Human beings have always found themselves thinking about life on other planes, other places, other worlds. Now, we know that these other worlds, orbiting other suns, exist and are common. In less than two decades, a revolution has taken place: thousands of planets outside the solar system have been discovered, and we've only just begun to study them.
A new field called astrobiology, or exobiology, has been busy asking questions about the possibility of life on these newly discovered planets. Today, let's understand what a planet needs to be habitable, and how common life must be in our universe. But for that, we will need to understand why the earth lives and breathes, and our sister planets have become just inhospitable corpses.
If I say the word alien, chances are something like this will fill your imagination: a little green, big-headed man with big eyes. But that's a really bad image. .
. because that's not an alien, that's a person, green. Literally.
The lack of creativity involved in this is appalling. For most people, aliens are basically us, again. And green, for some reason.
But what are the chances of something so similar to us happening again? When we look at the history of nature and realize the infinite amount of details that have determined how our world is today, it seems that life is an unreplicable experience. But it is a mistake to look at things backwards.
If one small detail had been different, and the butterfly effect had done its job, an entirely different end result would also have depended on chance. What I mean by that is that ours is not the only possible story, and we cannot look at events as if they happened so that we exist today, or that things were like this. Just to give you an example of all this, I want to introduce you to an animal.
This is Suminia. He was an ape before there were apes. And it wasn't a little earlier, but 240 million years earlier, whereas most modern apes are around 20 million years old.
It's not 2, not 20, it's more than 200 million years apart. It had large eyes, a strong, prehensile tail, and opposable toes, adapted for life in trees. It was the first arboreal tetrapod.
He lived in the forests of the Permian, the period that ended with the greatest mass extinction that life on Earth has ever suffered, with more than 95% of species lost. That is, if something lived in the Permian, it probably didn't survive until the Triassic. This was the case for Suminia, which was part of the most dominant group of continents at the time: the synapsids.
Some synapsids like Dimetrodon are mistaken for dinosaurs, but they are pelycosaurs, and are more closely related to us than to dinosaurs. Sumina was an Amonodont. We mistakenly called this group mammaliform reptiles, and although this conveys the idea well, it is wrong, because they are not reptiles, but synapsids.
The vast majority of them were wiped out in the pro-Triassic passage, but not all. This extinction opened niches that dinosaurs would later occupy. Only two lineages survived the catastrophe: the dicynodonts, which thrived in the early Triassic but also became extinct, and the cynodonts, which are the ancestors of all living mammals.
After many extinctions, the descendants of cynodonts exist in three major groups: placentals, like us, marsupials, and monotremes. Suminia, like platypuses, must have laid eggs, and had fur like a mammal. When he lived, true mammals were still 80 million years in the future.
Even so, its evolutionary convergence with apes is impressive . It means that two separate lineages of animals acquired similar characteristics without the need for a common ancestor to have them. It's the same evolutionary forces acting on independent histories.
This means that if the extinction that marked the end of the Permian had never happened, and dinosaurs had never existed, Suminia could have specialized and diversified more and more, giving rise to several species. And perhaps, one of their descendants could have explored the path our ancestors took: and become bipedal and humanoid. They could have been us, 260 million years before us.
But Earth, had other plans, and the civilization of Suminias descendants never happened. This imagination exercise involves a value in the history of nature: contingency. Things are absolutely chaotic and ruled by chance.
Evolution is blind, and has no direction. There may be patterns that repeat themselves, but because beings need to face the same challenges with the same physical and chemical limitations, but no preference that motivates nature to seek to reach a humanoid form, as if it were the end of a process, or the more advanced way. This is very anthropocentric.
We're going to need to get ourselves out of the center of things to be able to talk about another story of another life on another planet. The closest life has ever come to a humanoid form was with Suminia, yet this convergence happened within a specific group of tetrapods, and it didn't even fully roll out. So humanoid beings only actually evolved once on Earth.
How about thinking about the chance of a crustacean evolving into a humanoid form? That would indeed be evidence that this could be common. But as far as we know, crab man never existed.
So when we talk about aliens, it's good to replace the little green man with something more common and less sensitive: a bacteria. In the beginning, humanity believed that Earth was the only planet, and that the sky was a painted dome. Gradually, we discovered that the universe was getting bigger.
The discovery of other stars, other galaxies, shook humanity and launched the idea that life must be something common in this immense cosmos. Now, we have come to understand that although immense, the universe is an extremely dangerous and chaotic place, with events so powerful and destructive that it defies the imagination. That leaves very few places eligible for harboring life.
At least life similar to what exists on our planet, something we can recognize. We want to discover planets teeming with life, with large beings that run, fly, live in groups, and who knows, build civilizations! But the big chance is that life on other planets will closely resemble life on Earth during its first 4 billion years of existence.
This eternity of time that passed from the formation of the Earth to the first animals and plants is collectively called the Precambrian. Think about it: the entire history of animals is about 600 million years old, and for 4 billion years, days have come and gone, ages have come and gone, and Earth has remained the planet of bacteria. This can fundamentally mean two things.
The first is that the Earth for much of the time was inhospitable to macroscopic and multicellular life. And the second is that life doesn't necessarily tend toward complexity. Evolution was in no hurry to produce a tree or a fish once life began.
So the most common, least demanding and most resistant form of life that lives among us is bacteria. They are a good reference of what to look for. According to Peter Ward, author of the book “Rare Earth: Why Complex Life Is Uncommon in the Universe” microscopic and single-celled life must be more common than we think in the universe.
It could be that 99% of the time life arises, it remains very much like bacteria or archaea, until the planet dies, or conditions are no longer habitable. Perhaps the most common type of visible life is stromatolites. They are the ultimate living fossil.
Stromatolites are domes that look just like rocks but are alive. They were common for billions of years in the age of bacteria, and they still exist today in the brackish waters of Shark Bay, Australia. They are biosedimentary structures that form when a mat of photosynthetic algae is buried with sand, sends small filaments up, and recolonizes the surface.
This constant alternation between layers of biofilm and sand forms huge domes of stromatolites of various shapes. And perhaps, alien stromatolites are one of the most common forms of life beyond Earth. Animal life, in the sense of animation, of animated and macroscopic life, which is able to feel and move in the environment, must be something very rare, only reserved for an elite few of planets that won the lottery.
Because that kind of life is much more demanding and much more delicate. You are literally trillions of times bigger and heavier than the bacteria that live in your gut, which are much more like the first living things than you are. This makes Earth a rare place where conditions are stable and ideal long enough to sustain the long evolutionary history of many multicellular lineages such as animals, fungi and plants.
But what does it take to be an Earth? Surprisingly, one of the most important things for the possibility of life is the age of the universe itself. To be able to support life, the universe has to be of the right age, neither too young nor too old.
I'm going to call this fertile phase of the universe the "reproductive age". If the universe is too young, heavier elements like metals won't exist or won't be common enough to form rocky planets like Earth. Initially, the first atoms that formed the first stars were very simple, they had one proton, and one or two neutrons.
We call it hydrogen. In the core of the first stars, the pressure and temperature were so excruciating that the hydrogens began to fuse, releasing energy and forming a heavier element: helium. During the aging of the first stars, helium was transformed into carbon, then oxygen, until in their violent deaths in the form of supernovae, much of the periodic table came into being.
Most of the atoms that make up our planet and our bodies can only be produced in a star's core or during its death. That's why Carl Sagan said we're all made of stardust. This means that the sun formed from a cloud that had debris from explosions from past generations of stars.
If the universe were too young, the carbon and oxygen atoms on which life is based would be too rare for life to incorporate. As the universe ages, the fuel is being burned and converted into heavier elements , that is: it becomes more metallic. But the metallicity of the universe has to be ideal, it cannot be too low to the point of preventing rocky planets from being common, nor too high to the point of hindering the generation of stars.
If the universe is too old, it will be so metallic that few stars will be born, and stars are the source of all the heat and energy that powers life. A universe with fewer stars is a universe with less life. There is another problem that makes it difficult for an old universe to be habitable: radioactive decay.
Radioactive elements don't last forever, they go through phases of stability before breaking down into a lighter element. They have what we call a half life, which is the time it takes for half of the atoms of a given element to radioactively decay. This process releases radioactive energy, which produces heat.
This is actually the source of much of the heat in the interior of our planet. Without radioactive elements in the Earth's interior, our planet would have already solidified and cooled much more, and we would be much more like Mars. As much as the half-life of these elements is counted on the scale of millions and billions of years, it is important to put our moment in perspective.
The universe began 13. 8 billion years ago, which means it was 9. 2 billion years old when Earth came into existence, and dozens of generations of stars had already existed.
But how metallic and radioactive will the universe be in 100 billion years? We still don't know what is the optimal point at which all these factors end up producing the moment when the universe has more homes for life, that is, the most habitable point in time. Perhaps we are late, and much of the experience of evolution has lived and gone, and we are in a universe full of ghost planets.
Perhaps, these civilizations have visited us in the past, and found a completely different planet. But I believe that life on Earth is actually one of the first complexity experiments in evolution. The universe is so young, and the very existence of rocky planets so new, that perhaps the glory days of our universe are yet to come.
Perhaps the next tens of billions of years will be marked by a peak in the number of living planets in this universe. Perhaps this is what explains the lack of evidence of other civilizations. Maybe we're the first to the party, and there are cauldrons of life out there that won't be communicable until billions of years from now.
The universe is an extremely chaotic place, full of things that don't like life. Most places are too gassy, too vacuum, too hot, too cold, or have too much death rays. So where the solar system is is matters a lot.
Had it been too close to the center of the galaxy, Earth would never have developed life. The centers of galaxies squeeze most of the stars into a great orbital chaos. In this environment, stars would pass close to the sun all the time, destabilizing the orbits of all planets.
There would also be a storm of all kinds of electromagnetic radiation emitted by the multitude of stars and black holes. If the galaxy were a country, its center would be the metropolitan area of São Paulo, only being bombed. The solar system, like almost all stars visible at night, lies in the Orion Arm of the Milky Way.
A small arm between the arm of perseus and that of sagittarius. In the analogy with the country, we would live in an isolated house in the middle of the pastures. This is important, because it means that billions of years can pass without any stars passing through the vicinity, and the sun can follow its trajectory orbiting the center of the galaxy without disruptive gravitational perturbations.
So it's good for life that it evolves around a star on the periphery of the galaxy, or even outside of them, in intergalactic space. Some stars and their planets are flung out of galaxies in chaotic gravitational interactions, most notably during the merger of two galaxies. Intergalactic space is not a place where many stars are born, but planets that end up there may be free of a series of space tragedies that would sterilize the planet.
Small changes have huge consequences in the history of evolution, and the stability of Earth's climate was critical to the success of life here. Our sun has a cycle of 10-13 billion years, and during much of that time, it will undergo transformations so brutal that Earth will become uninhabitable. It means that today, we orbit a middle-aged star with a short lifetime.
Not as short as the lifetime of mega stars that last only a few hundred million years, but nowhere near as long as the lifetime of a dwarf star, which can shine for hundreds of billions of years. The bigger a star, the faster it lives, and the more violently it dies. It may be that our type of star is not the best fit for life.
It's likely that our sun gives about 5 to 6 billion years for life to enjoy itself, counting the 4 that have already passed. But what would life be able to do on a star that remained stable for 50, or 100 billion years? We don't have any examples of this yet, because the universe isn't even 14 billion years old.
But it may be that these are the great oases of the universe. Places even more livable and safer than Earth, where life can thrive and remain. Or perhaps Earth is the exception of the exception, being able to support life for billions of years, in a universe where the most common fate is early extinction on fast-dying planets.
Our own solar system is not ordinary. Most gaseous planets outside the solar system are hot, close to their star and less dense than Jupiter. Another unusual thing is that all the most gravitationally relevant orbits around the sun are very circular and not very elliptical.
This lessens the risk of large gradual changes in orbits caused by gravitational resonances. If Earth's orbit were more common in the solar system, it would be more eccentric, meaning that the difference between the distance minimum and maximum distance from Earth to the Sun over the year would be much greater. In the long winter, temperatures of -100 degrees and a much dimmer and distant sun, and a short summer of violent sun with temperatures of 200 degrees on the surface.
Not even most bacteria would survive a planet with a very eccentric orbit. The homes of living beings in the cosmos probably have circular orbits. Just because a planet is in the habitable zone doesn't mean it is habitable.
Look at Mars, look at Venus. Both are on the fringes of the habitable zone, but something has happened to these planets. Something was missing, which prevented them from staying alive.
In the first billion years of the solar system, Venus, Mars and Earth were the rocky sisters in which life could shelter. Venus wasn't always a carbon dioxide hell with sulfuric acid rains AND Mars once had rivers, oceans, active volcanoes and a cloudy sky. But it wasn't to last.
For a while, in the beginning, Earth was the troubled sister, and probably the most inhospitable to life. She was still recovering from a childhood trauma: a collision between two protoplanets, protoEarth and Theia, which ended up producing Earth and the Moon. This shock happened in the final phase of the formation of the planets, and placed the Earth in a state similar to its initial state of formation, delaying its cooling by hundreds of millions of years.
During that time, Mars and Venus were more stable and pleasant places, despite the annoying frequency of meteor impacts, which would have made animal life impossible but was tolerable for bacteria. This is the late bombardment phase, and it may have been important in transporting life from one planet to another. According to panspermia theorists, life could have arisen on Earth, but also on Mars or Venus.
Perhaps at some point, she inhabited all three planets simultaneously, and only Earth offered a future for living beings. It would be carried by fragments of ice and soil that are thrown into space during powerful meteor impacts, which were very frequent. When contaminated material landed on another planet, it ended up being colonized.
There is no solid and direct evidence for panspermia, but it is a possibility within the limits of science, which explains how life survived the most violent phase of Earth's history, and why it appeared so early. It seems that life appeared as early as possible, as soon as the oceans stopped boiling, while the Earth was still a toxic hell. Let's imagine that Earth is a house, and life is a happy family living in it.
Intuition says that first, land is deforested, then flattened, and then comes the foundation, the house is built, finalized, so that a family can live comfortably, right? That's not what happened to our family of life. It's as if the family were already in the middle of the bush, and had somehow survived the tractor going by, and hadn't left there during the entire job.
But we think that way because we tend to think that life only exists here because Earth is a benevolent and perfect place. When in fact, if we found a planet similar to what Earth was when life first appeared, we might even find it promising, but we wouldn't want to visit. And she could have stayed that way, or gotten worse!
If only I didn't have that brave family in the middle of the jungle. They were the ones who cut down the trees and built the house! it was not built for life, but for life.
The blue skies? They only got that way after photosynthesis evolved, about 2. 5 billion years ago.
Almost every aspect of the physical world around us has been shaped by life, and Earth would be another planet without it. If Mars and Venus could once have harbored life, what happened? Why did they die and the Earth live?
Walking on Venus would be a psychedelic experience. Its atmosphere hits nearly 500 degrees Celsius, and is 93x denser than Earth's. It's a strange state between liquid and gas that would allow you to almost float.
There, thunder rumbles in storm clouds of sulfuric acid, and little light reaches the surface through an atmosphere of dense smoke that envelops the entire planet. The red planet is now a desolate landscape, with a thin atmosphere just a fraction of the pressure of our own, made almost entirely of carbon dioxide. Some things that on Earth are disasters for people are needed here.
Mars has no earthquakes, no volcanic eruptions, and no tectonic plates to constantly renew the surface. Mars is biologically and geologically dead. Its oceans and atmosphere were slowly being lost to space, due to a combination of lower gravity and the intense flow of solar particles that bathe the planet.
For billions of years, the sun's light and heat acted as a constant rain that washed the atmosphere, causing it to be lost by the night side of the planet, generating an almost imperceptible tail. But if Earth is closer to the Sun and this flow is even stronger, why hasn't it also lost its gases? What kept Earth alive?
Our planet has some protective barriers against solar energy jets, and the first of them is an electromagnetic field that causes a large part of this energy to be deflected around the planet, like an immense protective force field. Mars doesn't have one of those, but it once did. Mars probably lost its magnetic field within the first 500 million years of its existence, and since then it has become an increasingly arid world.
Why did Mars lose and Earth still have a magnetic field? Electricity and magnetism are part of the same phenomenon called electromagnetism, very didactic. Magnetic fields are generated when an amount of electrically conductive material is in motion.
In the case of the Earth, this field is generated by the convective movement of the liquid core layer, made mainly of red-hot nickel iron. This constant movement distributes heat from the core to the surface, heats the mantle and moves tectonic plates, but it also produces the planet's protective magnetic barrier. Mars had a magnetic field while its metallic core had not solidified, but after that, it shut down forever.
If we look for promising places for living things in the universe, we have to rule out the magnetically unprotected. Our Moon may have been a decisive factor for the Earth to remain habitable for three reasons: 1. The Moon's orbit stabilized the Earth's rotation, preventing the climate from having too many brutal variations.
2. Its violent formation added heat to the Earth's formation process and delayed the solidification of the core. 3.
The gravitational friction of the Moon orbiting the Earth is a source of heat that keeps the core warmer, and must have interfered with the magnetic field, making it stronger and longer. Our Moon was only able to produce these effects because it is huge. So big that if it weren't around Earth, it would be considered a dwarf planet.
Our Moon is the largest in the solar system compared to the mother planet. With the exception of pluto and charon which are almost the same size, and are considered a binary system. Who knows, maybe one day the scientific consensus will decide that the Earth and the Moon are also a binary system?
This means that a pair of planets could be habitable longer than a lone planet, if gravitational friction is able to delay a world's geological death. Perhaps large moons orbiting gas giants in habitable zones are also good candidates for life's homes for the same reason. If a planet wants to live a long time, it needs a thermostat.
A self-regulatory system that keeps some key components in balance. You may have heard of the biogeochemical cycles of water, nitrogen, and carbon. Carbon is very important because it is the main greenhouse gas on Earth, which means it helps trap heat from the sun.
Basically, the more carbon dioxide in the atmosphere, the more energy the atmosphere will hold and the less will be reflected. It can exist in four forms: Dissolved in the atmosphere in the form of gases, mainly carbon dioxide. It may be dissolved in seawater It may be building up rock minerals Or being part of living beings and organic compounds.
And carbon is always on the move. I'm going to give two examples of how carbon movement has a stabilizing action on climate, one for each scale of the carbon cycle. It has a short-term and a long-term function.
The short term involves biomass and decomposition. Let's take a tree for example! When it vegetates and the leaves grow, the carbon that builds the leaves is being captured from the atmosphere.
Carbon leaves its gaseous form and turns into biomass. When this leaf falls and decomposes, the decomposing microorganisms throw this carbon dioxide back into the atmosphere. This also happens in an even more direct way when organic matter is burned.
Let's take the northern hemisphere to see what this means, because that's where the effects of this are most felt. During the summer, plants grow, taking carbon out of the atmosphere, and making the summer milder than it would be with more carbon dioxide in the air. During autumn and winter, leaves fall and decompose, increasing the amount of CO2 in the atmosphere, which helps winter not be as harsh as it would be with less CO2 in the air.
So the carbon cycle associated with biomass helps to reduce climate variations throughout the year, and in a world without forests, the differences between seasons would be brutal. The long-term carbon cycle is more important, and it plays with an even more powerful force: rocks. Carbon needs to move out of rocks and into rocks at roughly the same rate so that the climate doesn't slowly drift to extremes.
But this cycle also involves life. Carbon enters the system when the movement of tectonic plates causes rock to be constantly renewed and melted via subduction, the fuel of volcanic eruptions, the source of carbon in the atmosphere. If carbon is coming in through volcanoes, where is it going back into the rocks?
This is limestone. This is where carbon usually ends up when it leaves the atmosphere and ocean. Part of it is formed when small skeleton-forming organisms die and accumulate in marine sediment.
Over time and pressure, these trillions of skeletons form a very common rock. From tiny bacteria, to protozoa, and even seashells are doing an important climate service by sequestering carbon from the atmosphere and turning it back into rock. This brings us to two unusual conclusions: If we are looking for life, we will find it more easily on planets with tectonic plates that stabilize the climate.
And if a planet wants to be habitable for a long time, it has to be inhabited. Without life to complete the carbon cycle, our planet's climate would have already turned into something absolutely wild. These are just a few of the elements needed for long-term habitability, and as much as I 'd like to explore more in other videos, I don't want to miss out on one essential factor.
Luck. A planet can be perfect, until an asteroid destroys it, or a nearby stellar explosion kills it. The amount of huge screw-ups that can happen while a planet and its parent star orbit the galaxy is so large, it must be the norm.
Even the best planets are eventually killed by unpredictable space events, and however persistent life may be in the universe, its fate will always depend on chance. The universe has an absurd number of worlds and possibilities, but there is only one planet Earth. And this is the best place in the world for life, not because it was made for life, but because it was made for life.
Everything we do is irreversible and climate change is already underway . As much as the past has no return, our future is still uncertain. Humanity can end up being extinguished by its own greed, or it can have a promising future worthy of pride.
It all depends on how well we will understand and care for the only world that shelters us.
