Preface. So much research on why complex life is rare in the universe has come out since this book I’ve created another post: Rare Earth updates: recent research on why intelligent life is probably rare in the Universe. And intelligent life even rarer.  After all, there is no goal to evolution. .

I think that Ward & Brownlee’s 2000 book “Rare Earth: why Complex Life is Uncommon in the Universe” is one of the most profound books I’ve ever read.  What if we are the only intelligent species in the galaxy, or even universe?  There are dozens of reasons to think so.  Bacteria on the other hand, are probably a dime a dozen, splattered over planets within a reasonable Goldilocks zone from their star. 

But even the Goldilocks zone doesn’t guarantee life can exist. Mars is within this zone and almost certainly impossible to live on. Think again: “Escape to Mars after we’ve trashed the Earth?” And at the end of this post is a fiction story from the New Yorker, in which we can’t count on the Space Aliens to rescue us if they  show up.

Alice Friedemann  www.energyskeptic.com  Author of Life After Fossil Fuels: A Reality Check on Alternative Energy; When Trucks Stop Running: Energy and the Future of Transportation”, Barriers to Making Algal Biofuels, & “Crunch! Whole Grain Artisan Chips and Crackers”.  Women in ecology  Podcasts: WGBH, Financial Sense, Jore, Planet: Critical, Crazy Town, Collapse Chronicles, Derrick Jensen, Practical Prepping, Kunstler 253 &278, Peak Prosperity,  Index of best energyskeptic posts

***

Wikipedia. 2019. Rare Earth Hypothesis.

Requirements for complex life

The Rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as

1. a galactic habitable zone
2. a central star and planetary system having the requisite character
3. the circumstellar habitable zone
4. a right-sized terrestrial planet
5. the advantage of a gas giant guardian like Jupiter
6. a large natural satellite (the moon),
7. a magnetosphere and plate tectonics
8. the chemistry of the lithosphere, atmosphere, and oceans
9. the role of “evolutionary pumps” such as massive glaciation and rare bolide impacts
10. whatever led to the appearance of the eukaryote cell, sexual reproduction and the Cambrian explosion of animal, plant, and fungi phyla.

The evolution of human intelligence may have required yet further events, which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates.

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain many Earth-like planets. But if such planets exist, they are likely to be separated from each other by many thousands of light years. Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox: “If extraterrestrial aliens are common, why aren’t they obvious?”

The right location in the right kind of galaxy

Rare Earth suggests that much of the known universe, including large parts of our galaxy, are “dead zones” unable to support complex life. Those parts of a galaxy where complex life is possible make up the galactic habitable zone, primarily characterized by distance from the Galactic Center. As that distance increases:

1. Star metallicity declines. Metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets.
2. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, becomes less intense. Thus the early universe, and present-day galactic regions where stellar density is high and supernovae are common, will be dead zones.
3. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the Galactic Center or a spiral arm, the less likely it is to be struck by a large bolide which could extinguish all complex life on a planet.
4. Item #1 rules out the outer reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy’s habitable zone may be a ring sandwiched between its uninhabitable center and outer reaches.
5. Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an eccentric (elliptic or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10% of the stars in the Milky Way, about 20 to 40 billion stars. Gonzalez, et al. would halve these numbers; they estimate that at most 5% of stars in the Milky Way fall in the galactic habitable zone.
6. Approximately 77% of observed galaxies are spiral, two-thirds of all spiral galaxies are barred, and more than half, like the Milky Way, exhibit multiple arms. According to Rare Earth, our own galaxy is unusually quiet and dim (see below), representing just 7% of its kind. Even so, this would still represent more than 200 billion galaxies in the known universe.
7. Our galaxy also appears unusually favorable in suffering fewer collisions with other galaxies over the last 10 billion years, which can cause more supernovae and other disturbances. Also, the Milky Way’s central black hole seems to have neither too much nor too little activity (Scharf 2012).
8. The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma (million years), closely matching the rotational period of the galaxy. However, the majority of stars in barred spiral galaxies populate the spiral arms rather than the halo and tend to move in gravitationally aligned orbits, so there is little that is unusual about the Sun’s orbit. While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years. Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.

Orbiting at the right distance from the right type of star

According to the hypothesis, Earth has an improbable orbit in the very narrow habitable zone (dark green) around the Sun.

The terrestrial example suggests that complex life requires liquid water, requiring an orbital distance neither too close nor too far from the central star, another scale of habitable zone or Goldilocks Principle: The habitable zone varies with the star’s type and age.

For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper metallicity and size are also important to stability. The Sun has a low 0.1% luminosity variation. To date no solar twin star, with an exact match of the sun’s luminosity variation, has been found, though some come close. The star must have no stellar companions, as in binary systems, which would disrupt the orbits of planets. Estimates suggest 50% or more of all star systems are binary. The habitable zone for a main sequence star very gradually moves out over its lifespan until it becomes a white dwarf and the habitable zone vanishes.

The liquid water and other gases available in the habitable zone bring the benefit of greenhouse warming. Even though the Earth’s atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rain forest and ocean regions) and – as of February 2018 – only 408.05 parts per million of CO2, these small amounts suffice to raise the average surface temperature by about 40 °C, with the dominant contribution being due to water vapor, which together with clouds makes up between 66% and 85% of Earth’s greenhouse effect, with CO2 contributing between 9% and 26% of the effect.

Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as Sirius or Vega is wide, hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere. They may become red giants before advanced life evolves on their planets. These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification) as homes to evolved metazoan life.

Small red dwarf stars, one of the most common kinds of stars in our galaxy, have small habitable zones wherein planets are in tidal lock, with one very hot side always facing the star and another very cold side; and they are also at increased risk of solar flares (see Aurelia), coronal mass ejections, sterilization from ionizing radiation, and atmospheric erosion since their habitable zone is so close to the star (Hunt 2020). Life therefore cannot arise in such systems. Rare Earth proponents claim that only stars from F7 to K1 types are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9% of the hydrogen-burning stars in the Milky Way.

Such aged stars as red giants and white dwarfs are also unlikely to support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at a much greater distance may become habitable).

An energy output that varies with the lifetime of the star will likely prevent life (e.g., as Cepheid variables). A sudden decrease, even if brief, may freeze the water of orbiting planets, and a significant increase may evaporate it and cause a greenhouse effect that prevents the oceans from reforming.

All known life requires the complex chemistry of metallic elements. The absorption spectrum of a star reveals the presence of metals within, and studies of stellar spectra reveal that many, perhaps most, stars are poor in metals. Because heavy metals originate in supernova explosions, metallicity increases in the universe over time. Low metallicity characterizes the early universe: globular clusters and other stars that formed when the universe was young, stars in most galaxies other than large spirals, and stars in the outer regions of all galaxies. Metal-rich central stars capable of supporting complex life are therefore believed to be most common in the quiet suburbs of the larger spiral galaxies—where radiation also happens to be weak.

With the right arrangement of planets

Rare Earth proponents argue that a planetary system capable of sustaining complex life must be structured more or less like the Solar System, with small and rocky inner planets and outer gas giants. Without the protection of ‘celestial vacuum cleaner’ planets with strong gravitational pull, a planet would be subject to more catastrophic asteroid collisions.

Observations of exo-planets have shown that arrangements of planets similar to our Solar System are rare. Most planetary systems have super Earths, several times larger than Earth, close to their star, whereas our Solar System’s inner region has only a few small rocky planets and none inside Mercury’s orbit. Only 10% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable nearly circular orbits distant from their star. Konstantin Batygin and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present position. However, in the view of Batygin and his colleagues: “The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos.”

A continuously stable orbit

Rare Earth argues that a gas giant must not be too close to a body where life is developing. Close placement of gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce chaotic planetary orbits, especially in a system having large planets at high orbital eccentricity.

The need for stable orbits rules out stars with systems of planets that contain large planets with orbits close to the host star (called “hot Jupiters“). It is believed that hot Jupiters have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone. To exacerbate matters, hot Jupiters are much more common orbiting F and G class stars.

A terrestrial planet of the right size

It is argued that life requires terrestrial planets like Earth and as gas giants lack such a surface, that complex life cannot arise there.

A planet that is too small cannot hold much atmosphere, making surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere like Venus. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and surface temperature of 735 K (462 °C; 863 °F). Earth had a similar early atmosphere to Venus, but may have lost it in the giant impact event.

With plate tectonics

Rare Earth proponents argue that plate tectonics and a strong magnetic field are essential for biodiversity, global temperature regulation, and the carbon cycle. The lack of mountain chains elsewhere in the Solar System is direct evidence that Earth is the only body with plate tectonics, and thus the only nearby body capable of supporting life.

Plate tectonics depend on the right chemical composition and a long-lasting source of heat from radioactive decay. Continents must be made of less dense felsic rocks that “float” on underlying denser mafic rock. Taylor emphasizes that tectonic subduction zones require the lubrication of oceans of water. Plate tectonics also provides a means of biochemical cycling.

Plate tectonics and as a result continental drift and the creation of separate land masses would create diversified ecosystems and biodiversity, one of the strongest defences against extinction. An example of species diversification and later competition on Earth’s continents is the Great American Interchange. North and Middle America drifted into South America at around 3.5 to 3 Ma. The fauna of South America evolved separately for about 30 million years, since Antarctica separated. Many species were subsequently wiped out in mainly South America by competing Northern American animals.

Diamonds: bad for life. The planets circling some stars may be too diamond-rich, as much as 50% pure diamond. Their mantle might consist of a hard, brittle diamond that is incapable of flowing. Whereas iron and silicon trap heat inside our planet, resulting in geothermal energy, diamonds transfer heat so readily that the planet’s interior would quickly freeze. Without geothermal energy, there couldn’t be any plate tectonics, magnetic field, or atmosphere. Panero describes these diamond super-earths as “very cold, dark” worlds (Wilkins 2011).

A large moon

The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or only tiny satellites which are probably captured asteroids (Mars).

The Giant-impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, dubbed Theia, with the young Earth. This giant impact also gave the Earth its axial tilt (inclination) and velocity of rotation. Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. The Rare Earth hypothesis further argues that the axial tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. In this view, the Earth’s tilt is “just right”. The gravity of a large satellite also stabilizes the planet’s tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible.

If the Earth had no Moon, the ocean tides resulting solely from the Sun’s gravity would be only half that of the lunar tides. A large satellite gives rise to tidal pools, which may be essential for the formation of complex life, though this is far from certain.

A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet’s crust. The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust. It is possible that the large scale mantle convection needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity. A further theory indicates that such a large moon may also contribute to maintaining a planet’s magnetic shield by continually acting upon a metallic planetary core as dynamo, thus protecting the surface of the planet from charged particles and cosmic rays, and helping to ensure the atmosphere is not stripped over time by solar winds.

Most planets have moons, but Earth’s moon is distinct in that it is large compared to the size of Earth; the moon’s radius is larger than a quarter of Earth’s radius, a much larger ratio than most moons to their planets. And it now appears that only certain types of planets can form moons that are large in respect to their host planets, planets that are less than six times the size of earth, because in a collision with another planet forming, the potential moon will be vaporized since these collisions are too energetic to form a large moon (Nakajima 2022).

Atmosphere

A terrestrial planet of the right size is needed to retain an atmosphere, like Earth and Venus. On Earth, once the giant impact of Theia thinned Earth’s atmosphere, other events were needed to make the atmosphere capable of sustaining life. The Late Heavy Bombardment reseeded Earth with water lost after the impact of Theia. The development of an ozone layer formed protection from ultraviolet (UV) sunlight. Nitrogen and carbon dioxide are needed in a correct ratio for life to form. Lightning is needed for nitrogen fixation. The carbon dioxide gas needed for life comes from sources such as volcanoes and geysers. Carbon dioxide is only needed at low levels^] (currently at 400 ppm); at high levels it is poisonous. Precipitation is needed to have a stable water cycle. A proper atmosphere must reduce diurnal temperature variation.

One or more evolutionary triggers for complex life

Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life usually remains simple bacteria. Biochemist Nick Lane argues that simple cells (prokaryotes) emerged soon after Earth’s formation, but since almost half the planet’s life had passed before they evolved into complex ones (eukaryotes) all of whom share a common ancestor, this event can only have happened once. In some views, prokaryotes lack the cellular architecture to evolve into eukaryotes because a bacterium expanded up to eukaryotic proportions would have tens of thousands of times less energy available; two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into mitochondria that supplied the vast increase in available energy that enabled the evolution of complex life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple. An alternative view is that mitochondria evolution was environmentally triggered, and that mitochondria-containing organisms appeared soon after the first traces of atmospheric oxygen. Oxygen was needed for powering the process of aerobic respiration for both plants and animals.

The evolution and persistence of sexual reproduction is another mystery in biology. The purpose of sexual reproduction is unclear, as in many organisms it has a 50% cost (fitness disadvantage) in relation to asexual reproduction. Mating types (types of gametes, according to their compatibility) may have arisen as a result of anisogamy (gamete dimorphism), or the male and female genders may have evolved before anisogamy. It is also unknown why most sexual organisms use a binary mating system, and why some organisms have gamete dimorphism. Charles Darwin was the first to suggest that sexual selection drives speciation; without it, complex life would probably not have evolved.

The right time in evolution

While life on Earth is regarded to have spawned relatively early in the planet’s history, the evolution from multicellular to intelligent organisms took around 800 million years. Civilizations on Earth have existed for about 12,000 years and radio communication reaching space has existed for less than 100 years. Relative to the age of the Solar System (~4.57 Ga) this is a short time, in which extreme climatic variations, super volcanoes, and large meteorite impacts were absent. These events would severely harm intelligent life, as well as life in general. For example, the Permian-Triassic mass extinction, caused by widespread and continuous volcanic eruptions in an area the size of Western Europe, led to the extinction of 95% of known species around 251.2 Ma ago. About 65 million years ago, the Chicxulub impact at the Cretaceous–Paleogene boundary (~65.5 Ma) on the Yucatán peninsula in Mexico led to a mass extinction of the most advanced species at that time.

If there were intelligent extraterrestrial civilizations able to make contact with distant Earth, they would have to live in the same 12Ka period of the 800Ma evolution of life.

Snowball Earth (Ward & Brownlee)

It is possible that the extreme conditions of snowball earth were required to force multicellular life to evolve 650 million years ago when the Earth’s surface became entirely or nearly frozen at least once. 

Complex life evolved just once. All complex life is descended from a single common ancestor. Why? Nick Lane says that natural selection normally favors fast replication, keeping simple cells simple. Then a freak event occurred: an archaeon engulfed a bacterium and the 2 cells formed a symbiotic relationship. That transformed the dynamics of evolution, leading to a period of rapid change that produced innovations such as sex. The incorporated bacterium eventually evolved into mitochondria, the energy generators of complex cells.  So there was nothing inevitable about the rise of the sophisticated organisms from which we evolved. “The unavoidable conclusion is that the universe should be full of bacteria, but more complex life will be rare” (NS 2010).

Paul Simms. 2009. Attention, people of earth. The New Yorker.

We are on our way to your planet. We will be there shortly. But in this, our first contact with you, our “headline” is: We do not want your gravel.

We are coming to Earth, first of all, just to see if we can actually do it. Second, we hope to learn about you and your culture(s). Third—if we end up having some free time—we wouldn’t mind taking a firsthand look at your almost ridiculously bountiful stores of gravel. But all we want to do is look.

You’re probably wondering if we mean you harm. Good question! So you’re going to like the answer, which is: We mean you no harm. Truth be told, there is a faction of us who want to completely annihilate you. But they’re not in power right now. And a significant majority of us find their views abhorrent and almost even barbaric.

But, thanks to the fact that our government operates on a system very similar to your Earth democracy, we have to tolerate the views of this “loyal opposition,” even while we hope that they never regain power, which they probably won’t (if the current poll tracking numbers hold up).

By the way, if we do take any of your gravel, it’s going to be such a small percentage of your massive gravel supply that you probably won’t even notice it’s gone.

You may be wondering how we know your language. We are aware that there’s a theory on your planet that we (or other alien species from the far reaches of the galaxy) have been able to learn your language from your television transmissions. This is not the case, because most of us don’t really watch TV. Most of our knowledge about your Earth TV comes from reading Zeitgeisty think pieces by our resident intellectuals, who watch it not for fun but for ideas for their print articles about how Earth TV holds a mirror up to Earth society, and so on. We mean, we’ll watch Earth TV sometimes—if it happens to be on already—but, generally, we prefer to read a good book or revive the lost art of conversation.

Sadly, Earth TV is like a vast wasteland, as the Earthling Newton Minow once said. But, for those of you who can understand things only in TV terms, just think of us as being very similar to Mork from Ork, in that he was a friendly, non-gravel-wanting alien who visited Earth just to find out what was there, and not to harvest gravel.

Speaking of a vast wasteland, you might want to start picking out and clearing off a place for our spacecraft to land. Our spacecraft, as you will see shortly, is huge. Do not be alarmed; this does not mean that each one of us is that much bigger than each one of you. It’s just that there were so many of us who wanted to come that we had to build a really huge spacecraft.

So, again, no cause for alarm.

(Full disclosure: each of us actually is much bigger than each of you, and there’s nothing we can do about it. So please don’t use any of your Earth-style discrimination against us. This is just how we are, and it’s not our fault.)

Anyway, re our spacecraft: it’s kind of gigantic. The deceleration thrusters alone are sort of, like . . . well, imagine four of your Vesuvius volcanoes (but bigger), turned upside down.

We don’t want to hurt anyone, so, if you could just clear off one continent, we think we can keep unintended fatalities to a minimum. Australia would probably work. (But don’t say Antarctica. Because we’d just melt it, and then you’d all end up underwater. Which would make it virtually impossible for us to learn about your hopes and your dreams, and your culture, and to harvest relatively small, sample-size amounts of your gravel, just for scientific study.)

A little bit about us: our males have two penises, while our females have only one. So, gender-wise, if you use simple math, we’re pretty much identical to you.

And, as far as protocol goes, we’re a pretty informal species. If you want to put together a welcoming ceremony with all your kings and queens and Presidents and Prime Ministers and leading gravel-owners, that’s fine. But please don’t feel like you have to.

Technically, it would be possible for us to share our space-travel technology with you, so that you could build a spacecraft and travel to our planet also. But, for right now, it just feels like it would be better if we came to your place.

Speaking of gravel, one thing we can’t tell from our monitoring of Earth is how your gravel tastes. It’s just something we’re curious about, for no real reason. Is it salty? It looks salty.

Maybe you could form a commission of scientists/gravel-tasters to look into this and let us know. Just have them collect all the gravel you have and put it in one big pile. (There are some pretty big empty parts of Utah, New Mexico, and Russia that might be good spots for such a large gravel pile, but that’s just an F.Y.I.)

Then, if you could have your top scientists/gravel-tasters go through this gravel pile, tasting each and every piece, that would be great. Also, if it’s not too much of a hassle, have them put all the saltier-tasting pieces in a separate pile.

Anyway, that about wraps up this transmission! Looking forward to seeing you very soon. (Sorry we couldn’t have given you more notice, but we didn’t want you Earth people going crazy and looting stuff and having sex in the streets out of panic about losing all your delicious gravel, which is something that is definitely not going to happen, because, when it comes down to it, what is gravel really but just a bunch of baby rocks?)

Our E.T.A. on Earth is sometime in the next 450 to 500 years, which we know is a blink of an eye in your Earth time, so start getting ready! Let’s have fun with this.

Yours,

A Species from a Galaxy You Haven’t Even Noticed Yet

P.S.—We saw that you sent some people to your moon recently. Good job! But, just to let you know, don’t waste your time with the moon. There’s no gravel there. We already checked.

References

Gribbin, J. 2018. Why we are probably the only intelligent life in the galaxy. Scientific American.

Hunt K (2020) Observations of our closest neighboring star dampen hopes of a potentially habitable planet. CNN.

Nakajima M et al (2022) Large planets may not form fractionally large moons. Nature Communications.

NS. 2010. An unlikely story. New Scientist.

Scharf, C. 2012. The benevolence of black holes. Scientific American.

Wilkins, A. 2011. The galaxy could be full of diamond planets. Gizmodo.

Williams, O. 2016. Brian Cox Explains Why He Thinks We’ll Never Find Aliens. His answer doesn’t bode well for the future of humanity. Huffingtonpost