Rare Earth updates: recent research on why complex & intelligent life are rare in the Universe

Preface. These are updates to Ward & Brownlee’s book “Rare Earth: Why Complex life is Uncommon in the Universe”. If we are one of the few planets with intelligent life, what a shame it would be if we destroyed ourselves and millions of other species in the 6th mass extinction we are causing, or nuclear winter, or continuing to exceed planetary boundaries. Maybe we aren’t so intelligent after all.

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Balbi A, Frank A (2023) The oxygen bottleneck for technospheres. Earth & Planetary Astrophysics. https://doi.org/10.48550/arXiv.2308.01160

For combustion to occur, a planet needs an atmosphere of at least 18% oxygen. Less than that might allow complex multicellular life, but without a concentration of over 18%, there can be no fire, no combustion from diesel or gasoline engines, no steam turbines. So no technology, let alone propelling starships to other galaxies. You couldn’t even build them: it takes oxygen to generate high heat to extract metals from ores and make steel, copper, and other products such as glass, ceramics, bricks and more.

The article doesn’t go into this, but today that high heat is provided only by fossil fuels, yet another limiting factor for industrial technological civilizations – a planet would need both oxygen and fossil fuels. See the posts in this category for details: https://energyskeptic.com/category/fastcrash/industrial-heat/

A planet with oxygen of 30% and above would be a planet on fire since combustion would be too easy.

Perhaps thermal features could provide heat in a low-oxygen atmosphere, but such a civilization would have to remain very local, since geothermal heat isn’t transportable like wood, coal, and oil and impossible in a water world as well where the heat is coming from sea floor hydrothermal vents.

Scientists looking for life in the universe would be wise to look for planets with atmospheres containing 18 to 21% oxygen, the optimum level for useful combustion. Though today the telescopes viewing exoplanets can’t do that. And there is a chance of false positives where there is oxygen but no life exists, but Krissansen-Totton et al (2021) have found ways to rule them out (here).

If oxygen is the bottleneck that limits intelligent life, that might help solve the Fermi paradox which asks why there is no evidence of intelligent life given the size of the universe.

Sauterey B et al (2022) Early Mars habitability and global cooling by H2-based methanogens. Nature Astronomy https://doi.org/10.1038/s41550-022-01786-w

Re-creating Mars as it was four billion years ago using climate and terrain models, researchers concluded methane-producing microbes could once have thrived centimeters below much of the Red Planet’s surface, consuming hydrogen and CO2 while protected by the sediment above. But freezing temperatures of its own making may have driven them deeper, triggering global cooling that caused the surface to be covered in ice, killing them.

Koberlein B (2022) Venus’ atmosphere stops it from locking to the sun. universetoday.com

Of the thousands of exoplanets we’ve discovered, most of them closely orbit red dwarf stars. Part of this is because planets with short orbital periods are easier to find, but part of this is that red dwarf stars make up about 75% of the stars in our galaxy. Most are likely tidally locked, which means that only one side of the planet faces the sun which happens from tidal forces created by rotating around their sun so closely. But there may be exceptions. Earth rotates every 24 hours, Venus every 243 days. So instead of “fire and ice” tidally locked planets, these would be planets with hot, dense atmospheres (my comment: neither sounds friendly to life!)

Voosen (2022) The Planet Inside. Scientists are probing the secrets of the inner core and learning how it might have saved life on earth. Science 376: 18-22.

Earth’s inner core generates the protective magnetic field shielding our planet from damaging radiation. The magnetic field was sputtering to just 10% of what we have today 565 million years ago. Then miraculously, over just tens of millions of years, it regained its strength and not long after the multicellular life of the Cambrian explosion occurred with the birth of the inner core, a sphere of solid iron that spins independently from the rest of the planet. If this hadn’t happened Earth’s developing life in the ocean would have been exposed to far more radiation from solar flares, and rising oxygen levels would have escaped to space from the increased ionization.

**Ziyi Zhu et al (2022) The temporal distribution of Earth’s super mountains and their potential link to the rise of atmospheric oxygen and biological evolution, Earth and Planetary Science Letters. DOI: 10.1016/j.epsl.2022.117391**

Giant mountain ranges at least as high as the Himalayas and stretching up to 5,000 miles (8,000 km) across entire super-continents played a crucial role in the evolution of early life on Earth, which only formed twice in Earth’s history—the first between 2,000 and 1,800 million years ago and the second between 650 and 500 million years ago. Both mountain ranges rose during most important periods of evolution.
The first range coincided with the appearance of eukaryotes, organisms that later gave rise to plants and animals.
The second range coincided with the appearance of the first large animals 575 million years ago and the Cambrian explosion 45 million years later, when most animal groups appeared in the fossil record.
When the mountains eroded they provided essential nutrients like phosphorous and iron to the oceans, supercharging biological cycles and driving evolution to greater complexity. The super-mountains may also have boosted oxygen levels in the atmosphere, needed for complex life to breathe. There is no evidence of other super-mountains forming at any stage between these two events, making them even more significant.

“The time interval between 1,800 and 800 million years ago is known as the Boring Billion, because there was little or no advance in evolution,” co-author Professor Ian Campbell said. “The slowing of evolution is attributed to the absence of super mountains during that period, reducing the supply of nutrients to the oceans.”

2021 Longer days on early Earth set stage for complex life. Extra light spurred oxygen release by mats of photosynthetic bacteria. Science 373: 607-608

Scientists have struggled to find a satisfying reason of what triggered oxygen buildup and why it took so long. It may be that the increasing length of a day as Earth’s spin slowed enabled more photosynthesis from bacterial mats, allowing oxygen to build up in ancient seas and diffuse into the atmosphere. 4.5 billion years ago the length of a day was only 6 hours long. But by 2.4 billion years ago the pull of the Moon had slowed the spin to 21 hour days, which is when the Great Oxygenation Event occurred, and then again a billion years later.

2021 Stars less than half as hot as our Sun can not sustain Earth-like biospheres: not enough energy to sustain photosynthesis

A new study of exoplanets — planets beyond our solar system — has revealed that none of them, despite previously thought to be habitable, may have the right Earth-like conditions needed to sustain life. The research evaluated the amount of energy these Earth-like planets received from their host star and if it was enough for living organisms to “efficiently produce nutrients and molecular oxygen” that are critical for complex life. So planets orbiting cooler stars known as red dwarfs that smoulder at roughly a third of the Sun’s temperature, don’t receive enough energy to even activate photosynthesis. Although the number of planets in our Milky Way galaxy is in the thousands, those with conditions similar to Earth and in the habitable zone are not very common, the study stated. A habitable zone means the region around a star where the temperature is just right for liquid water to exist on the planet’s surface. There is only one exoplanet Kepler-442b, a rocky planet with a mass twice that of the Earth 1,200 light-years away, comes close to receiving the radiation necessary to sustain a large biosphere. Covone G et al (2021) Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone. Monthly Notices of the Royal Astronomical Society.

O’Callaghan J (2021) Venus’s surface may always have been too hot for oceans. New Scientist.

If this is so, then the window of time for planets to become habitable is even narrower than astronomers had thought. Even earth was only able to condense water early in its history because the sun was 25% dimmer, seemingly solving the “faint young sun paradox” when Earth was thought to have been too cold to support liquid water. Had it formed today, our planet might well have been a “steam Earth” like Venus.

Oluseyi H (2021) Intelligent life probably exists on distant planets — even if we can’t make contact, astrophysicist says. Washington Post.

But just four in our galaxy: If only one in a hundred billion stars can support advanced life, that means that our own Milky Way galaxy — home to 400 billion stars — would have four likely candidates. Of course, the likelihood of intelligent life in the universe is much greater if you multiply by the 2 trillion galaxies beyond the Milky Way.

It also helps that Earth’s atmosphere is transparent to visible light. On most planets, atmospheres are thick, absorbing light before it reaches the surface — like on Venus. Or, like Mercury, they have no atmosphere at all. Earth maintains its thin atmosphere because it spins quickly and has a liquid iron core, conditions that lead to our strong and protective magnetic field. This magnetosphere, in the region above the ionosphere, shields all life on Earth, and its atmosphere, from damaging solar winds and the corrosive effects of solar radiation. That combination of planetary conditions is difficult to replicate.

At the low end of consensus estimates among astrophysicists, there may be only one or two planets hospitable to the evolution of technologically advanced civilizations in a typical galaxy of hundreds of billions of stars. But with 2 trillion galaxies in the observable universe, that adds up to a lot of possible intelligent, although distant, neighbors.

Unfortunately, we’re unlikely to ever make contact with life in other galaxies. Travel by spaceship to our closest intergalactic neighbor, the Canis Major Dwarf, would take almost 750,000,000 years with current technology. Even a radio signal, which moves at close to the speed of light, would take 25,000 years.

2021 Water is essential for life – not just the medium but active participant

Water is often seen as the background in which other chemicals like DNA and protein are dissolved, but of 6500 reactions, 40% made or destroyed a molecule of water. When E. coli divides to form 2 new cells, every water molecule it contains is either transformed or drives a chemical reaction 3.7 times on average. This may also be key to the origins of life, since water would have determined which chemicals survived — the ones that were soluble in water and able to react with it.

Abstract: Water, the most abundant compound on the surface of the Earth and probably in the universe, is the medium of biology, but is much more than that. Water is the most frequent actor in the chemistry of metabolism. Our quantitation here reveals that water accounts for 99.4% of metabolites in Escherichia coli by molar concentration. Between a third and a half of known biochemical reactions involve consumption or production of water. We calculated the chemical flux of water and observed that in the life of a cell, a given water molecule frequently and repeatedly serves as a reaction substrate, intermediate, cofactor, and product. Our results show that as an E. coli cell replicates in the presence of molecular oxygen, an average in vivo water molecule is chemically transformed or is mechanistically involved in catalysis ~ 3.7 times. We conclude that, for biological water, there is no distinction between medium and chemical participant. Chemical transformations of water provide a basis for understanding not only extant biochemistry, but the origins of life. Because the chemistry of water dominates metabolism and also drives biological synthesis and degradation, it seems likely that metabolism co-evolved with biopolymers, which helps to reconcile polymer-first versus metabolism-first theories for the origins of life. Frenkel-Pinter M et al (2021) Water and Life: The Medium is the Message. Journal of Molecular Evolution volume 89: 2–11

2021 The right mixture of salts is needed to start life

Chemically speaking, RNA is closely related to DNA. However, in addition to storing information, RNA can fold into complex structures that have catalytic activity, similar to the protein nanomachines that catalyze chemical reactions in cells. These properties suggest that RNA molecules should be capable of catalyzing the replication of other RNA strands, and initiating self-sustaining evolutionary processes. In order to fold correctly, RNA requires a relatively high concentration of doubly charged magnesium ions and a minimal concentration of singly charged sodium, since the latter leads to misfolding of RNA strands. Thes right salt balance might have happened under the conditions on Earth 4 billion years ago. Matreux T et al (2021) Heat flows in rock cracks naturally optimize salt compositions for ribozymes. Nature Chemistry.

2021 Stellar winds can evaporate the atmosphere of planets

Most stars, including the sun, generate magnetic activity that drives a fast-moving, ionized wind and also produces X-ray and ultraviolet emission (often referred to as XUV radiation). XUV radiation from a star can be absorbed in the upper atmosphere of an orbiting planet, where it is capable of heating the gas enough for it to escape from the planet’s atmosphere. M-dwarf stars, the most common type of star by far, are smaller and cooler than the sun, and they can have very active magnetic fields. Their cool surface temperatures result in their habitable zones (HZ) — the range within which the planet’s surface can remain liquid — are close to the star, and because of that especially vulnerable to the effects of photoevaporation which can result in partial or even total removal of the atmosphere. Harbach LM et al (2021) Stellar Winds Drive Strong Variations in Exoplanet Evaporative Outflow Patterns and Transit Absorption Signatures, The Astrophysical Journal.

2021 Planets too small to host life, such as Mars

“Mars’ fate was decided from the beginning. There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics, with mass exceeding that of Mars,” said Kun Wang, assistant professor of earth and planetary sciences in Arts & Sciences at Washington University. Water is essential for life on Earth and other planets, and scientists have found ample evidence of water in Mars’ early history. Tian Z et al (2021) Potassium isotope composition of Mars reveals a mechanism of planetary volatile retention. Proceedings of the National Academy of Sciences

2021 Planets with a tilt, like Earth, more likely to develop complex life

It is believed that to sustain even basic life, exoplanets need to be at just the right distance from their stars to allow liquid water to exist; the so-called “Goldilocks zone.” However, for more advanced life, other factors are also important, particularly atmospheric oxygen. Oxygen may be one of our most important biosignatures in the search for life on distant exoplanets because it plays a critical role in respiration, the chemical process which drives the metabolisms of most complex living things. Some basic life forms produce oxygen in small quantities, but for more complex life forms, such as plants and animals, oxygen is critical. Early Earth had little oxygen even though basic life forms existed. The researchers found that increasing day length, higher surface pressure, and the emergence of continents all influence ocean circulation patterns and associated nutrient transport in ways that may increase oxygen production. They believe that these relationships may have contributed to Earth’s oxygenation by favoring oxygen transfer to the atmosphere as Earth’s rotation has slowed, its continents have grown, and surface pressure has increased through time. Most of all, how a planet tilts as it circles its star increases photosynthetic oxygen production in an ocean, partly by increasing the efficiency of recycling biological ingredients — similar to doubling the amount of nutrients that sustain life. Too much tilt, like the 98 degrees of Uranus would likely limit the proliferation of life.

Klatt JM et al (2021) Possible link between Earth’s rotation rate and oxygenation. Nature Geoscience.

Cyanobacteria, also called blue-green algae, evolved more than 2.4 billion years ago, churning out oxygen when Earth was inhospitable. Yet it still took an awfully long time — 2 billion years — for oxygen levels to rise enough to enable an explosion of aerobic life on earth. New research shows that earth’s day used to be just 6 hours, not much time to generate oxygen, but when the earth-moon system formed, the rotatation of Earth slowed down to our 24 hours, giving cyanobacteria longer days and more time to generate oxygen

Patel NV (2021) How lightning strikes could explain the origin of life—on Earth and elsewhere. MIT Technology Review.

For life to evolve, lightning may be necessary. Here’s why: “You can have all the ingredients in one place—water, a warm climate and thick atmosphere, the proper nutrients, organic material, and a source of energy—but if you don’t have any processes or conditions that can actually do something with those ingredients, you’ve just got a bunch of raw materials going nowhere. So sometimes, life needs a spark of inspiration—or maybe several trillion of them. A new study published in Nature Communications suggests lightning may have been a key component in making phosphorus available for organisms to use when life on Earth first appeared by about 3.5 billion years ago. Phosphorus is essential for making DNA, RNA, ATP (the energy source of all known life), and other biological components like cell membranes.”

Starr M (2021) We’ve Found The Best Time And Place to Live in The Milky Way… And It’s Not Here. ScienceAlert. Scientific paper here.

More and more, it seems that the existence and persistence of life on Earth is the result of sheer luck. According to a new analysis of the history of the Milky Way, the best time and place for the emergence of life isn’t here, or now, but over 6 billion years ago on the galaxy’s outskirts with the best protection against the gamma-ray bursts and supernovae that blasted space with deadly radiation capable of causing mass extinctions. In fact, two of Earth’s mass extinctions may have been due to supernovae or gamma-ray bursts, including the end-Pliocene extinction 2.6 million years ago, the Ordovician mass extinction 450 million years ago, and the Late Devonian extinction 359 million years ago.

Tyrell T (2021) How has earth stayed habitable for billions of years? Inverse.

It took evolution 4 billion years to produce Homo sapiens. If the climate had completely failed just once evolution would have come to a crashing halt, and we would not be here now. This is not a trivial problem. Current global warming shows us that the climate can change considerably over the course of even a few centuries. Over geological timescales, it is even easier to change the climate. Calculations show that there is the potential for Earth’s climate to deteriorate to temperatures below freezing or above boiling in just a few million years. We also know that the Sun has become 30% more luminous since life first evolved. In theory, this should have caused the oceans to boil away by now, given that they were not generally frozen on the early Earth. Using a model that took into account factors like asteroid impacts, supervolcanoes and more, there was just one time out of 100,000 that a planet had such strong stabilizing feedbacks that it stayed habitable all 100 times, irrespective of the random climate events. On nearly every occasion in the simulation when a planet remained habitable for 3 billion years, it was partly down to luck. But luck alone was never sufficient, and planets that had no feedbacks at all never stayed habitable. By implication, Earth must therefore possess some climate-stabilizing feedbacks but at the same time good fortune must also have been involved in it staying habitable. Theory #1: The Earth has something like a thermostat – feedback mechanisms preventing the climate from ever wandering to fatal temperatures. Theory #2: Luck. The most likely explanation (read the article for why).

2021 Changes in Earth’s orbit enabled the emergence of complex life

Changes in Earth’s orbit may have allowed complex life to emerge and thrive during the most hostile climate episode the planet has ever experienced when most of Earth’s surface was covered in ice during a severe glaciation, dubbed ‘Snowball Earth’, that lasted over 50 million years.

Strickland A (2021) Record-breaking flare erupts from neighboring star. CNN.

A giant flare over 100 times more powerful than any flare our sun has ever released erupted from the nearest star Proxima Centauri (PC) a red dwarf 25 trillion miles / 4 light-years away. Red dwarf stars are common in our galaxy, and often have exoplanets. PC has two, one potentially earth-like. PC is about the same age as our sun, but has been blasting its planets with high energy flares for billions of years, several times a day, potentially stripping the atmosphere away, and making the evolution of life challenging, if not impossible.

Mann A (2020) There’s Something Special About the Sun: It’s a Bit Boring. New York Times.

The sun seems a little less active than hundreds of similar stars in our galaxy, which could play a role in why life exists in our solar system.

Heller, J. 2020. Habitability of the early Earth: Liquid water under a faint young Sun facilitated by strong tidal heating due to a nearby Moon. Earth and Planetary Astrophysics.

The sun is thought to have been 70% dimmer its first 100 million years, so faint the Earth should have been a frozen snowball for up to 2 billion years. That it wasn’t has long plagued astronomers, but now we might have an answer: the moon helped keep Earth warm. When the moon and Earth formed about 4.4 billion years ago, the moon was about 20,000 kilometres away versus 380,000 km now. Earth was also rotating much faster, as quickly as once every 3 hours. These two factors mean the gravitational interaction between the two bodies would have been much stronger – enough to produce tidal heating from the gravitational squeeze. This would have slightly warmed Earth and could have triggered the eruption of volcanoes, giving our planet a thicker atmosphere that could trap more heat. But this may not be the right explanation. Other theories include that Earth had a thicker carbon dioxide atmosphere as a result of the planet being molten following the giant impact that formed the moon, trapping more heat. Another is that the planet’s orbit brought it closer to the sun at times, warming it up, or that the sun had more mass at the time and was brighter than we think.

Longrich, N. 2019. Evolution tells us we might be the only intelligent life in the universe. Phys.org

Complex animals evolved once in life’s history, suggesting they’re improbable. Surprisingly, many critical events in our evolutionary history are unique and, probably, improbable. One is the bony skeleton of vertebrates, which let large animals move onto land. The complex, eukaryotic cells that all animals and plants are built from, containing nuclei and mitochondria, evolved only once. Sex evolved just once. Photosynthesis, which increased the energy available to life and produced oxygen, is a one-off. For that matter, so is human-level intelligence. As far as we can tell, life only happened once. Curiously, all this takes a surprisingly long time. Photosynthesis evolved 1.5 billion years after the Earth’s formation, complex cells after 2.7 billion years, complex animals after 4 billion years, and human intelligence 4.5 billion years after the Earth formed. That these innovations are so useful but took so long to evolve implies that they’re exceedingly improbable. These one-off innovations, critical flukes, may create a chain of evolutionary bottlenecks or filters. If so, our evolution wasn’t like winning the lottery. It was like winning the lottery again, and again, and again. On other worlds, these critical adaptations might have evolved too late for intelligence to emerge before their suns went nova, or not at all. Imagine that intelligence depends on a chain of seven unlikely innovations—the origin of life, photosynthesis, complex cells, sex, complex animals, skeletons and intelligence itself—each with a 10% chance of evolving. The odds of evolving intelligence become one in 10 million. But complex adaptations might be even less likely. Photosynthesis required a series of adaptations in proteins, pigments and membranes. Eumetazoan animals required multiple anatomical innovations (nerves, muscles, mouths and so on). So maybe each of these seven key innovations evolve just 1% of the time. If so, intelligence will evolve on just 1 in 100 trillion habitable worlds. If habitable worlds are rare, then we might be the only intelligent life in the galaxy, or even the visible universe.

2019. New study dramatically narrows the search for advanced life in the universe. Phys.org.

Scientists may need to rethink their estimates for how many planets outside our solar system could host a rich diversity of life. In a new study, a UC Riverside–led team discovered that a buildup of toxic gases in the atmospheres of most planets makes them unfit for complex life as we know it. Accounting for predicted levels of certain toxic gases narrows the safe zone for complex life by at least half—and in some instances eliminates it altogether. Using computer models to study atmospheric climate and photochemistry on a variety of planets, the team first considered carbon dioxide. Any scuba diver knows that too much of this gas in the body can be deadly. But planets too far from their host star require carbon dioxide—a potent greenhouse gas—to maintain temperatures above freezing. Earth included. “To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today,” said Edward Schwieterman, the study’s lead author and a NASA Postdoctoral Program fellow working with Lyons. “That’s far beyond the levels known to be toxic to human and animal life on Earth.” Carbon dioxide toxicity alone restricts simple animal life to no more than half of the traditional habitable zone. For humans and other higher order animals, which are more sensitive, the safe zone shrinks to less than one third of that area. What is more, no safe zone at all exists for certain stars, including two of the sun’s nearest neighbors, Proxima Centauri and TRAPPIST-1. The type and intensity of ultraviolet radiation that these cooler, dimmer stars emit can lead to high concentrations of carbon monoxide, another deadly gas. Carbon monoxide cannot accumulate on Earth because our hotter, brighter sun drives chemical reactions in the atmosphere that destroy it quickly. If life exists elsewhere in the solar system, Schwieterman explained, it is deep below a rocky or icy surface. So, exoplanets may be our best hope for finding habitable worlds more like our own. “I think showing how rare and special our planet is only enhances the case for protecting it,” Schwieterman said. “As far as we know, Earth is the only planet in the universe that can sustain human life.” Source: Schwieterman, E. W., et al. 2019. A limited habitable zone for complex life. The Astrophysical Journal.

Gribbin (2018) Chain of Improbable coincidences

Many things had to go right for us to exist. Serendipity in the timing and location of our home star and planet as well as lucky conditions on earth and fortuitous developments in the evolution of life, resulted in human beings.

Timing. If the sun and earth had been born any earlier in galactic history, our planet would likely have had too few metals to form life. These elements are created during stellar deaths, and it took billions of years for enough stars to form and die to enrich the materials that built our solar system.

Location. The sun lies in a goldilocks zone within the milky way – not too close to the galactic center, where stars are more crowded and dangerous events such as supernovae and gamma-ray bursts are common, and not too far, where stars are too sparse for enough metals to build up to form rocky planets.

Technological Civilization. Once multicellular life arose, the development of an intelligent species was far from assured, and our species may have come close to extinction several times. Evolution doesn’t have a goal of creating intelligence, and if you asked an elephant what the goal of evolution was, she would probably tell you to evolve here extraordinary trunk with its thousands of muscles and consequent exquisite flexibility. And without fossil fuels, we would have the civilization of what existed in the 14^th century. To become who we are today required language, an opposable thumb, the invention of fire, and much more, all very unlikely to have happened, yet here we are.

Williams (2016) If an alien civilization does arise, it will wipe itself out

‘Stargazing Live’ presenter Brian Cox believes the search for celestial life will ultimately prove futile. Cox believes that any alien civilization is destined to wipe itself out shortly after it evolves.

“One solution to the Fermi paradox is that it is not possible to run a world that has the power to destroy itself and that needs global collaborative solutions to prevent that,” Cox said.

The physicist explained that advances in science and technology would rapidly outstrip the development of institutions capable of keeping them under control, leading to the civilizations self-destruction: “It may be that the growth of science and engineering inevitably outstrips the development of political expertise, leading to disaster. We could be approaching that position.”