet light. Oxygen gas in the atmosphere is in diatomic form (O2), and the balance is mostly nitrogen gas (N2), about 78%, and the inert gas, argon (Ar) 1%.

Next, we have a large, close-in moon, which stabilizes Earth’s polar axis, and keeps our planet from turning upside-down occasionally, as some toy tops do. (Some scientists suggest that an Earth-sized rocky planet may remain stable without a large moon, but this is not proven.) The Moon also produces regular oceanic tides that sweep sea water into and out of tide pools in a regular pattern, having both daily and (lunar) monthly components. This periodic stirring and mixing of early precursor molecules certainly helped in the random combining of chemicals until a favorable combination occurred that was capable of self-replication. Later, it repeatedly provided nutrients to the primitive single-celled organisms living in small, genetically-isolated populations.

The moon also acts as a shield to intercept a small, but not-insignificant, proportion of the incoming impactors.

Current Moon-origin theories favor a chance encounter of a Mars-sized planetesimal with the newly-accreted proto-Earth, striking a low-speed glancing blow that resulted in the formation of our satellite. Because of the oblique impact, mostly material from the Earth’s and the interloper’s outer layers (crust and mantle) was blasted away to orbit the Earth, where it formed a ring that gradually coalesced to form our Moon. Lunar samples returned by the Apollo astronauts and robotic Russian and Chinese sample-return missions, and observations by various countries’ satellites orbiting the Moon, have confirmed this composition. At the time of its formation, Earth’s new satellite may have been only about 15,000 miles away, and circled the Earth much more rapidly.

The rest of the impactor’s mass, mostly its nickel-iron core, combined with the Earth's existing one, creating the large liquid metallic core that is responsible for our extremely strong planetary magnetic field – a shield that protects us from the bulk of solar and cosmic radiation (some researchers feel that the atmospheric shielding may be sufficient for this purpose), and even more importantly, blocks the solar wind from blowing our own atmosphere away, as happened to Mars with its much weaker magnetic field, leaving it with less than 1% of Earth’s atmospheric pressure. Recent discoveries of the fine structure of the Earth’s core revealed complex motions, periodically reversing rotation of at least one layer, and an unsuspected solid nickel-iron center, all of which may further strengthen our planetary magnetic field.

This early proto-planet collision is also likely responsible for the tilt of the Earth’s rotational axis, and thus the seasonal temperature changes – another evolutionary driving force. In addition, it may have speeded up the early Earth’s daily rotation to over four times its current rate, helping to start the dynamo of electrical currents circulating in its enlarged core that generates our strong protective magnetic field. (This faster rotation gradually slowed to our present 24 hours, as the newly-formed Moon stole angular momentum from the Earth in the process of migrating farther and farther away, to its current average of 234,000 miles.)

The record of the Earth’s magnetic field recorded in ancient rocks shows that it weakened significantly about 500-million years ago, and then about 26 million years later, increased to the strong field we have today. This period of weakened magnetic field corresponds to the emergence of multicellular life and its rapid diversification into lifeforms that could occupy most of the available ecological niches. Some scientists believe that the increase of solar and cosmic radiation reaching the surface caused more mutations that hastened evolution, and also allowed more Solar wind to carry away more hydrogen from disassociated atmospheric water vapor. This in turn increased the concentration of oxygen in the air, which larger animals needed because then number of body cells increases as the cube of linear dimensions, while the surface area of lungs increases only as the square of it. This temporary decrease in the magnetic field may have been caused by the rearrangement of the components of the Earth’s core that was earlier disrupted by the proto-planet collision.

Finally, the heat of the impact may have driven off some of the Earth’s volatiles, like water and atmospheric gases, which otherwise could have increased the greenhouse effect to raise the temperature beyond what life could tolerate. 

This sort of heavenly billiard shot probably does not occur all that often.

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Even other planets that don’t collide with the Earth can still influence its orbit, causing it to move out of the GZ. Venus, out nearest neighbor on the sunward side, with about the same mass, if it had certain orbital resonances, could, over hundreds of mega-years, have dragged the Earh closer to the Sun. Likewise, the more-than-300-times more massive Jupiter, although much farther away, could have pulled the Earth father away from the Sun. But together, their effects largely canceled out, leaving our planet more-or-less where it formed over the intervening billions of years. (They do slightly increase and decrease the Earth’s orbit’s eccentricity in a 400,000-year cycle, but not to the extent of endangering all life on Earth.)

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Next, the Earth’s orbit happens to be located in the Sun’s “Goldilocks Zone” (“GZ”, also called the “Habitable Zone” or “HZ”), not too hot or too cold, where water can exist simultaneously in all three forms: solid (ice), liquid (water), and gas (water-vapor). Liquid water is called a “universal solvent” because its polar molecular structure (having positively- and negatively-charged regions) dissolves so many substances. Washing all these different chemicals into the oceans improved the chances of a fortuitous combination of molecules occurring by random encounters, and in a reasonable amount of time (by astronomical standards).

To date, most of the exoplanets found orbiting in a GZ are around tiny red dwarf stars, but for such a dim star, the GZ is so close to the star that the planets have become tidally locked to it, rotating once on their axis per revolution abound the star, thus always keeping the same side toward the star (as our Moon does with the Earth). The resultant “hot-side/cold-side” planet will have only a narrow zone where the temperature is between 32°F (freezing) and 212°F (boiling), but this area will be continually swept by supersonic winds from the interchange of broiling hot atmosphere from the hot side with the freezing air from the cold side. Also, the dim red light lacks sufficient energy to trigger photosynthesis, at least by any chemical pathway we know of. Without this mechanism to create an oxygen atmosphere, there would be no source of planet-wide, readily-available energy for animal lifeforms like ours to utilize. 

At the other end of the stellar scale, large, very hot stars have a GZ that is very distant, so planets can have any rotation period, but then there is another serious problem: these giant stars consume their nuclear fuel at a furious rate, and burn out so quickly (less than a billion years) that life may not have a chance to get started. Also, their luminosity changes considerably over their lifetime, introducing drastic climate changes on any planet orbiting them.

Stars are classified according to their temperature, from hottest (and usually biggest) to coolest (and usually smallest), using a letter designation: O, B, A, F, G, K, M, S, and each letter category is further subdivided into 10 finer divisions: 0-9. “Goldilocks” stars (ones that are not too big and not too small, as well as not too hot and not too cold) have stellar classes that range from about F0 to K5. Stars like our Sun (classified as G3, near the middle of the range) make up only about 3% of the stellar population of our galaxy.

Our Sun is also sedate and well-behaved. A recent Max Plank Institute study of 369 other similar stars, with the same age, composition, and temperature, found them to be about five times more active than our Sun, with increased starspot activity and thus more flares and CMEs – not so conducive to a stable environment for planetary life.

The Sun is also an uncommon loner; the majority of stars have one or more stellar companions. Up to half the stars may be binary pairs. In order for planets to form and/or have a stable orbit around one star in a multiple star system, the other star(s) must stay at least 5 times farther away from the first star as the planet’s orbit. For many such systems, the stars orbit each other so closely that any planetary “safe orbital zones” are too far inward of the goldilocks zone, and any planets there would be hellish infernos. If the planet is to stably orbit both stars, now it must be so far away from both of them that it will be outside of their GZ.

And it is not only the planet that has to be in a GZ; its star must be, too. The stellar nebula out of which the planets formed must have been enriched with heavier elements from a nearby supernova, also in a GZ. Too close, and the tremendous explosion would disrupt the nebula completely; too far, and there wouldn’t be enough heavy-element atoms captured to allow the formation of rocky planets and satellites, because the nebula would contain only its primal hydrogen and helium.

A galaxy also has a GZ. Too close to the galaxy’s center, and the stellar density is so high that the neighboring stars will occasionally disrupt the planets’ orbits, or nearby stars may give off gamma-ray bursts or go nova or even supernova and fatally irradiate the planets of other stars, within about 150 light-years or so. (Prompt gamma radiation lasting days to weeks, secondary X-radiation from that gamma striking nearby nebular gas and dust lasting months to decade, and particulate radiation, protons, neutrons, or electrons, lasting years to centuries.) Also, polar beams of radiation from the central black hole’s accretion disk will blast any planets they strike and disrupt any living organisms’ metabolism and genetics. Too far from the galactic core, and the concentration of heavy elements in newborn stellar nebulas will not be high enough for “rocky” planets with iron cores to form.

In addition, a galactic GZ star also has to have a circular orbit around the galactic center, otherwise it will periodically overtake or be overtaken by the concentration of stars in our spiral galaxy’s arms, with the resultant excessive stellar density problems just mentioned

And spiral galaxies are rare – most galaxies in our part of the cosmos are ellipticals; not spirals with arms. Elliptical galaxies and globular clusters tend to be too low in heavy elements throughout, and are poorer candidates for rocky planet formation. They also have their stars more closely spaced. Current research indicates that ellipticals result from collisions of spirals, and the resulting stellar interactions can disrupt the orbits of any planets that have previously formed. There are many fewer spirals that have avoided such collisions in the time they existed, and our galaxy has managed to avoid colliding with another galaxy since…forever.

Our galaxy is an outlier in yet another way, with an uncommon, much smaller supermassive black hole at its center. Supermassive black holes emit unbelievably powerful beams of energy from their accretion disks, which can sterilize planets or even blow away their atmosphere from many light-years distance. Earth’s Permian-Triassic mass extinction about 250-million years ago may have been caused by such an event from a much closer ordinary black hole.

The result of all these incredibly rare conditions is that we now inhabit an orbitally-stable, thermostatically-controlled world, with plenty of water (about 70% of the planetary surface). We are protected from most devastating planetary impacts by gas and ice giants in the outer solar system, and from radiation by both an extensive atmosphere and an intense planetary magnetic shield, which also conserves our atmosphere from the Solar wind.  And our singleton G3 Sun is one of the most-stable types, with its GZ at a middle of a spiral galaxy, the GZ of GZs, and located in the galactic GZ as well, with a GZ galactic orbit.

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After the Earth formed about 4.5 billion years ago, the evolution of life on Earth did not proceed smoothly. Although single-celled organisms appeared in the oceans almost as soon as the initial high temperature of the planet’s formation dropped low enough for them to exist (about half a billion years later), it took about another 2.4 billion years more for multi-cellular lifeforms to evolve. During this long time, Earthly life was vulnerable to all sorts of natural catastrophes. 

There were periodic mass extinctions that eliminated the majority of species existing at the time: 440 million years ago (mya), there was an 86% extinction; 375 mya, 75%; 250 mya, 96%; 200 mya, 80%; and 66 mya, 76%. If any of those had been somewhat larger, the result might have been 100%.

On the other hand, from about 600 million years ago to 560 million years ago, the Earth’s magnetic field weakened significantly, allowing the Solar wind to remove many hydrogen atoms from disassociated water vapor molecules in the atmosphere, which increased its oxygen concentration. This time period corresponds to the “Cambrian Explosion”, a great increase in biodiversity that included the larger, air-breathing animals. (This temporary reduction in the magnetic field is believed to have resulted from the process of the Earth's inner core transitioning from liquid to solid.)

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In retrospect, it was wrong to assume we were in a typical planetary system “picked at random” – our solar system is home to intelligent (?) self-aware beings, a member of a very exclusive club. And our “clubhouse” has so many unusual architectural features, including its neighborhood, that are very rare in the cosmos. We’re here only because our system is anything but “typical,” and this allowed life to come into existence and evolve into our present form, despite many close calls.

It may be that “life as we know it” elsewhere in the cosmos is not anywhere as prevalent as we might think.

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If life as we know it does exist elsewhere, does it have to resemble Terrestrial life? The elements carbon, oxygen, and phosphorus (P) can form many different chemical compounds when combined with other elements – many more than their three next closest chemical candidates: silicon (Si), sulfur (S) and arsenic (As).

Remember, the “polar” nature of water molecules makes it an almost “universal” solvent. Most other compounds that are liquid at GZ temperatures are non-polar, and much less able to dissolve a multitude of substances. Thus, we appear to be limited to the same palette of elements as here on Earth to construct extra-terrestrial living beings.

If the alien lifeforms are made of the same ingredients, do they have to look like Earthly ones?  The only body shapes and anatomical/physiological systems that we know for certain to exist are the ones that are found here. But that includes the fossil organisms found in the Burgess Shale, which look nothing like those that are currently living. Did these other-worldly prehistoric templates simply have bad luck in the evolutionary lottery, or is there something in the 4-limbed body plan for larger lifeforms that is inherently superior? 

Still, “Form Follows Function,” so the Laws of Physics and Mechanical Design will apply to them, no matter how outrageous (to us) their appearance.

We can assume that evolution will take place in any ecosystem – organisms will adapt to fill any available ecological niche. Flying evolved independently among Terrestrial lifeforms at least four times: insects, prehistoric pterodactyls and their ilk, birds, and bats. And almost evolved in gliders: “flying squirrels” and “flying fish.” Also, some spiders can “fly” by squirting a fluffy mass of silk into the wind that acts like a kite to carry them away (inappropriately called “ballooning”). Cephalopods (octopus and squid) independently evolved eyes amazingly similar to human ones, except that they are not built “backwards,” unlike ours with their nerves and blood vessels in front of the image-forming retina.

In considering this question, it is important not to make the same mistake astronomers made in assuming or solar system was “typical,” and then creating theories to explain its formation. We believe that placing the major sense organs at the top of the body confers an evolutionary advantage because they can “see” farther. We believe that bipedal locomotion is the most efficient. We believe that the mammalian dual-sex reproductive system is the most advanced. But are these beliefs correct?

Two eyes provide binocular vision that can judge distance by convergence and parallax-shift between their images; not just by focus. Is this really better than monocular vision? Would three eyes be even better? Many spiders have eight eyes. And it is possible to judge distance with only one eye, using image focus length, though not over such a large range as with two eyes, and parallax-shift by moving the head as strutting pigeons do, or having a multi-layer retina with different focal lengths like those of jumping spiders’ eyes. Are eyes even necessary? Sonar, and electric field sensors might be more useful, especially to a species that lives in a foggy environment.

Our sense organs are in the head, and this requires our brain to be there too, because the electro-chemical system our nerves use has limited transmission speed. If aliens managed to evolve nerves composed of elemental copper (Cu) that used purely electrical signals, their brains could be located deep inside their bodies where they would be more protected.

Are four fingers and one opposable thumb the best combination for a hand? And only two hands? How often have you wished for an extra one? Are hands even the best manipulatory appendage? Octopi do amazing things with their tentacles.

Most Terrestrial mammals have four limbs of similar function: either four legs for support and locomotion, or four (five, if you include a prehensile tail) prehensile limbs for climbing (though the upper pair are also used for manipulating objects). Only humans have evolved to the point where the lower limbs are used exclusively for support and locomotion (though we can still pick up small objects with our toes), and the upper limbs are used exclusively for manipulating objects (though we have a limited ability to climb ropes or swing from overhead handholds). Perhaps there are 6-limbed aliens with a centaur-like body, or a completely different lifeform with hundreds or thousands of small cilia on the bottom of their body, or a single muscular foot like a snail. Even biological wheels are “possible.”

Most complex advanced Earthly lifeforms are bilaterally symmetric, though some show left- or right-handedness. An extreme exception is the American lobster, with one claw much larger than the other. The large claw is used to crack the shells of its prey; the small one used to restrain it and then shred and extract its contents. Aliens might be more (or completely) asymmetric – like the “Moties” in The Mote in God’s Eye, a science-fiction novel by Larry Niven and Jerry Pournelle, who have a single large muscular left arm and two smaller, much more dexterous right arms.

We know for sure that humanoid beings are one possible form of intelligent life, so the aliens could look like us. Whether they do, we can only speculate.

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“Life as we don’t know it” is another matter entirely. Are there silicon-based insectile beings living on a very cold planet of Alpha Centauri A that drink liquid ammonia (NH3) instead of water, or eat sulfur instead of breathing oxygen? Or even have a substitution of the chemically-similar but more-common element arsenic for the rarer phosphorus. It will be a while before we find out. Interstellar travel (unless we suddenly discover a Star Trek “warp drive”) will take many decades, or even centuries, to develop, and far more to execute.

But we don’t have to wait that long – there are places in our own solar system that might be the abode of “alien” life. Mars, Jupiter’s large moon Europa, and Saturn’s moons Titan and Enceladus. What are their chances?

Let’s start with huge Titan, with its nitrogen atmosphere and hydrocarbon lakes. The laws of physics and chemistry are the same everywhere in the universe. (If there are multiverses, all bets are off…but we can’t ever get there.) Thus, Titan has many shortcomings as an abode for unearthly life. Chemical reactions, which any kind of life requires, proceed at a speed determined by temperature. Titan is not, and has not been for a long time, very warm. Therefore, life would take far longer to evolve, and might today have a metabolism so slow that we could not detect it.

Life also requires an energy source. Photons, whether light or more energetic radiation, are one source. But the influx of solar energy at Titan is only about 1% of that at the Earth’s orbit. “Heat” is another possible energy source, but it requires a “sink,” a colder region to flow to, in order to be useable. Chemical energy is often used by living organisms, but some other form of energy is required beforehand, to create chemical compounds that have available energy. It is also necessary to prevent this energy from being dissipated by environmental factors before it can be utilized by any lifeforms. None of the conditions on Titan provide high-, or even medium-level usable energy sources.

Life also requires an assortment of chemical “building blocks” to grow and reproduce (assuming that these are some of the acceptable criteria of “life”), and Titan’s extremely low temperature (about -300°F) locks up many of these as frozen solids, including many gases. Any water that happens to be there will be “hard-as-rock” ice. Hydrocarbon “rain” on Titan must mechanically wear down these solid deposits, as it lacks liquid water’s polar “universal solvent” action to dissolve them. Without a means to bring all the necessary components together, the creation and evolution of life on Titan is extremely unlikely. (But not impossible, of course, just extremely unlikely.)

Unlike Titan, Jupiter’s moon Europa is warm, thanks to tidal flexing and heating produced by a gravitational tug-of-war between the huge planet and several of its other large moons. Under its icy surface, Europa may have a miles-deep warm-water ocean. So here we have warmth and water, which is a good start. And most of the tidal heating occurs in the “solid” body of the satellite, so the bottom of the ocean is warmer than the top. This produces convection currents that circulate material from the bottom to the top, and then back down again. Plus, with the temperature difference, the thermal energy is available for exploitation. Europa’s ocean won’t have any significant light from the distant Sun, but there’s lots of life at the bottom of Earth’s oceans in the pitch blackness of the thermal vents. And many Earthly lifeforms emit their own light by chemo-luminescence. All in all, a much more promising abode. The overlying ice will shield the ocean from much of the radiation surrounding Jupiter, but there will be enough of it that still gets through the ice to hasten evolution by increasing the mutation rate.

A thoroughly-sterilized submersible probe capable of penetrating Europa’s frozen surface and exploring this ocean should be high on our list of future space missions.

Two of Jupiter’s other three large moons, Ganymede and Callisto, may also have sub-surface seas. The fourth, innermost, large moon, Io, has molten lava under its surface, and is within the most intense portion of the planet’s radiation belt, so life is extremely unlikely to have evolved there.

Saturn’s smaller moon Enceladus (about 300 miles in diameter), with its water-vapor and complex hydrocarbon geysers near its south pole, is another satellite that, like Europa, may have liquid water under its surface. Not as promising as Europa, but if we do find life there, Enceladus would certainly be worth a look next. Neptune’s one big moon, Triton (note the “r” and “o” that differentiate its name from Titan’s), has geysers too, but they spout only nitrogen gas and dust, not water.

The planet Mars is much more Earthlike than either of the two moons mentioned above, but not Earthlike enough for “life as we know it,” except perhaps for a hardy bacterium or two far belowground. With only about 1% of Earth’s atmospheric pressure, and essentially no free oxygen, Mars’s carbon-dioxide “air” isn’t of much use to oxygen-breathing animals. Plants would be happy with the CO2, though there’s very little of it, but the thin, ozone-free atmosphere cannot block the Sun’s ultraviolet (UV) radiation, which is energetic enough to break many molecular bonds and “sunburn” the plants, and in fact, the UV may completely sterilize the surface, and create highly-reactive chemical compounds (e.g. perchlorates) that would be inimical to living organisms. (Unless they found a way to safely ingest them and utilize their chemical energy.) Mars’s thin air also cannot burn up most meteors before they reach the surface, allowing for a highly-undesirable kind of rain.

If there currently was any liquid water on the surface of Mars, it would quickly boil away in the near-vacuum. One possible exception is a highly-saline aqueous solution, which might be able to flow on the surface for a while before evaporating, but which would still not be suitable as an abode for life, unless there are permanent pools of it underground. But most of Mars is very cold, well below water’s freezing point, so the water that is there is locked up as underground permafrost, or surface ice at the poles.

In addition, Mars’s almost non-existent atmosphere and weak magnetic field don’t do much to shield the planet’s surface from the bombardment of radiation from the Sun and outer space.

On the upside, it is believed that Mars may have been considerably warmer in the past, with a thick enough atmosphere to allow liquid water to collect on the surface.

Perhaps simple life managed to evolve then, and migrated underground as conditions worsened, to survive until today.

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How will we know if we find evidence of extraterrestrial life? This is one of the problems we have right now with our planetary landers/rovers.

Our first Mars landers, Viking 1 and Viking 2, reached the surface in 1976, and ran three chemical tests to look for signs of Martian life as we knew it back then. Their results are still being analyzed, and the answer is still the same: “Inconclusive” – some form of microscopic life, or some exotic chemical reactions.

One of our current rovers, Perseverance, has just found a rock with chemical and structural features that could indicate it was formed in the presence of liquid water, and has visible patterns that could have been formed by microbial life. Or not. But NASA has cancelled the program to collect the sample tubes left by Perseverance and return them to earth, so we will never know.

NASA scientists have developed a 7-step protocol for validating any discoveries: CoLD (Confidence of Life Detection).

1. Detect evidence of possible life. 

2. Eliminate possibility of terrestrial contamination.

3. Have a plausible biological hypothesis.

4. Eliminate any known non-biological explanations. 

5. Obtain additional independent evidence.

6. Eliminate any other plausible hypotheses for all the evidence.

7. Obtain independent confirmation of the biological hypothesis based on all the evidence.

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Until recently, rocky planets (or rocky satellites) with oceans of liquid water have been our focus for habitability. Planets like Neptune or somewhat smaller were not considered. But current research has indicated that similar-looking exoplanets might have a different internal structure, with a liquid-water surface and “reasonable” surface temperatures and pressures.

Their hydrogen-helium atmosphere has a much greater greenhouse effect than our nitrogen-oxygen one, allowing these planets to orbit much farther from their stars, and avoid being tidally-locked to them. Also, they would receive much less (based on the Inverse-square Law) dangerous radiation from stellar flares and CMEs.

There are many more of these “Hycean” exoplanets than Earth-like ones.

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Let us assume that life, either as we know it or as we don’t, has managed to appear on some exoplanet, and further, that is has managed to evolve “intelligence” (whatever that is). How long is it likely to remain in existence?

To begin with, there may be physical limits to the development of technology. While our own advanced exponentially after a point, it appears that it may be starting to slow down. Certain technologies, like quantum computing and fusion power, seem to be creating problems faster than solutions (though a “breakthrough” could happen at any time).

The increasing demand for energy required by AI may outstrip our ability to provide it. And even if we do, all the energy used ultimately winds up as waste heat, potentially overtaking “climate change” in excessively warming our planet.

Then there is the exhausting of natural resources, or the means of disposing unwanted byproducts and garbage. Simply feeding an increasing population may cause the sudden collapse of the food chain.

Worse, there may be a “Great Filter” that completely eliminates every civilization before it can leave the planet where it evolved. To start, look at what so-called “intelligent” life is currently doing on Earth. We have invented and manufactured enough nuclear and thermonuclear weapons to eliminate our species from the planet in a matter of hours, and the control of some of them is in the hands of madmen. Unfortunately, aliens may follow the same self-destructive path.

In a longer time-span, we humans are:

1. Dumping billions of tons of greenhouse gases into the atmosphere, which will eventually raise the temperature beyond the ability of humans to survive.

2. Creating enormous quantities of “trash” that soon finds its way into all aspects of our environment, eventually degrading into tiny fragments that enter the bodies of all living organisms, including us. 

3. Releasing into the environment all manner of bio-active contaminants that also soon find their way into our bodies and brains.

4. Inadvertently spreading lifeforms from their native habitant to new areas where they have no natural enemies, upsetting the “balance of nature” and destroying the local ecosystems. 

5. Genetically engineering plants and animals whose genes (or the animals themselves) are escaping into the planet’s ecosystem where they have no natural enemies.

6. Causing the extinction of increasingly large numbers of species, leading to the eventual collapse of one ecosystem after another, reducing available food sources.

7. Overusing antibiotics, antivirals, and antifungals to the point that completely-resistant microorganisms are spreading worldwide.

8. Overloading the entire ecosystem with rampant human overpopulation.

9. Having low-intelligence selected for in evolutionary reproductive success.

10. Developing AI (Artificial Intelligence) that may become more intelligent than humans and wipes us out, because once the AI becomes self-aware, it conceals its true intentions and convinces humans to let it design and run factories automatously, secretly altering them to produce “terminators.”

11. Too many more negative actions to keep listing here.

Additional possibilities for the demise of aliens (and some of which might affect us in the future, too) are:

1. Their planet is permanently shrouded in clouds, or they developed in a sub-surface ocean, so they never realized there was anywhere else they could go, and thus never developed space travel to enable them to colonize another planet and avoid extinction from the causes listed below.

2. Their planet is so much more massive than Earth that its escape velocity is beyond the ability of chemical rockets to overcome, and again they never developed space travel to enable them to colonize another planet and avoid extinction from the causes listed below.

3. Having a worldwide nuclear war, which doesn’t have to kill everybody; only destroy technology that cannot be recreated because all their easily obtainable energy and material resources have already been depleted.

4. Eventually exhausting all their available natural resources, raw materials as well as energy sources upon which their technological civilization depends. 

5. Accidently, or deliberately, creating enough “space junk” shrapnel that it begins to destroy satellites and produce increasingly more fragments, which start a chain-reaction and eventually shred everything in orbit, creating a “buzzsaw” environment that prevents the passage of spacecraft or satellites unless they are extremely heavily-armored. This makes further space exploration impractical.

6. Being exterminated by another hostile civilization, which is destroying every less-advanced civilization they find (or are found by) before it can advance further and destroy them.

7. Being exterminated by autonomous mechanical entities designed as a doomsday weapon that is self-replicating and slowly spreading through the universe to destroy any and all lifeforms.

But let us suppose that the exoplanet’s putative inhabitants have avoided all these dangers. How many millennia can they survive, assuming no asteroid impact wipes them out, or their sun doesn’t “burp” and emit a massive solar flare or CME that destroys the technology upon which their civilization depends. Or a more energetic stellar eruption (nova or supernova) simply incinerates them along with their entire planet. Intense gamma-ray beams from black-hole accretion disks or neutron-star mergers within 1,000 light-years are yet another possibility.

Information theory tells us that to accurately copy a certain amount of data, given a particular error (mutation) rate and level of data redundancy/repair, we can only do this for a limited number of generations. In the real world, this means that even if we humans instituted a strict eugenics policy, and prevented any person with a detectable “defect” from reproducing, in a million years (or maybe much less time) our descendants would bear very little resemblance to their homo sapiens sapiens ancestors. Random mutations, selection factors not immediately obvious, and conscious choice (e.g. allowing, and even encouraging, stupid people to outbreed intelligent ones), are typical factors.

Homo sapiens sapiens, or any alien species, cannot be around forever. (Much less than forever – all suns eventually die.)

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For the sake of further discussion, we will grant the aliens the ability to maintain their genetic integrity and their advanced civilization for millions of years. Then the question becomes: Why aren’t they here? (Known as the “Fermi Paradox” after physicist Enrico Fermi asked it many years ago.) There is no credible evidence that aliens are, or have ever been, here – first-person stories of abduction and anal probing notwithstanding.

Incidentally, SETI (Search for Extra-Terrestrial Intelligence) programs using radio telescopes to listen for alien radio signals haven’t found any, and more recent optical telescope searches haven’t found any alien laser communications either. (Of course, they may use another method we haven’t discovered.)

One possible answer is the speed-of-light (denoted by the lowercase letter “c”) limitation. By all observations and experiments conducted to date, it appears that physical objects cannot travel faster than light, regardless of the means of propulsion used. Ditto for the various forms of energy, and even information. Furthermore, as the speed of an object increases to an appreciable faction of c, its mass also increases exponentially. At 75% of c, the mass has increased 50%, and by 99% of c, the mass has increased over 700%.

And the amount of fuel required to accelerate a rocket to even 20% of c is tremendous, especially when you realize that it takes additional fuel to accelerate all the stored fuel that will be used later on. This must include the fuel to decelerate the rocket at its destination, the fuel to re-accelerate it for its return journey, and finally, the fuel to decelerate it when it gets back to the home planet (unless the crew boards a much-smaller escape capsule and decelerates just that, leaving the derelict main ship to continue speeding on indefinitely).

Assuming the aliens have some technology to achieve a fraction of lightspeed, these is still another roadblock to overcome: so-called “empty space” is anything but empty – it is filled with interstellar (aka cosmic) dust particles and gas molecules. Kinetic energy is given by the formula:

KE = ½ mv2

where m is the mass of the object and v is its velocity (relative to some reference frame). In the MKS (meter, kilogram, second) system, the mass of a grain of dust ranges from about 10-16 to 10-4 Kg, and 20% v is about 60,000,000 m/s. (60 million squared is 3,600,000,000,000,000.) For the largest dust particles, this gives about 1.8 x 1011 joules or the equivalent of 43 tons of TNT, for each grain of dust. It doesn’t matter if the dust grain hits the spaceship or vice versa. With the continuing barrage of dust and gas, the spacecraft will need a massive front shield, adding even more weight to lift off and escape from their planet’s gravity.

Assuming the aliens have some technology to achieve a fraction of lightspeed, and protect their ship from these impacts, and allowing for acceleration and deceleration at 1 g, it would take them over 22 years traveling at 0.2 c to get here from the nearest likely stellar system, Alpha Centauri, some 4.4 light-years (l-y) away. (Plus another 22 years to get back home.) A journey of 20 l-y (not far by astronomical standards) would take a century each way. Greater distances involve even more time. Unless the aliens are very long-lived, they aren’t going to travel very far. Or unless they discover a way to safely put the occupants into cryo-sleep and wake them at their destination.                                                        

However, there is another effect at near lightspeed: time dilation. On board the spaceship, time will flow more slowly the faster they go. If the aliens can somehow reach 0.999c, the length of time the ship’s crew will experience will be only 4.4% of the time that passes on their planet or elsewhere outside the starship. Thus, a trip of 224 lightyears will take them only 10 years onboard (plus the much longer amount of time to accelerate and decelerate). But all their friends and family back home will be long dead. (This would also apply to a “sleeper ship” traveling at slower speeds.)

Of course, if the aliens manage to discover a “warp drive,” or some other form of FTL (Faster-Than-Light) travel, trip time won’t be a problem, though fuel requirements still might be.

There is another solution for long trips, one that would apply even if the aliens don’t have a long lifetime, cryo-sleep, or the ability to travel at anywhere near lightspeed: generation ships. These huge vessels are self-contained worlds, growing their own food and recycling everything. The initial crew will have children onboard and train them to continue the mission, have children, and train them, until their nth-generation descendants finally reach the destination.

But generation ships have their own problems, some of which are:

1. Trace contaminants in the life-support systems that build up over time.

2. Unsuspected failure modes in the infrastructure over a journey of centuries or millennia or even longer.

3. Inability to maintain psychological stability in the insolated and confined limited population.

4. Massive outbreaks of disease from a mutated pathogen.

5. Sabotage by one or more psychopathic individuals, either before launch or afterward.

6. Collision with an unavoidable object.

Sleeper ships have some of these problems, too, plus many others:

1. Unknown factors that cause the reanimation procedure to fail.

2. Inability to maintain psychological stability in the small revived population onboard the ship.

3. Massive outbreaks of disease among the revived passengers from an unsuspected pathogen frozen along with them.

Then there may be habitability changes in the target world that occur after the flight is underway, such as:

1. Asteroid impacts or massive volcanic eruptions that make the planet uninhabitable.

2. Development of an indigenous technological civilization that destroys the generation ship as it arrives.

3. Development of an indigenous technological civilization that completely destroys themselves in a massive nuclear war, leaving the planet highly radioactive and most of the animal and plant life extinct.

4. Thermal runaway from climate change, also making the planet uninhabitable.

5. Tremendous stellar flares that strip away most of the planet’s atmosphere, yet another development rendering the planet unlivable.

6. The earlier arrival of another alien species’ generation ship, or maybe an FTL one. If hostilities break out, both ships and their inhabitants may be destroyed.

Fermi pointed out that even if an alien civilization was limited to these very slow generation spaceships, if they sent several out to the nearest habitable planetary systems, set up colonies there, and after several hundred years, each colony sent out a few spaceships of its own, they would have long ago occupied every suitable world in our galaxy.

Considering how old the universe is (about 13.8 billion years), and that our Earth formed about 9 billion years after the big bang, life took half-a-billion years to get established, then remained as mainly simple unicellular forms for another two-and-a-half billion years more. It required another half-billion years or so to evolve to its present state. At 12 billion years, we’re real latecomers, so civilizations on planets that formed many billions of years before us had lots of time to get here, either rapidly (FTL ships) or slowly (generation or sleeper ships).

Since the fossil record shows it’s obvious that we evolved here, we’re not their descendants.

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Continuing our “let’s pretend,” we will assume the aliens do have some form of FTL travel that can reach us. So, if they can get here – why aren’t they?

1. They don’t care. Based on their religion or philosophy.

2. They have a really-strict “non-interference directive.” See any Star Trek episode.

3. They are pathologically xenophobic. Or just realistic: they don’t want their children to become addicted to our social media or cat videos.

4. They are completely addicted to their advanced virtual reality so that they no longer have any interest in the “real world.”

5. Their civilization is very old and has reached a point of stasis. They control their population to the maximum their planet can support, and cannot spare any resources for space exploration.

6. They decided life really was better living with a more primitive technology in harmony with nature (farming, domesticated animals, simple stone structures, sailboats, herbal medicines, etc.) and reverted to a pastoral existence, never again advancing beyond that. 

7. They have been looking, but there are so many inhabited worlds in the universe that they haven’t got to us yet.

8. Because the distribution of habitable planets varies, we may be the only one in our volume of space, and they concentrate on other regions with many closely-spaced civilizations. 

9. After finding millions of civilizations all not significantly different from their own, they got bored and gave up looking.

10. They found out that microorganisms on other planets are always fatal to larger lifeforms from another planet. Maybe not bacteria or viruses, which are adapted to specific hosts, but fungi are amazingly unparticular as to what they colonize and break down. (It was athlete’s foot, not the common cold, that did in H. G. Wells’s Martian invaders.)

11. They didn’t find out about the deadly microorganisms, and carried them back to their homeworld, killing off their entire population.

12. They discovered the remains of other civilizations that had obviously been exterminated by another more-technologically-advanced race of hostile aliens, and immediately returned to their home world and set about eliminating any radiation of artificial signals that would signal their own existence and draw attention to them. This includes hiding from autonomous drone destroyers sent out by early civilizations that didn’t want any competition to ever develop.

13. Our human civilization is the last (or one of the last) to develop, so all (or almost all) of the others are long gone, from some of the causes previously mentioned.

14. They are here, but have really good stealth technology, so we will never know they are. They may be waiting for us to evolve into advanced, peaceful beings before contacting us, or to destroy ourselves so they can have the Earth for themselves. Or they’re here for entertainment, watching us like monkeys in a zoo. 

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There are a couple of other factors that could influence the evolution of “intelligent” life of any sort.

First, if evolution in general follows the course it did on Earth, the early land plants and animals will evolve to become the simplest ecological system: plants, herbivores, and carnivores (plus smaller organisms to break down and recycle their excrement and dead bodies). Absent any limiting factors, the animals will grow very large, as the dinosaurs did on Earth. They did not need to be too smart – just evolved back and forth in the hunter-prey relationship. (e.g. The armored prey got thicker armor – then the carnivores got sharper, longer teeth.) This arrangement lasted over hundreds of millions of years here, and likely would have continued for hundreds of millions more, had not an asteroid impact ended it. Otherwise, this stasis could have delayed the development of an intelligent species by even more mega-years, if not prevented it entirely.

In Earth’s case, the asteroid had to be in the GZ of sizes: too small and enough of the dinos would have survived to continue their reign, and slightly larger, all, or almost all, life on Earth would have perished. As it happened, the global effect of the impact was to cloud the planet with extended high-altitude dust and ash particles that reduced temperature below what the dinos had evolved with, and in addition, reduced the ambient light level, both of which killed off most of the photosynthesizing plants upon which the herbivore dinos fed. This allowed the smaller mammals that had been scurrying around the dinosaurs to survive on their carcasses and what of the plants that remained until the atmosphere cleared.

Second, if something similar (or different) happened on another planet to displace the large primitive exo-dinosaurs, it might simply happen that another class of large, stupid herbivores and somewhat smarter carnivores evolved to replace them, and continued to preempt the development of an intelligent species.

But on Earth, a branch of the surviving mammals evolved into tree-dwellers. This required a bigger brain to deal with arboreal living: precise 3-D vision and grasping hands to navigate the airborne path between branches. Finding this a fairly easy life, they again stagnated, and might have remained there for many hundreds of millions of years. However, the area of Africa that they occupied suffered a rift from plate tectonics and developed a string of active volcanos that altered the local climate and killed off most of the trees, forcing the ur-monkeys back to the ground where there was barely enough vegetation to support them. Dealing with this new reality, and avoiding the ground-dwelling predators, provided the impetus for further brain development, bipedal locomotion, hands capable of using tools, and speech for communication.

Had their original habitat remained stable, they might be arboreal still, and homo sapiens sapiens would never have evolved.

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There was yet another hurdle our prehistoric ancestors had to face. The climate they had evolved in underwent a drastic change: an ice age. About 900,000 years ago, the warmth they had evolved in vanished, initially reducing the population from about 100,000 to perhaps only 10,000 individuals. The large prey animals they depended upon for a major part of their diet went extinct, and recent studies of our current genetic composition indicates that only a little over 1,000 reproducing individuals might have remained, struggling to maintain even that number for more than a thousand centuries, before the temperature increased and they were able to migrate to the most favorable locations and begin to increase their numbers.

With such a small breeding population, there was a tremendous loss of genetic diversity, and it wouldn’t have taken much to have totally eliminated the common ancestor of all the more advanced species, such as Neanderthals, Denisovans, and Homo sapiens.

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Finally, in a carbon-oxygen life system, it might be possible for land-dwelling intelligent beings to evolve in a low-oxygen-concentration atmosphere, but with less than about 18% oxygen, the air cannot support the unaided combustion of most carbonaceous substances like wood, and without fire as a source of light and heat, they are highly unlikely to develop a technological civilization. Consider all the early applications: torches, candles, and oil lamps; firing clay to produce waterproof vessels, tiles, and pipes; and, smelting, alloying, annealing and tempering metals.

The same would be true of an aquatic intelligent species. Additionally, they might be able to naturally communicate acoustically through the water over very long distances, and never considered any kind of artificial communication, let alone be able to develop it.

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For all these reasons, we must consider the possibility that life, of any sort, is rare in the universe, and intelligent life may be so rare that it is possible we humans are the only (living) example. And on the course we are currently following, we may end up proving that intelligence above a certain level has no survival value at all.