Preserving our Species Beyond the Solar System - Part I
(Originally posted September 01, 2017 on Blogger)
Is Our Extinction on the Horizon?
NASA administrator Charles Bolden, physicist Stephen Hawking, entrepreneur Elon Musk, and a host of others have bluntly stated that if we are to survive as a species over the long term, then we must venture beyond Earth to become a multi-planet species.
"I believe that the long-term future of the human race must be in space... ...it will be difficult enough to avoid disaster on planet Earth in the next hundred years, let alone the next thousand, or million. The human race shouldn't have all its eggs in one basket, or on one planet. Let's hope we can avoid dropping the basket until we have spread the load." - Hawking (in an interview with The Big Think).
As we'll see below, they're fundamentally right; if we are to ensure the survival of our species over geologic timescales (tens to hundreds of millions of years and more), then we'll need to find ways to survive beyond the current cradle of our solar system.
It sounds far-fetched, but they have a point, and in the grand scheme of things it's a strong one. Earth will not always be suitable for habitation. One way or another, life on this planet is ultimately doomed. To be blunt, our only chance over the long term is to find our way beyond the limits of our solar system, and into the cradle of another more favorable star system.
For clarity, we'll consider "long-term" to be on the order of a billion years and more. This number isn't entirely arbitrary. I've based it off recent three-dimensional climate models that suggest Earth's climate should be hospitable to life for another billion years. For those who've read my blog in which we discuss higher-Kardashev-level civilizations, a billion years is just the sort of added time necessary for our species to spread across a galaxy such that no star or other extraterrestrial calamity could singularly wipe us out. We'd have redundancies upon redundancies in terms of shear numbers (population), and stellar energy sources, among other things.
(Please note: In reference to a predicted billion-year habitable climate on Earth; Earth's climate being hospitable to life does not necessarily include human life. What might be habitable by some species, may not be so for others. Those interested in reading about natural and anthropogenic drivers of the current accelerated global climate warming trend, sans Bill Nye-level scare tactics, are invited to read my blog on the subject here.)
In order to (hopefully) make this blog a more interesting read, we'll take the position that interstellar travel will one day be possible; albeit at sub-luminal speeds. Star Trek-style warp drives though convenient, are a bit too fanciful for this blogger.
I want to take the stance that interstellar travel will some day be possible, because for me to sit here and simply list the myriad of ways it isn't, is too easy given today's technology and sociopolitical environment. Let's face it, to succeed in interstellar travel will not only take the right tech (which we're within reach of), but also—and perhaps more importantly—require an extraordinary amount of money from a cooperative of nations with this common goal in mind. Also, skepticism to the point of outright denial is unfairly shortsighted, and unscientific by nature.
This harks back to two of Arthur C. Clarke's three laws: One of them states, when a distinguished but elderly scientist states that something is possible, s/he is almost certainly right. When s/he states that something is impossible, s/he is very probably wrong. The second law says, the only way of discovering the limits of the possible is to venture a little way past them into the impossible. And so there are no cliff hangers, the third law states that any sufficiently advanced technology is indistinguishable from magic. That latter one is most famous of the three.
We'll start by looking at some of the natural threats that ultimately limit our time on Earth. We won't delve much into potential anthropogenic threats like climate change, accidental or purposeful nuclear war, or even post-antibiotic-world pandemics to name a few. Though they're all real threats to human survival, they are (I'd like to think) still within our control to varying extents. The natural threats we'll discuss however, are (as of yet) beyond our control.
Once we've established the threats, we'll consider a handful of other star systems as potential new homes; look at inherent problems these systems might pose to life, and try to come up with reasonable, albeit generous estimates as to what sort of star systems we should be looking for, and how far we'll need to travel to get there.
We'll then look at some of the problems interstellar travel might pose, briefly consider possible propulsion systems and spacecraft that could get us there, and discuss what sort of preparations would need to be made prior to departure.
Once we've covered that, we'll consider the philosophical implications of what I see as an oft-overlooked paradox of what it is to be human on another planet over large time scales. Will there be a point at which we no longer think of ourselves as human in the same sense as our ancestors did?
We'll finish up by considering a possible path we can take as a species, in order that we might avoid this paradox, but then ask perhaps the most important question of all; "what is life?".
I know that all seems rather vague, but I hope to clarify everything being laid out here by the end of this blog. I hope to show that the long-term survival of our species beyond our solar system may well be a classic situation where we can't have our cake and eat it too. In fact, I tend to believe this is the case for any other advanced space-fairing civilization out there should they exist. That is unless they come up with, or have come up with a way to avoid this paradox by taking that mysterious path I allude to above.
If we or 'they' take or have taken this path, it would give entirely new meaning to what it means to be a Type-II or Type-III civilization, capable of surviving the course of trillions upon trillions of years. It is after all, most folks' deepest hope that they (or their 'souls') might live forever.... even after death.
It all sounds weird at this stage, but please read on...
- The Sun
The radiant flux of our Sun is increasing with time; its luminosity (a unit of radiant flux) is increasing at a nearly linear rate of about 1% per 110 million years (Schroder and Smith, 2008). We can generally think of luminosity as the total power output of the Sun. Higher solar output will ultimately equate to higher global temperatures here on Earth. (For more on how the Sun has affected global climate over the past few billion years, I invite you to read this blog.)
The Sun is ~30% more luminous today than it was over 3 billion years ago. Fortunately for life on Earth in the millions of years prior to our evolution, carbon dioxide levels were quite high relative to today, thus compensating for the lack of solar output by today's standards. Again, folks are welcome to read more on that in my climate blog linked above.
So why is the Sun effectively getting hotter? In a very general sense, outward gas pressure (radiation pressure in high mass stars) is in equilibrium with the inward effect of gravity. However, this outward pressure decreases as more and more hydrogen fuses into helium within the Sun's core. As this happens, equilibrium is lost between outward gas pressure and the inward gravitational effect, thereby allowing gravity to win out and contract the core.
This contraction increases the core's density and raises its temperature. These higher core temperatures in turn, increase the fusion rate in unspent layers of hydrogen above the core. This increased rate of fusion re-establishes equilibrium between the outward gas pressure and the inward effect of gravity. Of course, this doesn't last, nor does it likely happen in stages like this. As with most things in nature, everything operates along a continuum. It's an ongoing thermonuclear/gravity balancing act inside the Sun.
Though it is more complicated than this—a teeter-tottering of pressure, temperature, fusion, density and gravity in constant response to each others' fluctuations—the net result can fairly be generalized as a hotter Sun with time as the core continues to become denser and therefore hotter with time.
Some 600 tons of hydrogen fuses to 596 tons of helium every second. The lost 4 million tons is largely accounted for in photons and electron neutrinos. Using Avogadro's Law to convert this weight to a more recognizable unit for gas, that's well over 6 billion liters of hydrogen per second.
Today about half the hydrogen within the Sun's core has been converted to helium. As such, our Sun is halfway through the stable main sequence stage of its life; with about 5.4 billion years to go. From that point it will enter the subgiant phase, doubling in diameter over the course of the next ~500 million years. Its rate of expansion will increase substantially, expanding over 200 times its current size, and becoming a thousand times more luminous. At that point our Sun will enter the red-giant phase and likely engulf Earth into its thermonuclear abyss (Schröder and Smith, 2008), (Boothroyd and Sackmann, 1999).
Over the course of the next billion years or so, the Sun will experience significant mass loss, a violent helium flash, and four thermal pulses, among other not-very-friendly-to-life events before becoming a white dwarf. This latter version of the Sun will last trillions of years before devolving to a (theoretical) black dwarf; something that does not yet exist in our universe, as the universe hasn't existed long enough for one to have been formed.
With the Sun about halfway through its stable lifetime, means there are about 5.4 billion years left before the fury of changes listed above come to pass. But this doesn't mean we have 5.4 billion years left to figure out ways to leave Earth and find a home on another planet.
Long before the Sun gobbles up our world, it will have long since boiled away our precious world ocean.
The most recent 3-dimensional climate models predict the Sun will begin to boil away Earth's surface waters in about a billion years; a point at which the mean solar flux is predicted to reach 375 W/m2 as compared to today's mean value of 341 W/m2 (Leconte et al., 2013). At that point, mean global surface temperatures would exceed 70 degrees Celsius... assuming certain partial pressures of various greenhouse gases.
For clarification, this is a mean global value. Just as today's mean global surface temperature is ~14 degrees Celsius, yet there are many regions of the world that experience 40+ degree C days, a world with a mean global surface temperature of 70 degrees Celsius would have regions experiencing 100 degree C days and hotter... hot enough to boil fresh water at sea level.
Even without 100-degree C days, global evaporation rates will skyrocket and as a result a large volume of water vapor will enter the atmosphere. Water vapor is a potent greenhouse gas, and this cataclysmic event will invariably amplify global heating in a positive feedback loop that even Dante Alighieri couldn't have imagined. What's worse is that by the time this occurs, 27% of the modern ocean will have already been subducted into Earth's asthenosphere (Bounama et al., 2001), reducing the capacity of our ocean as a heat sink while also exposing thousands upon thousands of square miles of dry land; the latter of which of course has a much lower heat capacity than water. Imagine hot day out on the lake... you jump out into that lake from a boat and it feels cool and refreshing. Now imagine an equally hot day out in the middle of the desert... you jump out of a vehicle onto the desert floor and your feet burn... that's the difference between a surface with a high heat capacity, and one with a low heat capacity in a nutshell.
There's a consensus within the scientific community that we do not have 5.4 billion years left on Earth. But we at least have a billion years left here... that's what the latest models predict right?
Sadly, the models predict a habitable climate. As mentioned above, this does not necessarily mean habitable by us. Reality is, we simply don't have a billion years left here either. Long before our waters boil away, mean global surface temperature will exceed levels hospitable to us and many species we depend on. This will come long before that 70-degree Celsius surface mean is reached.
Heat stress puts an upper limit on our bodies' ability to adapt to increasing temperatures. Mammals (that includes us) will have to contend with the ongoing threat of hyperthermia, as well as the inevitable losing battle of our bodies' struggle in dissipating its metabolic heat load. The threshold at which we'll lost that battle is debatable, but is generally expected to fall within the 20-26 degree Celsius range (Sherwood and Huber, 2009).
Also, a very recent 16-year-long study on over a quarter-million subjects shows that temperature alone can be a trigger for heart attacks in humans (www.sciencedaily.com/releases/2017/08/170828093807.htm).
With regard to mean global surface temperature, the limit at which species of the animal kingdom (particularly mammals) can survive, may be tens of millions of years (if not more) before the billion-year limit for climate "habitability" as predicted by recent three-dimensional climate models. And it can't be reiterated enough; "habitability" does not necessitate parameters suitable for human life. There are numerous species more hardy than us largely tech-reliant humans.
For tech optimists, imagine the amount of energy it would require to keep any computing system cool in a hotter environment... any vehicle.. and building. Let's face it.. most of us can't handle the heat of where we live during summers as it is. Opting to retreat to air-conditioned buildings or vehicles.
Let's consider that the inferred mean surface temperature during the Devonian (~400 million years ago) was about 20 degrees Celsius... a full 6 degrees C warmer than today's mean. It was a time in Earth's deep past when most, if not all, animal life lived beneath the cool, protective waves of the global ocean.
Whether it's in a few hundred years, or a few million, there will be a point at which our Sun will be the trigger that makes life on Earth impossible for us; even with the oceans intact. We have to face it; our time here is limited because of the Sun. But the Sun isn't the only looming doom perched upon the horizon...
- Earth-Approaching Impactors
The Chicxulub impactor was an asteroid 10-15 kilometers in diameter that struck Earth about 66 million years ago. The estimated 1.3 to 58 septillion Joules of kinetic energy converted to heat it unleashed caused the mass extinction of no less than 75% of all species on Earth (Durand-Manterola & Cordero-Tercero, 2014).
According to folks at NASA's JPL in Pasadena, an impactor of that magnitude is expected to occur once every 50 to 100 million years. To be fair, I didn't read that in a peer-reviewed paper, but on their website. What is published in peer-review is the estimate that impacts by object 1 km or larger in diameter occur once every 500,000 years (Bostrum, 2002). Impacts by objects 5 km or larger in diameter occur once every 20 million years (Marcus, 2010). Impacts by objects this size could certainly kill millions, if not billions when the aftermath of their impact winters compounded by smoke from regional or even global fires are factored in; the latter of which can be set off by falling fiery debris.
Whether we're in for it in the coming decades or we have a few tens of millions years before another extinction-level asteroid hits Earth is up for conjecture. Point here is that Earth will be struck again if we are unable to detect, approach, and successfully deflect the incoming threat in a timely and efficient manner.
Of course, such a feat would require cooperation among nations which history has shown is never an easy task. But on the bright side, when there is a common enemy, nations have put aside their differences (temporarily) to head that enemy off. Let's hope that's the case if and when the time comes, because despite the hyperbolic delusions of grandeur suffered by some of Hollywood's unimaginative screenwriters, an American-only team of oil drillers armed with a nuke and a rig, aren't going to be the kind of coordination we'll need to save ourselves from an Earth-approaching impactor.
The best we can do is continue to monitor the heavens; a daunting task shared by citizen astronomers and professionals alike, including folks at NASA's Center for Near Earth Object Studies and JPL's automated Sentry System. NASA may utilize kinetic impactors to deflect Earth-approaching asteroids or comets, but the success of this technology depends on detecting them early enough to prepare and launch such missions.
Though the annual threat of impact by asteroids and comets is very small, the consequences of any such collision would be catastrophic on levels no human has ever experienced... and hopefully never will. As such, the threat should be taken seriously despite its rarity.
Of course, the Sun, and Earth-approaching impactors aren't the only natural threats to our long-term survival on Earth. There are other threats lurking beneath our feet. Under Earth's surface exist enormous caches of potential energy that are arguably the more immediate threat...
- Ultra-Plinian Class Supervolcanoes
Not all volcanoes are equal. They range from the relatively docile to the frighteningly explosive. There are three basic types of volcanoes in the world; basaltic, andesitic, and rhyolitic types. These are types of igneous rocks that have different viscosity when melted. (Geology experts who might be reading this, I'm ignoring the phaneritic cousins of the aphanites listed above for simplicity.)
Basaltic shield volcanoes like Kilauea in Hawaii ooze fluid lava; this is a function of the melt's relatively low silica content. Because magma/lava from these volcanoes is so fluid, they rank quite low on the Volcanic Explosivity Index (VEI); an index that scales the explosive power of a volcano with numbers 0 through 8, with 8 having the most kinetic energy (most devastating). Basaltic melts are fluid enough that they ooze out of Earth easily without building up too much pressure. The majority of Kilauea's Holocene eruptions have ranked 0 on the VEI; very low explositivity.
But this isn't to say such volcanic activity isn't threatening to life on Earth. The Deccan Traps are arguably an example of weakly-explosive volcanic activity that can have a negative impact on life (Keller, 2014), (Schoene et al., 2014); due to the long-term effects outgassing has on global climate.
A quick side note: Molten rock is termed "magma" when it is underground, and "lava" when it is above ground. The two terms are not interchangeable. The chemistry of the melt in fact changes at the surface due to outgassing (and other things).
The second type of volcano is the andesitic stratovolcano. Andesite is a type of igneous rock that has a higher silica content than basalt. This higher silica content results in a more viscous melt which in turn does not ooze to the surface as easily (generally speaking). As such, this thicker melt builds more pressure before erupting. Eruptions from these volcanoes are more explosive and therefore rank higher on the VEI.
The nuclear winter caused by the 1815 Tambora eruption (trachyandesitic melt) brought the year without a summer, forcing author Mary Shelley and her family to spend many of their otherwise warm days by Lake Geneva indoors instead. Her residential confinement led to her penning the great novel, Frankenstein.
The third, and most powerful type of volcano is the rhyolitic volcano; this is the type that is the greatest threat to the relative harmony of human life on Earth.
Yellowstone is a prime example of this sort of volcano. Rhyolite is a type of igneous rock with high silica content, and therefore acts as a very viscous melt. Copious amounts of pressure builds before rhyolitic eruptions, placing these Goliathan supervolcanoes into a class all their own; ultra-Plinian. These eruption rank at the very top of the VEI.
The mega-colossal eruptions from these volcanoes are easily capable of ejecting over a thousand cubic kilometers of material into the atmosphere in an instant. Finer particles from this ejecta would remain aloft in the stratosphere for a decade or more, resulting in a volcanic winter lasting just as long.
Tremendous ash fallout across America's farm belt would accumulate 15 to 60 cm, depending on distance from the blast. Such accumulation would choke off crops en masse, negatively affecting soil chemistry, and sterilize the ground via oxygen deprivation.
Natural waterways would become contaminated; acidifying and increasing in turbidity, the latter of which can reduce the amount of light reaching lower depths, which can inhibit growth of submerged aquatic plants, which would affect the animals that live and feed on those plants. Such trophic cascades would affect nearly every niche on the planet, or at the very least in the northern hemisphere.
Fish gills would no longer be able to dissolve oxygen in such ash-ridden waters, young trees would likely die across vast regions, and economies around the globe would suffer. If such an eruption occurs in our lifetimes, sell your airline stock, as the skies will be completely off limits for air travel for many years. That sounds silly, but the socioeconomic complexities that will arise from such an eruption will only compound through the length of the volcanic winter. Not to be pessimistic of human nature in large groups, but expect rampant corruption and war.
According to Mason et al., 2004, M8 (VEI 8) events—though rare—occur far more frequently than equivalent massive asteroid impact events (in terms of kinetic energy released). This is why ultra-Plinian class supervolcanoes are arguably the more immediate threat. Will such an eruption wipe out the human species? Not likely. But the aftermath will drastically affect our species, and many others as well, and be a wake-up call as to just how vulnerable we are. Certainly a population bottleneck is possible.
- Other Threats
There are plenty of other threats to human population. Consider the very real threat of a post-antibiotic world. Thanks in large part to the free-for-all giveaway of antibiotics (ABs) to poorly-diagnosed people who either don't need them, or don't finish the supply they were given, bacterial strains with physiologically or genetically enhanced capacity to survive high doses of ABs evolve and wreak havoc on our population on a global scale. This has been known since the 1940s (Luria and Delbruck, 1943), and yet doctors (and "doctors") continue to push ABs for cases in which they're either not needed, or outright inappropriate. Also, companies that produce ABs push this stuff not to save lives, but to make money. None of us were born yesterday... companies never sell their products soley to save lives and improve the environment. Less plastic in those water bottles has less to do with the environment and more to do with using the former as an excuse to cut costs on plastic. Anyway, the net result of rampant AB distribution are and will continue to be the births of superbugs that may well be the spawn of pandemics that could make the Black Death look like just another regional outbreak of the common cold by comparison.
Other potential global catastrophes linger precariously close, such as anthropogenic global warming (discussed in this blog, sans Bill Nye/Al Gore-level scare tactics), and the ever-looming threat of global nuclear war which can and has nearly been triggered completely by accident.
I mentioned above I'd not delve into human-induced threats, but they do warrant mentioning nonetheless.
I like to believe we have control over the human-induced threats, but we have no control over the threats of an increasingly-luminous Sun, or the eruption of supervolcanoes, despite NASA's slightly naive claims to the contrary. I tentatively say it's naive for a myriad of reasons ranging from the rational roadblocks posed by the physics of multiple magma chambers, to the availability of water, to political roadblocks posed by such peculiar social phenomena as NIMBY.
So given everything written so far, it seems our time frame here on Earth is more limited than most think. Whether it's a few centuries or several tens of thousands of years, it's all just a blink of an eye in the grand scheme of things.
I'm no Aeschylus, but that's quite tragic. We've come all this way as a species, only to realize we have to either accept our fate here on Earth, or leave and try our collective hand among the stars. Despite all that's going on in the world, I like to think our time here is longer than some believe. But it's undeniably clear that our time here isn't forever.
Some might say, "well it's far too soon and pessimistic to start thinking this way". To that it can be said, if we have the mind to imagine it, then we ought to also imagine ways to prevent or avoid it. "It" being our ultimate end on Earth. Perhaps we have a hundred thousand years or more (doubtfully), but why put off for tomorrow that which can be thoughtfully considered today. As they used to say in the old days... "Procrastination is the thief of time".
Carl Sagan once said, "we are a way for the Cosmos to know itself". I find that profound. It seems only natural for us to venture deeper into the cosmos, and establish ourselves among its stars. This of course leads us to the question... where will we go?
PER ASPERA AD ASTRA
- Proxima Centauri & Red Dwarfs at Large
The closest star system to our Sun is Alpha Centauri at about 4.37 light years distant. There are three stars in that system, two of which are locked together; orbiting around a common barycenter as a binary star. A third star, a faint red dwarf dubbed Proxima Centauri, is ~13,000 astronomical units (AU) away from the binary star. For perspective, 1 AU is the distance from our Sun to Earth, the Voyager 1 spacecraft that was launched in 1977 and traveling over 38,000 miles per hour relative to the Sun, is just barely over 139 AU from Earth.
Technically, the red dwarf does get within 4,300 AU (periastron) of the binary star, but this is over the course of a 550,000-year orbital period. 13,000 AU is the distance at apastron (Kervella et al., 2017). We have to give or take a thousand or so AU on these numbers, but point is we can consider any planet orbiting the red dwarf as being largely unaffected by the binary system of that, er, system. We'll get back to this red dwarf in a bit. But first let's squash any ideas about the binary star in that system...
Life in the midst of two stars orbiting each other at distances that range from our Sun to Pluto, to our Sun to Saturn may be difficult for human beings at best. The gravitational and radiative chaos that'd bring to any planet in orbit around such dancing stellar banshees would probably be unlivable... At least by fragile beings like us.
So back to their the far-flung red dwarf, Proxima Centauri, which happens to be slightly closer to Earth than the binary star at about 4.24 light years away. Closer to Earth is good... even better, Proxima Centauri hosts a planet only slightly more massive than Earth. Another plus is the fact that red dwarfs are long-lived; on the order of trillions of years; plenty of time for evolution to occur should other parameters viable for life exist.
However, there is considerable debate about the habitability of planets orbiting red dwarfs. We won't get into this here, as I think life could exist on planets tidally-locked around red dwarfs. But given the environmental conditions on such planets (assumed to have an atmosphere), including but not limited to high stellar variation, (see also: (https://www.sciencedaily.com/releases/2017/06/170606123342.htm), I think human arrival on a red-dwarf-orbiting planet would be met with misery and death... miserable death. Remember, habitable doesn't necessarily mean habitable by humans.
We can go on and on (and have here, and here) about "Earth-like" planets in other star systems... we can also discuss why most planets being discovered happen to orbit red dwarfs, but for this blog, we'll jist say that our chances of living long and prospering on a planet orbiting Proxima Centauri or any other red dwarf are slim to none.
Generally speaking, stars are given letter designations to categorize them by effective temperatures, with designations of O, B, A, F, G, K, and M representing stars from hottest to relatively coolest (O being hottest, M being coolest). They're further sub-categorized by the addition of the Arabic numerals 0 through 9, where 0 is hottest and 9 is coolest. So a G2 star is hotter than a G3 star for example. They are sub-categorized further still by the addition of Roman numerals (I, II, III, IV...) which are used to designate a star's luminosity class. Our Sun is categorized as a G2V star.
There's a lot of different stars out there, and many real and theoretical exotic types that don't fall under any of the above designations. Finding a star similar to our Sun will be difficult, if for no other reason than a younger solar analog will likely be cooler just as our Sun was when it was younger.
Finding the perfect solar analog may not be necessary, or practical given the fact we want a younger star. It'd be illogical to leave our solar system to inhabit another system whose star has just as much stable life left in it as our Sun.
And on the point of stability, we can say with confidence that the type of star we should be looking for ought to be a main sequence star (designated by the Roman numeral V). I discuss more on what main sequence stars are in this blog for those interested. All main sequence stars are in hydrostatic equilibrium as discussed above (fusion/gravity balance stuff). There are varying degrees of 'solar' activity on these stars, but for all intents and purposes they are stable.
Stability is of paramount importance if we hope to enjoy as a long-lived civilization on another habitable planet; a real habitable planet, not an uninhabitable undersized stranded planetary embryo like Mars. I love you Mars, but you're not future home for humankind.
The last thing we want to do is attempt to colonize a planet in orbit around an unstable star outside the main sequence on the Hertzsprung-Russell diagram. I can't imagine life on a planet orbiting a hypergiant! Nevermind radiation and solar storms, those things will just kill us dead... But imagine living on a planet where a year might take longer than all of written human history to complete! No birthday celebrations, we'll never reach retirement age, and telling someone you'll see them "next year" would be tantamount to telling them to bugger off. Tsk tsk.
Tau Ceti is widely considered a solar analog. It's quite close by cosmic standards, at just under 12 light years away, and it may even host two planets within its, ahem, "habitable zone" (Tuomi et al., 2012). I use scare quotes because I feel the term 'habitable zone' is rather poorly-defined for reasons I explained here and here.
At any rate, the Tau Ceti star system is a popular destination —at least with many science fiction writers. From the mid-20th century to present day, authors have penned this star system as the go-to place for either finding alien life, or establishing our own permanent colonies. This star is so popular that even SETI consistently lists it as a target for technologically-advanced extraterrestrial life.
Well, good luck with that SETI, because the Tau Ceti system is chalk full of crap in the form of a dense circumstellar debris disc. This disc could host numerous large bodies that might threaten any planets that may exist there. "Could", "might", "may"... I'm wording this carefully aren't I?
The frequency of asteroid impacts in such a system would likely be quite high relative to what our planet experiences (post late heavy bombardment), and this frequency could reset the clock for any life in that system such that it never has the chance to evolve beyond yeast. This doesn't bode well for advanced intelligence, nor does it bode well for long-term habitability fragile sentient beings like us.
Even if this debris disc were sans large bodies, small debris particles would still pose a serious threat for any incoming spacecraft carrying hopeful colonizers. It may also render the use of necessary communication and weather forecasting satellites impossible. It'd be like the Kessler syndrome on steroids.
Tau Ceti is also a mere ~52% the luminosity of the Sun (Pijpers, 2003). And as if that weren't enough, the star has another factor working against it... at least as far as we're concerned; it's over a billion years older than our Sun meaning it's stable main sequence phase has less life left in it than our Sun. Given this, we have to ask ourselves, "why leave one solar system whose end is nigh, only to arrive at another system whose end is nigher?"
Sigma Draconis & Kappa1 Ceti
Sigma Draconis is a G-type star just under 19 light years distant. The star is younger than our Sun, and it may even host a planet (yet to be confirmed). But its luminosity is only 41% that of our Sun, and if the planet is confirmed, it'd be Uranus sized. Uranus-sized planets have Uranus-sized gravity with Uranus-sized atmospheres. I feel pretty confident in saying we probably wouldn't fair well on such a planet.. which probably doesn't even have a solid surface anyway.
Kappa1 Ceti is a G-type star as well. It has been measured to have 95% the Sun's radius and 85% its luminosity... not bad. This star is a little less than 30 light years away, which is in our immediate stellar neighborhood. But the star lacks any planets; at least so far as can be inferred. The search is ongoing. However, even if a planet is discovered, chances an Earth-sized rocky planet exists and is habitable is highly unlikely. Kappa1 Ceti is known to erupt superflares; coronal mass ejections that can be up to 10,000 times more energetic than the largest X-class flares from our Sun (Schaefer et al., 2000).
Imagine the Carrington Event times 10,000 inflicted upon a modern human civilization who is trying to get a foothold on a new planet ~30 light years away from the nearest help desk.
Right Star, Wrong Planet(s)
Ok, so we need to find a nice stable main sequence star similar, but not exactly like our Sun if we want to make our interplanetary transition as smooth as possible. Problem is, finding Earth-sized planets in comfortable orbits around Sun-like'ish stars is rare.
As I wrote and referenced in this blog, small rocky planets like Earth and Venus insofar as observations have determined, are not common. This may have been true in our own solar system in its earlier years as well (please see my blog for more on this). Fortunately for us, Jupiter migrated about our solar system in the early days and chucked those super-Earths into the Sun. Super-Earths are rocky planets with masses considerably greater than Earth's; up to 10 times more massive (Charbonneau et al., 2009).
When you hear about the discovery of "Earth-like" (which should be termed "Earth-sized") planets being discovered, it's a fairly safe bet they're tidally-locked around a red dwarf. But when it comes to solar analogs, or quasi-solar analogs, it seems the terms super Earths, hot Jupiters, and Uranus-sized are the norm.
Life for a human on a planet with super-Earth mass might be miserable, as the acceleration due to gravity would be so great our muscles would fatigue, particularly the heart muscle. To be fair, we could theoretically evolve to adapt to the gravitational effects of a planet on the lower end of the super Earth spectrum (about 1.5 Earth masses), but that's if they have a surface upon which to stand.
New evidence suggests that super-Earths cling to extremely thick hydrogen-rich atmospheres such that their rocky cores never evolve into terrestrial planets with thin atmospheres like Earth (or even Venus, whose atmosphere is thin by comparison).
These super-Earths could theoretically be stripped of much of this enveloping atmosphere should they migrate closer to their parent star, becoming a theoretical Chthonian planet (Hébrard et al., 2003).
We can do better.
Back to the Drawing Board
Even if we find an Earth-size, Earth-like planet in comfortable orbit around a younger solar or quasi-solar analog, we'd need to contend with the likely fact that younger star will be much cooler than our Sun, just as our Sun was in its youth. An Earth-distant orbit (1 AU) around a cooler star that is otherwise like our Sun, would probably be a spinning ball of ice (if it has water).
Life around a cooler star would be a frigid experience; playing host to an environment ranking quite low on the diversity scale for life. An environment's biological diversity is directly proportional its resilience and inversely proportional to its members' risk for extinction.
So it seems finding a younger version of our Sun comes with a catch; bring your jackets and be prepared to tough it out a few million years before getting that much deserved tropical vacation. That is unless we can find an Earth analog.
Long ago when our own star (the Sun) was much younger and less luminous than it is now, Earth prospered. This was possible, despite the cooler Sun, because carbon dioxide levels in Earth's deep past were considerably higher than they are today (as inferred by proxies). These higher concentrations of this important greenhouse gas are what kept Earth from completely freezing over (save for that one time). I invite readers to read this blog for more on the important role carbon dioxide has played throughout Earth's climate history.
If we can find an Earth-like planet with the right atmosphere around a younger Sun-like star, we'd be in fat city.
But would higher carbon dioxide levels be problematic to breath? At worst CO2 is only a minimally-toxic at relatively-high concentrations (ie. 2,000 ppm), but it is still an asphyxiant, and could be a bad news for us if the partial pressure of oxygen isn't up to speed. There's little doubt members of the Plant Kingdom would love higher concentrations of carbon dioxide, but would we? It's a complicated matter, but one to be considered long before departure. We'd also have to consider the fact that an Earth-like planet with high enough concentrations of greenhouse gas(es) in an Earth-like orbit around a Sun-like star may be habitable "now", it may not be later as its host star become more luminous with time and said planet is unable to rid itself of that excess supply of greenhouse gases.
I often think we underestimate just how extraordinarily rare our situation on Earth is, and has been since the beginning.
If we are unable to find an Earth-analog orbiting a quasi-solar analog, we'd still need to send missions to thoroughly investigate the goings on there. We don't want to arrive to what seemed like a nice planet from afar, only to discover it has extreme weather conditions, or hosts unimaginably virulent strains of alien microbes capable of taking down the healthiest humans first via cytokine storms the likes of which medical science has never seen before.
We have to remember that we'd be arriving on a planet upon which we did not evolve. We are completely vulnerable to the microbes, viruses, flora, and fauna of that planet. There's also a flip side to that coin, at least with regard to the planet's flora and fauna; They may be completely vulnerable to us. Either way, the entire venture is extremely risky. We may arrive on a beautiful planet only to die from its many hazards within the year, or... we may arrive on a beautiful planet only to see its life die thanks to the microbes we'll bring with us. If one thinks we can fully sterilize ourselves before setting foot on such a planet, they're 100% wrong.
We also must consider the possibility that all flora is inedible either due to toxicity, or lack of nutritional value. The same could hold true for its fauna; ciguatoxic titan triggerfish sushi anyone?
The sad reality may very well be, no matter how Earth-like and beautiful another planet's environment is, we may need to enjoy that paradise within the protective confines of space suits and hermetically-sealed habitats indefinitely. This sort of stuff should be seriously addressed, if for no other reason than it will invariably add to the amount of time it will take to research an exoplanet prior to any preparations for a one-way trip to 'live' there. If that planet is light years away, which it will be if we can find it, we could easily be talking about many centuries of added time.
All the more reason to start thinking about this stuff now, rather than putting it off for the generations to come. Let's face it, it won't be our generation that makes the move, nor will it be the next.
The Starchip Enterprise
That's right... starchip. Research is underway on electromagnetically-accelerated, ultra-low-mass nanoprobes equipped with a camera, navigation system, communication transmitter, photon thrusters, and a power supply. The nanocrafts (a thousand of them) would be accelerated to as much as 20% the speed of light, and directed towards the Alpha Centauri star system. At this velocity, the nanocrafts would reach the system in about 20 years from launch date, with an additional ~4 years to communicate their successful arrival back to Earth.
How they'll come up with a 100-Gigawatt phased laser array capable of propelling-without-evaporating wafer-mass probes in space while having a communication system small enough, yet powerful enough to communicate effectively back to Earth is beyond the scope of this humble blog, but they're on it, and I'm inclined to give them the benefit of the doubt. If this technology can be proven, it'd be a paradigm shift for space exploration.
Ventures like Breakthrough Starshot are fascinating, and my fingers are crossed the folks behind the technology are able to pull it off. Accelerating gram-mass objects to relativistic speeds is one thing, but accelerating kilogram-mass objects to such speeds is a whole other animal. As can be seen in the graph below, the larger the mass the more energy is needed to accelerate that mass:
The kinetic energy (KE) of a mass will increase with speed, and according to Lubin's white paper, a 1 kg mass accelerated to 0.3 c would have the kinetic energy equivalent of a detonated 1-megaton strategic thermonuclear weapon. This matters, because that's a lot of KE to contend with when slowing the spacecraft down upon destination arrival.
The speed at which we'll be able to accelerate interstellar spacecraft in the future, will dictate how critical the relativistic effects of time dilation, length contraction, photon energy change, and effective mass increase will be, should that speed be a substantial fraction of light speed. Suffice it to say, it's going to be an extremely complicated business trying to figure out how to send humans to other star systems at relativistic speeds without killing them in one way or another.
Unless we can figure this out, we'll be stuck within the confines of our own ultimately-doomed solar system. And I don't think opening a wormhole (Einstein-Rosen bridge) near Saturn will much of a backup plan as I'd imagine anything capable of bending spacetime to that extreme would probably have some sort of noticeable affect on our solar system. Not to mention entering it probably wouldn't get us anywhere but dead.
Perhaps, like the movies depict, we'll have to figure out how to put the human body into sustained hibernation within which the effects of aging are minimized. Or, maybe we'll need to accept that there will be entire generations of people who will be forced to live out their entire lives on a spacecraft. Their centuries-long transit being carried out by their children, and their children's children.
At that point it may be easier to assemble something like an O'Neill cylinder or Stanford torus in orbit around Earth (mostly out of Earth's gravity well), and then figuring out a way to propel them to our new home. Light from the Sun would be unavailable en route, but perhaps some long-lasting radioactive energy source could be used for artificial sunlight. Now I don't mean the cool glow of Cherenkov radiation obviously, but converting that nuclear energy to electrical energy for use in a lightning system that emits a warm light with a color temperature like the Sun.
There will be all sorts of problems to contend with en route; from the dangers of space debris breaking through the hull/windows of the spacecraft, to HZE ions bombarding everyone on board, to maintaining a livable in-ship atmosphere, to simply keeping from killing each other from the psychology of it all wearing the human mind and spirit down to a nub.
There may be another way, other than boarding and breeding on our way to a new home aboard these complicated generation ships.
Self-Replicating Seeder Ships
One way of getting humans onto habitable planets in other star systems might be to use replicating seeder ships. These are a variation of the Von Neumann probe. The idea for self-replicating probes was conceived by mathematician/physicist Jon von Neumann. Replicating probes use local material from asteroids, moons, or other planets to make copies of themselves, then send those new ships out into the galaxy to do more of the same. An exponential growth of probes results, and if traveling at relativistic speeds, could theoretically explore every star system in the Milky Way within a half million years. This is a blink of an eye in the grand scheme of things, and vastly increases our chances of finding what we're looking for.
Remember, we don't need to explore every star system; we just need to explore the ones that fit our basic parameters based on what the experts think is best (or what we discussed above... but I'm no expert).
A variant of the Von Neumann probe is the replicating seeder ship. Rather than explore the universe by replicating itself, it seeds the most habitable moons and planets of the universe through self replication. The parent ship would have the genomes of our, and other species encoded into it, and when it finds a suitable planet or moon, it replicates us and whatever other species we've chosen (plant and/or animal) either from stored embryos or the use of molecular nanotechnology to build zygotes using local raw materials (Tipler, 1980).
This is one way to send gram-scale spacecraft at relativistic speeds to exoplanets without having to send an adult crew along with all the food and supplies necessary to ensure they survive the journey. These seeder ships would be an alternative to slow-going Generation ships, and establish our species on other planets (or moons).
But then we have to ask a few obvious questions... who will care for the human infants when they're born? One rather grim option is to include an artificially intelligent caretaker on the journey. One to raise, and protect the children, and teach them about where they came from, why they're there, human history on Earth, as well as the arts and sciences.
But the morality of sending embryos or a machine capable of using local raw materials to spawn human infants is in my opinion pure shit. It's wrong on so many levels I doubt there could be any arguments that would make it sensible, even in the face of extinction otherwise.
Another thing to consider, is that these artificially-intelligent self-replicators could theoretically run rampant after having been replicating for tens of millions of years. We'd have a locust swarm of replicators harvesting the local raw materials of every planet in the galaxy at some point, out competing local flora and fauna and causing mass extinctions on a galactic scale... that is if we're not the only planet harboring life in our galaxy.
Artificial intelligence can be of great benefit to humankind, but if we're not absolutely and tediously careful when developing it, that intelligence can make 'moral' decisions that seem logically correct, but have horrific consequences. AI is an extraordinarily complicated matter, and one far beyond the scope of this blog. However, we'll consider it again as we look at yet another alternative to preserving our species towards the end of this blog.
BEING & STAYING HUMAN
Evolution & Social Psychology
Anthropogenesis has been a gradual process believed to have given rise to anatomically-modern humans about 200,000 years ago, based on homo sapiens skeletal remains in Africa as well as comparisons of mitochondrial DNA sequences among the living (Cann et al., 1987). Although findings in a cave in Morocco may push this back to 300,000 years. Irregardless, the point here is that we've evolved, are evolving, and will continue to evolve to the selective pressures experienced here... on Earth.
Of course, if future generations were to be born-live-procreate-die-repeat on another planet, then they would naturally evolve to the selective pressures experienced in the world over millions of years (assuming they don't screw up the environment of that planet in the meantime).
Let's face it, if we want a 'Sun' with an added billion years of life left in it, and expect to enjoy all that extra time, we can expect to look and act quite differently after a few hundred-thousand generations.
Evolution occurs along a continuum, with gradual changes that occur over long periods of time. If we consider this, then the "clever" question of, "what came first, the chicken or the egg?" is a non sequitur. In other words, at what point will a human no longer be human over the course of a billion years of gradual evolutionary change?
There will be a point along that evolutionary continuum where we really won't be human anymore. An indohyas is no more a whale, than a human is to whatever we evolve into after a billion years. I believe few evolutionary biologists would argue with that.
Unfortunately I don't think it will ever come to that. I say 'unfortunate' because I believe social psychology will play a role in all this long before we physiologically change enough such that we're no longer genetically human. Being human may be as much about genetics as it is about psyche... whatever that is.
Future generations born and raised to parents who themselves were born and raised on another planet light years from Earth, will probably develop new and unique customs, languages, and perhaps even religions... even if they're all based off ones imported from Earth. Their world will build its own separate and unique histories. New cultures will arise, and most certainly new nations.
I would think the first colonizers would arrive and establish a single unified nation of sorts. One that exists in cooperation and overall harmony. A nation without borders because all lands and seas are a part of that nation.
But I wasn't born yesterday.
Call me pessimistic, but such an establishment won't likely last, and we don''t need to read Lord of Flies to figure that out; we can simply look at the state of world affairs from the start of agriculture to now to see there's simply no precedent for it.
Infighting for one reason or another will eventually lead to societal fracturing, and it will devolve into an "us" and "them" global system of inefficient political squabbling that invariably leaves the downtrodden in the metaphorical dust.
But I suppose all this is all just a part of what it is to be human. But will they think of themselves as such? Or will some misplaced pride or prejudice give them reason to believe otherwise? Consider this; if we were to ask anyone what they are, the response will likely be something other than "human". They'll state their ethnicity, their sex, or perhaps their religious affiliation. Sometimes I wonder if large portions of society have already forgotten what it means to be human. The strange thing is however, I believe every individual knows.
I suppose that's some food for thought... and a good point for me divest from this tangent I've gotten us onto. One thing is certain, preserving the human species over geologic timescales will be a tricky venture to say the least.
As we've covered so far, a lot is stacked against us; finding the right star, then the right moon or planet, then figuring out safe & successful interstellar travel, and being able to endure the long-term societal and selective pressures that await us out there.
But maybe there's a way to avoid all that, and still be able to preserve our species...
... but this blog is WAY too long, so we'll get to that alternative in the next one.
As always, thanks for reading.