The Fermi Paradox: Air — Atmospheres as a Great Filter

What if the answer to why we don't hear from alien civilizations is floating around us every day? Today we'll dive into the atmospheric mysteries that might explain the great silence of the universe. Welcome back to Science and Futurism with Isaac Arthur for another look at the Fermi Paradox, the big question of where all the aliens are. Often we reason along the lines of those ancient alien civilizations we would expect to exist simply do not, and that there are a lot of hard steps in the road from inanimate stardust gathering to form a planet, and evolving life on that planet that one day can build rockets to escape that planet and several new ones. We call those hard steps filters, hurdles that must be passed on the road to the stars, and they can be everything from what makes a planet hospitable to life, to that life evolving in a complex brain and tool use. Today we'll be looking at a neglected filter, the role of atmospheres and air retention in the Fermi Paradox and Earth's Origins. Everyone knows it's important to have air, but it's a surprisingly complex topic and also a surprisingly under-discussed one. None sing hymns to breath, but oh, to be without it! Says Yama, the incarnation of Death in Roger Laszny's classic sci-fi novel, Lord of Light, after drowning an opponent who seemed immune to all his other attacks. It's a truly epic novel, set in a distant and isolated colony of Earth in an equally distant future of advanced technology that blurs the lines between science and magic at times, where the ancient settlers of that planet who fought to make it livable live on still in a world peopled by their technologically primitive descendants. The novel is worthy of all the praise it gets, and I won't spoil it further, but the phrase always stuck with me. When we talk about post-scarcity civilizations, higher-tech ones with access to vast resources, I often note that this is likely to always be category-specific, that they have a lack of scarcity of certain resources or certain problems, like they defeated famine or disease, but still have concerns over personal happiness or long-term security. They all live like kings but have a scarcity of leadership positions. Air is a great example of something we have no scarcity of already. Humanity has often had an abundance of water, even fresh water in some places, and deserts in others. But air was everywhere, and no one really thinks about it, or if they do, it was concerns over air quality, which while they could get pretty serious, even pre-industrial era and smog never included simply running out of it. You cannot live without certain things, but can go without for a while, some longer than others, and water is one you can't last long without, even compared to food. But air is orders of magnitude more time-urgent, you can only last minutes, not hours or days without it. Interestingly enough, as we'll explore today, it's also probably the least stable aspect of planetary habitability. We think Earth is in its third atmosphere by now, if not more, repeatedly replacing it in composition since our planet formed. And as you probably know, we think we'll lose Earth's atmosphere in maybe a billion more years, and think places like Mars once had thicker ones themselves and lost them. How long a planet can keep an atmosphere, and of the same type, that can support life is obviously important to intelligent life eventually arising there. But as you may also have heard, life is twice as old as our modern oxygen-rich atmosphere, and so it's quite possible that atmospheres like our own cannot occur without life, or they might be very inimical to early life forming and thriving. This could mean that there's a very narrow range of planets that not only have the right kind of mass and sunlight and age to be good for life, but also the right atmosphere composition, durations, stabilities, and even instabilities to foster life. When we talk about the Fermi Paradox, we are essentially grappling with why we don't see other advanced civilizations in a Universe that seems fertile for life. A key part of this paradox might lie in subtle but critical influences of atmosphere, air retention, and gravity on planetary habitability, especially as they shape the conditions under which life can emerge and thrive. Earth itself may hold many clues to help us understand why the night skies remain so silent of signals from, or clues of, ancient alien civilizations. We also need to understand from the outset that the atmosphere is far more than just the air we breathe. It's a delicate, dynamic system that does change over time, and acts as both a shield and a cradle for life, from protecting the surface from harmful radiation to regulating temperatures and helping sustain liquid water. The atmosphere is one of the most important features for any planet hoping to host life. However, holding on to an atmosphere, especially one that can sustain life, is a tricky balancing act that may tip one way or another depending on gravitational forces, proximity to the parent star, and many other factors. Now we do not know that you need surface oceans for life to exist, indeed we currently believe that life probably began deep beneath the sea and far from air or sunlight. But after photosynthesis evolved, life on Earth rapidly grew vastly more abundant from organisms being able to harness the sun's light directly as a power supply. And they could only do that where water and other chemical nutrients were also abundant. And to this day, even though for most of Earth's history life exists in the oceans but not on land, most biomass is on land, by an overwhelming margin. The remainder is virtually all in the relatively small parts of the waters that are shallow, like the coastlines, rivers, or lakes, so that the nutrient below is well mixed into the water or reachable by sunlight or by plants that can bridge the gap to get sunlight from above and nutrient from below. The large majority of Earth's surface does not qualify, as it's deep oceans with only very dilute nutrients on the surface. They are essentially deserts while simultaneously being the exact opposite. They have plenty of water and get plenty of rain usually too. Planets without as much storms, which is largely dependent on planetary spin rate and thickness of atmosphere, might see that ecological niche, filled by floating raft organisms, that built upon techniques we see in coral and algae for capturing dilute nutrient from ocean water, and then became the proverbial bedrock of a more complex ecosystem attached or upon it. But that's a topic for another day. Air thickness or density is very important for storm power and erosion strength too. Mars is well known for its dust storms and fast winds, but overhyped in that regard. The wind often moves quite fast, but a hurricane-speed wind that would knock you or I over will feel much weaker on Mars, where the atmosphere is only around a percent or two of Earth's. If a gust of wind is striking you with 1% of the particles, even though it's moving the same speed, it exerts only 1% of the force. On the flip side, a planet with a thicker atmosphere is carrying even more force, contributing to the erosive forces on the landscape. Indeed if we imagine a planet a bit wider than Earth, and decently more massive, with a 24 hour day like we have, it is very likely you would have vastly stronger storms from a thicker atmosphere, retained by that higher gravity, and every raindrop or pebble that hit the ground, or rolled down a hill, would carry even more force from that gravity. That wider planetary diameter with equal day length means it will have a higher Coriolis force, making for more powerful wind and storms too. Indeed as we discussed in our Fermi Paradox episode on Large Moons, unless this bigger planet itself had a big moon, it would probably be rotating even faster than once a day. Earth used to rotate faster and our Moon be much closer to us, but the effect of time and tidal forces slowly caused the Moon to drift away and leeched angular momentum off of Earth, giving it a longer day and slower and less powerful Coriolis storms. We think Earth had a 12 hour or shorter day originally, and the other planets larger than us have shorter days too. Uranus, the next largest planet, takes approximately 17.2 hours to rotate once, Neptune is next biggest and its day takes about 16.1 hours. Saturn rotates in around 10.7 hours and Jupiter, biggest of planets, has the shortest rotation period, completing a day in just about 9.9 hours. It does not actually follow that a planet's spin rate is determined by its mass, there's no simple correlation there, but there are a lot of overlapping factors. The same will be true of atmosphere and also atmosphere leakage, as we'll get to in a moment. For now, it is important to see all the different ways atmosphere characteristics can shape a planet and the life forming or evolving on it, not just by giving it air to breathe or liquid water on its surface. And you can only have liquids where there's a pressure present. In basic terms, the three normal, basic states of matter, gas, liquid, and solid, are really just gas and solid. Molecules often have some force attracting them together when near but it's not very strong. As a result, things with higher temperatures, meaning they are moving around with more speed and momentum, tend to bounce off each other rather than stick together and form nice stable crystalline lattices as many solids have. If you lower their speed and thus their temperature, they don't collide as hard or often, and those attractive intermolecular forces can overpower that bumping around and colliding and bind it into a stable and solid object. Hence a cloud of steam can be turned into an ice cube, in space or other near vacuums, that's it, a material is solid until it gets hot enough for the individual molecules to fly off and disperse in a cloud. If it's cool enough gravity can grab it and pull it together to form stars and planets, but more to the point, if a bunch of other matter all around you in a cloud, you no longer just have the heat energy of those individual molecules bouncing around versus intermolecular forces trying to bind it into a rigid solid. You can get an in-between state where the outside pressure of other molecules shoves back against those free molecules trying to bounce around, so that you now have an external pressure pushing that material together on top of that intermolecular force. Thus we get a phase of matter, liquids, where free molecules glob together, pulled on by intermolecular forces, and pushed outside by other materials, like a pressurized atmosphere. If that combined force is not enough to keep things together, the matter boils into a gas. If it's high enough, you get a solid, and in between, not too hot not too cold, just right, then you get a liquid. This is a generalization incidentally, water is a weird duck in that its frozen form is less dense than its liquid form, which is why ice floats on water, so that outside pressure still makes it a liquid instead of a gas. But in ice format, you actually lower its freezing point with pressure. Indeed this is what makes ice slippery, you step on it, and that brief local increase in pressure liquefies that surface layer. Water's weird properties in relation to the Fermi Paradox merit their own episode, but not this one. So we raise it in regard to water evaporating to replenish the atmosphere that leaks away. Liquids are obviously very important to allow chemistry and thus life to occur, but the handy thing about them is that the boiling temperature decreases, and the evaporation rate increases as we lower pressure. Water boils at 100 Celsius or 212 Fahrenheit, and freezes at 0 Celsius or 32 Fahrenheit, at normal Earth Sea Level Pressure. As we lower pressure, by going up higher in the atmosphere like on mountaintops, or by atmospheric loss over time, that boiling point will drop too. If you're curious, by the time you're down to 6% normal Earth Pressure, it will boil at room temperature, 20 Celsius or 68 Fahrenheit, and this is a very common Physics 101 experiment to do for classes that many of you have probably seen, and was always a favorite of mine to conduct. We get out a glass of water and a bell jar in a pump, and lower the pressure in there till the water starts boiling and frothing. So, if a planet begins losing its air, and the pressure drops a bit, liquids on its surface, presumably water, will begin to evaporate faster and replenish that pressure with their vapor. A planet losing air will eventually end up as a desert, as the water will keep replenishing the atmosphere, but while the seas will eventually disappear, this doesn't mean all the water does. Mars has ice caps and probably subterranean water too, and lots of icy moons probably have seas below, like Jupiter's moons Europa and Callisto are thought to. Now, how does atmospheric loss work? Well, it's several different mechanisms in play for both stripping atmosphere away and for mitigating it, and some are losing it into space while others are sequestering it into rock below. Most rock, dirt, and sand, is oxygen plus something else, often more than half oxygen by weight. Water is 89% oxygen by mass, and sand, mostly silicon dioxide, is 53% oxygen by mass. Oxygen likes to interact with other things and bonds a lot, so molecular oxygen, two oxygen atoms alone together, is actually rather unusual for oxygen. We also need to keep in mind that most of our atmosphere is not oxygen. Oxygen molecules are only 21% of the particles in our air, or 23% by mass, as most of the remainder is made of molecular nitrogen, which is slightly lighter than oxygen, 78.1% of the particles in the atmosphere but only 75.5% of the mass. Most of the tiny amount remaining is argon and carbon dioxide which are both heavier than air, and of course temporary water vapor. That mass fraction versus particle fraction tends to result in a lot of confusion about the makeup of objects, be it sea, sky, or the stars themselves, when we will say that a certain percent something or other but not specify if it's a percent of the particles, the mole fraction, or a percent of the mass. Our Sun for instance is 92% hydrogen by particle, but only 73% by mass, and slowly loses hydrogen as it fuses into helium but also as it blows away in the solar wind, as hydrogen being lighter can escape more easily from the Sun, and similarly from Earth, which is why hydrogen is the most abundant atom in the Universe but relatively uncommon on Earth, and helium, which bonds to nothing, is the second most abundant in the Universe, and in our Sun, but virtually non-existent on Earth. In the Sun there are no molecules, too hot, just atoms or nuclei. On planets you get molecules and they are heavier as they're combined mass, not that of individual atoms in them, that control so they can zip off into space. Unfortunately, radiation and solar wind are good at breaking molecular bonds, often causing hydrogen to find itself all alone in our upper atmosphere, where it can more easily leak into space. Oxygen is much heavier and falls back down to bond with something else, but it leaks out of the atmosphere a lot too, just mostly downwards, as it will bond to other atoms to form rock and water. Indeed its presence and abundance in our atmosphere is relatively new. As mentioned, Earth is already in its third atmosphere, if not more. That itself is a testament to how fragile and changeable these systems can be over geological timescales. Even now, Earth is slowly leaking its atmosphere into space. This loss is mainly due to the interplay of gravity and solar wind, streams of charged particles emanating from our Sun. Solar wind plays a significant role in stripping away atmospheric particles, particularly lighter elements like hydrogen and helium which are critical for life as we know it. At least hydrogen is. But why is it that Earth has managed to retain enough of its atmosphere to support life while a planet like Mars, which is smaller and has weaker gravity, has lost most of its once-thicker atmosphere? Gravity holds the key here. Planets with higher mass exert a stronger gravitational pull, allowing them to hold onto their atmospheres more effectively. Earth's gravity combined with its magnetic field helps retain the atmosphere by shielding it from solar wind. Without that magnetic field, particles in the upper atmosphere would be bombarded by high energy solar wind particles and gain enough momentum to escape the planet's gravitational pull. We talked about that more in our Fermi Paradox gravity episode last year, including that it isn't really the molten core of a planet that generates its magnetosphere, but rather its spinning outer core. Even widening that core by a tiny amount has a large effect on the strength of that magnetosphere. This gets misconstrued as some rapidly spinning metal ball in our center. Our core does indeed rotate faster than the surface does, at an angular speed, but it isn't a lot. We're not sure the exact speed, but we think it's only about one or two more rotations per millennium than Earth's surface gets, which means its own day is about a tenth of a second shorter than the surface. It isn't that it rotates separately than the surface, the difference there is trivial, though the speed it turns at is not and is based on how fast we spin, which rises as you get out from the core in terms of tangential velocity. The mantle would generate tons more magnetic field except it is made of silicate minerals that are not nearly as conductive as the liquid metals in the outer core. The inner core doesn't spin as fast as it is thought to be solid too, which doesn't help. The Earth's magnetic field is primarily generated by the geodynamo, which occurs in the outer core. The rapid convection of electrically conductive liquid, iron and nickel, combined with the Earth's rotation, creates circulating electric currents. These currents generate a magnetic field according to the principles of electromagnetism. This matters a lot because people would toss around the idea of nuking Mars' core so it starts its dynamo back up. This would not work. Beyond being nigh impossible, even by this channel's high ball for throwing out that word, and we discussed better methods in our episode giving Mars a magnetosphere and also in our more recent episode focused on Restoring the Martian Core, the magnetosphere deflects incoming hydrogen particles from the Sun that are moving with far more speed than Earth's own escape velocity. If they hit another particle in the upper atmosphere, they usually share to it enough momentum to let both of them blast into space. If on the way out one of them bumped into another particle, it might slow them enough to stay or bounce them back toward Earth. So escape happens best in the upper atmosphere, way, way over even the mountaintops, where the air is ultra-thin. But that magnetosphere not only deflects ionized hydrogen from the Sun away, it can also deflect a struck hydrogen particle in our atmosphere back down in many cases, thus magnetospheres do double duty in atmospheric retention and the stronger the better. Hydrogen generally goes first along with helium, but heavier atoms like nitrogen can go too in weaker gravity and as the air thins. Note that the speed of solar wind is not significantly diminished the further those particles get from the Sun, they thin out in numbers by cross-section but not by individual potency. Thus this particular type of atmospheric stripping, and there are many kinds, works even on distant frozen worlds. Temperature matters, so does gravity, but often the sheer kinetic energy of a solar wind particle is just too immense for those to matter as much. Thus on smaller worlds like Mars, this hydrogen leakage process happens much more quickly, as do the other leakages. Mars, being just 10% the mass of Earth, lacks both the gravity and the magnetic field to effectively shield its atmosphere. As a result, it has lost most of its atmosphere over time, leaving it cold, dry, and inhospitable. Venus, though similar in size to Earth, tells a different story. Its atmosphere is thick and heavy, filled with carbon dioxide, yet it is also hydrogen depleted. Despite its thick atmosphere, Venus has lost most of its hydrogen due to the Sun's radiation and the planet's slightly lower gravity. Venus turns very slowly, taking 243 Earth days to turn, which contributes to it having a very weak magnetic field. The exact state of Venus's core, liquid or solid, is unknown. On Earth, the geodynamo effect of the movement of the liquid outer core generates a strong magnetic field. Venus does not have a strong magnetic field, which might suggest its core does not have the same convective motion as Earth's liquid outer core. Regardless, Venus clearly has an atmosphere, indeed it has about 100 times more atmosphere than Earth has, and also again one very depleted of hydrogen. As a result, it has plenty of nitrogen, and oxygen, and carbon for an Earth-like atmosphere, but doesn't have anything we could breathe. It is only 3.5% nitrogen, versus our own atmosphere which is almost all nitrogen. But since it has 100 times our atmosphere, it has about 4 times more nitrogen than we do. But the rest is mostly carbon dioxide, there's little free oxygen, and of course it's boiling hot and rains sulfuric acid. Venus has visible clouds so we used to think it was a paradise planet, later we realized they were made of sulfuric acid and that Venus was less a paradise and more a suburb of hell. Either way, Venus's example shows that atmospheric retention is not just about size, gravity, magnetosphere, or even temperature. Atmospheric retention is a complex equation involving many variables, including proximity to the parent star, atmosphere composition, and whether the planet has a magnetic field to shield it from stellar winds. The Fermi Paradox may in part be explained by this, many planets, here they began with atmospheres capable of supporting life, may have lost them before life had a chance to evolve. Or worse, life may have emerged, only to be wiped out by the planet's failure to maintain an atmosphere long enough to sustain complex organisms. So what did that atmosphere used to look like? Unsurprisingly it began with a lot more hydrogen. As we discussed in our episode Primordial Planets, we have limited details on the atmosphere before the Moon's theoretical formation since the geological records were destroyed by our impact with a dwarf planet that ripped our whole crust off, but we suspect it went through a change of atmosphere even before then. Hydrogen is the most abundant element in the Universe and a key building block for life. It is also the first to escape a planet's atmosphere. Lighter gas like hydrogen and helium move faster at a given temperature, and on planets with weaker gravity they can easily reach escape velocity and leave that planet behind. They also absorb more speed from collisions with other particles transferring momentum, like solar wind or even high energy photonic radiation, so they escape fastest, and this is particularly true for planets that are closer to their stars or have hotter climates, where atmosphere particles gain more energy and are more likely to escape. Earth's hydrogen loss has been mitigated by its magnetic field and its ability to keep much of its hydrogen bound in molecules like water. Water vapor, when it rises into the upper atmosphere, is subject to being broken apart by ultraviolet radiation from the Sun. The hydrogen, being lighter, can then escape into space, while the oxygen may remain in the atmosphere. As discussed, over time this process slowly erodes the amount of water a planet can retain, a problem Earth may face in the distant future. And while we can imagine other types of life, like those based around ammonia or methane, over water and carbon, ammonia and methane, like water, are made with hydrogen. A planet lacking them, or any abundant solvent liquid, that has also exposed the Sun for energy, as a planet without an atmosphere cannot have, cannot plausibly have a robust and diverse ecosystem. Only small and shallow ecosystems that would have problems supporting much life where evolution may be slowed by that smallness too. Life in the oceans, without sunlight penetrating the thick atmosphere, would likely remain primitive, clustered around hydrothermal vents at the ocean floor. Instead of vibrant ecosystems like Earth's, these worlds might harbor only sparse, microbial life in the dark depths of their oceans. This would severely limit the potential for intelligent life to evolve, much less develop technology or spacefaring capabilities. Effectively serving as one of the many filters proposed in the Fermi Paradox. Regardless, on earlier Earth we would expect more hydrogen initially, though don't assume we ever had a thick blanket of it. As planets form, their escape velocity rises as they clump up bigger and grow in mass. But every particle coming down on this forming planet begins landing with more and more energy as it experiences that increased gravity. This is why celestial bodies can't form in hot gas regions, where the particles move quickly and are low in density, but instead in thick and cold clouds. Now objects falling in obviously can't have more energy on average than they got while dropping. Balls don't bounce higher than they fell from unless they not only fell but were thrown down. So planets form hot but keep most of their material. Still much of them will consist of hot hydrogen which moves faster than bigger particles the same temperature. So the upper atmosphere of a newly formed planet will probably be blowing off hydrogen even while the planet is still forming. The planets are also younger than our Sun, and by millions of years. Jupiter is thought to be the oldest, Uranus and Neptune may be the youngest, and Earth is probably about 50 million years younger than the Sun. As such, it was already experiencing solar wind, atmospheric erosion, before it was done forming, and so we probably had already lost countless tons of hydrogen before we even stabilized as a planet, and would have been ultra-hot then too. Often the Fermi Paradox is a question about missing history. What makes Earth unique that we seem to be alone? And if you want a grounded, visceral reminder of how much history can fit inside one planet, watch Joe Scott's documentary, Oldest and Newest Places on Earth. From a 4 billion year old rock in Canada, long predating our current atmosphere, to brand new land forming right now in Hawaii. Many of you already know Joe's work, he's a long time friend of myself and this show, and he's just one of the many creators who also helped create Nebula, home to tons of amazing content from him, me, and many others. Right now Nebula's earning a huge discount, with my link or the QR code on the screen, you get 50% off a yearly plan, which is $30 for the whole year, about $2.50 a month, and new members get a 3-day free trial, so you can check out all of SFIA's content, including our monthly exclusives, full-length SFIA episodes only on Nebula, like Proto-Engineering, for free. Head to go.nebula.tv slash IsaacArthur and start watching today. We don't know much about this era yet, but it clearly wouldn't be hospitable to life and was temporary anyway, so need not concern us today. Except that if the pre-lunar atmosphere was very different in composition than what was restored after the dust settled, or after the hypothetical cometary bombardment millions of years later, we might have a very different atmosphere today, or at least for many millions of years after, which may have delayed or prevented life arising. And since dwarf planets standing to newly formed planets is probably fairly uncommon, this would presumably impact the Fermi Paradox and the strength of atmospheres as a filter. Though emphasis on fairly uncommon, it probably isn't a rare event either. I think we need to acknowledge that air breaks up into a few separate filters that are probably only loosely related. Initial mix, what the mix is or needs to be when life emerges, susceptibility to local sunlight, which will vary by the star and also change over time as stars slowly brighten with age, susceptibility to geological and hydrological cycles, which will also vary by planet and change over time, and susceptibility to change with biology. The latter would seem reasonably safe as biology ought to alter it only to what it can survive, but we also need to keep in mind that biology, without even contemplating intelligent tool users like humans, can radically change an environment and quickly. One new microbe or plant that's amazingly good at storing water and keeping it, or can survive in permafrost, could suddenly green whole deserts or tundra and suddenly might be mere centuries. That all by itself could cause a huge change to atmosphere mix, same for if an organism managed to adapt to grow on top of oceans like a raft as we mentioned earlier. We mustn't ever assume atmosphere is static, even on short timelines, and you could have a distant binary partner sun that orbited once every thousand years on a strongly elliptical orbit that massively altered that planet's atmosphere both by its radiation and by tugging along Kuiper or Oort Belt objects that reigned on the planet. Or even organisms that lived in those falling meteors like we see in Anne McCaffrey's classic Pern series. As mentioned, our atmosphere got ripped off when the Moon formed, and later was restored, we think, by a mix of volcanic outgassing and cometary bombardment. Later it would gain free oxygen as the land and sea would get so saturated with it as a byproduct of photosynthesis that it couldn't absorb any more oxygen, or at least not as fast as it was accumulating then. The actual oxygen content of Earth's atmosphere has fluctuated a lot since then. We tended to assume it was pretty static, same for ocean salinity, but evidence seems to point otherwise, and same for things like carbon dioxide content and overall planetary temperature, which atmospheric makeup can impact, as things like volcanoes form tectonic plates colliding, which itself can alter atmospheric makeup too. Earth is thought to have had entire apocalyptic eras of tens of thousands if not millions of years after major new tectonic collisions occurred, or the seas froze over, or we got whacked by a big asteroid or passed near a supernova. A lot of planets would have different frequencies of such events, quite likely much higher, and as we discussed in our looks at our Sun and solar flares, our Sun is much more stable than most stars, though we don't really know how stable for how long, at least not with great certainty. This brings up our last two air filters, whether or not technological civilizations can wreck their atmosphere, and will, and whether or not planets can keep their atmosphere long enough for life to get to such a stage. The former is easier to speak to, though mostly in the sense of vagueness. It is obviously possible to wreck an atmosphere if you want to, we discussed creating or altering them with terraforming technologies in many other episodes and there are brute force methods that will work. Doing it by accident or because your planet is very much on the tipping point will depend on a lot of factors specific to a given planet or a given technology. For instance, some planets might simply be too tectonically active to have any massive supplies of fossil fuels or deep methane or natural gas pockets. Those worlds have their own possible specific filters, like difficulties developing big technological and industrial bases without such fuels, but that's a different topic for another time. Without having more exoplanet data, specifically atmospheric and geological data, we really can't say how big a filter this is. It might be minimal or a major filter, obviously getting that geological data is at least centuries ahead, since you have to go there in person and start digging and those are long trips. We also examined the fossil fuel question in more detail last month in our Fermi Paradox and Fossil Fuels episode. The other air filter is also a bit hard to speak to, and indeed most of the Fermi Paradox filters around air are as hard to grasp and look at as the air itself. This is if most stars can allow an atmosphere on a planet that includes both water and oxygen long enough to allow advanced life to form. Any star with a lifetime much less than 5 billion years can probably be ruled out, though that's only a percent or two of stars as our Sun is itself on the large side, the top 5% or so in mass, and the bottom 5% in lifespan. As stars age they grow hotter and their solar wind intensifies, which accelerates the rate at which planets lose their atmospheres. Even Earth, with its robust gravity and magnetic field, is not immune to this process. Over time the Sun will get hotter, causing Earth to lose more of its atmosphere and oceans, eventually rendering the planet uninhabitable. This is a process that will unfold over billions of years but the implications are profound for planets orbiting stars that burn hotter and faster than our own Sun. Planets around more massive stars experience faster solar wind and higher radiation levels, which can strip away atmospheres much more quickly than on Earth. In these systems, life may only have a brief window to emerge before the atmosphere is lost or rendered uninhabitable by runaway greenhouse effects or radiation. This rapid loss of habitable conditions might explain why we see no signs of advanced civilizations, but again that's a small minority of stars. The problem is those smaller stars that live longer tend to have higher variability in their solar wind output, and planets need to be closer to them to be warm, often a lot closer. Most stars in this Universe are only about 1% of our Sun's brightness or less, which would mean being 10 times closer to their Sun, which makes for more tidal locking and narrower windows of habitability. Indeed, planets close to stars will tend not just to tidally lock but bleed off their eccentricity, either end up in near-circular orbit or strange residents like Mercury or getting kicked out of their solar system or swallowed by that star. If they do end up tidally locked or even just with very long days, that's going to make maintaining an atmosphere harder too. But there's a lot of unknown unknowns there too, and we looked at colonizing red dwarfs in their own episode. It is possible that you don't need hydrogen or water on your surface, and to evaporate to later rain on land. It is also possible you don't need free oxygen in your atmosphere for animal life. We've looked at options like non-carbon based life, silicon life, ammonia based life, and even crystal based life in their own episodes, but as we saw in them, it's a heavy lift. Especially for fast evolution to intelligent animals like us. Some might reason air composition isn't critical, and indeed that may be the case, but I wouldn't hold my breath on the matter. In the end, atmospheric retention, makeup, long-term evolution of that atmosphere, and air loss are probably some of the most critical factors in determining whether a planet can support life, let alone intelligent life. The delicate balance of conditions that allow Earth to foster life and maintain its atmosphere may be far rarer than we think. Many planets, even those with the potential to start life, may lose their atmospheres before life has a chance to evolve into intelligent species, or the conditions of their atmosphere may inhibit the development of complex ecosystems. In the context of the Fermi Paradox, these planetary filters, whether they be from atmospheric loss or the rapid destabilization of environments, could mean that intelligent life is far less common than our imaginations might hope. Earth itself may be the result of a series of lucky breaks from its initial conditions to its ability to retain an atmosphere long enough for intelligent life to emerge. If these processes are as finely tuned as they seem, then the silence of the Universe may simply reflect how fragile and rare the conditions for life truly are.