What makes Earth perfect for life?
Our planet is a lot more special than you might think.
The third rock from the Sun, the pale blue dot, the World, home.
Whatever you chose to call it, the fact remains that out of the trillions of celestial bodies in the universe, it is the only one where we’ve ever observed something incredibly special: life.
When scientists look for life beyond Earth, there’s often a lot of talk of liquid water. In fact, we hear so much about it that we could think water is all it takes to get those tiny microbes up and running.
But the truth is much, much more complicated.
While liquid water on Earth was vital for life to start, it is far from being the only necessity.
Our little rock has numerous other characteristics which through billions of years made it possible for me to write these lines and for you to read them.
The star, the planet and the moon
Life stood a chance to develop on Earth not because of Earth itself, but because of its surroundings, in particular its star.
The Earth orbits the Sun in what we call the habitable zone — a region just the right distance from the star to allow for liquid water on the planet’s surface. A little closer would be too hot — the water would evaporate. A little farther would be too cold — the water would freeze.
The Sun’s heat regulates weather on Earth and the temperature of water bodies. Its light triggers photosynthesis in plants that produce oxygen as a byproduct. You know, the one we breathe.
Our Sun is an average sized yellow star and might not seem all that special. But we think about 70% of all the stars out there are red dwarfs, and they are much smaller and dimmer than the Sun.
Since a red dwarf star emits less energy, a potential planet needs to orbit very close to it to be in its habitable zone — much closer than the Earth orbits the Sun. This means the planet is exposed to potentially stronger radiations coming from its star.
One point for Earth: our star is sufficiently big that we can be at a safer distance from it and still have liquid water on the surface.
But it doesn’t matter how far from the Sun we are: the charged particles — solar wind — it sends in our direction would instantly kill us if we didn’t have good protection. One of the most important things for living organisms on our planet is the Earth’s magnetic field.
The magnetic field is generated by electric currents due to the motion of molten iron and nickel in the Earth’s outer core. It extends from the Earth’s interior out into space, where it interacts with the solar wind. It is this interaction that causes astounding auroras on the North and the South Pole.
A magnetic field is not such a trivial thing: out of all the rocky planets in our system, the Earth is the only one with a substantial field.
Another point for our planet: it successfully protects its inhabitants from being fried by the Sun’s radiation.
Then there’s also our precious atmosphere. On Mars it is almost non-existent, on Venus it is very thick, but also very toxic. At least from our point of view.
But on Earth, it is 78% nitrogen, 21% oxygen and less than 1% other gasses, including argon and carbon dioxide. Basically, the perfect mixture for the life we know and love to thrive.
Of course our atmosphere is not a static brew. From the Earth’s beginnings until now, it evolved quite a lot. And it’s still changing: this time, unfortunately, because of humanity’s arrogant behavior.
Although carbon dioxide represents only 0.04% percent of the Earth’s atmosphere, it has a very strong influence. Its primary role is to keep some of Sun’s heat trapped on Earth which warms the planet. Without it, the oceans would freeze.
CO2 is being naturally released into the atmosphere by volcanos and the movement of tectonic plates — two vital geological activities for the apparition of life.
But too much of it, even just a tiny bit, can cause big problems: melting of glaciers, rising ocean levels, disastrous floods and devastating draughts, just to name a few.
0.04% seems negligible. But it’s actually the ideal amount for countless species, including ours. So let’s give it our best shot to keep it at this level.
Our atmosphere has many layers, but one of the most important ones is the ozone layer.
The ozone layer is part of the Earth’s stratosphere and acts as a protective bubble: it shields us from most of the Sun’s ultraviolet radiation. You know, the one that ages your skin and gives cancer and whatnot.
The destruction of the ozone layer would be catastrophic for life on our planet. The situation became particularly worrisome in the 1970s when ozone started rapidly decreasing above Antarctica. Luckily, member countries of the UN ratified the Montreal protocol in the eighties and committed to reducing ozone-depleting chemicals in products like aerosols and solvents.
The situation improved significantly over the last decades, but evidence shows that the layer remains fragile and future action is required.
Summer vacation, autumn strolls, winter sports, spring breaks. We love our seasons. They’ve been dictating the lives of most living species for millions of years. But why do we have them? And how are they important in the evolution of life?
Billions of years ago, when Earth was a newly-born planet, large bodies collided with one another. Earth underwent several of these collisions, each of which tilted its axis of rotation.
One of the last giant collisions was with a Mars-sized planet called Theia which set the final angle of the Earth’s axis. Today we have seasons because our planet’s axis of rotation is tilted at an angle of 23.5 degrees relative to the orbital plane — the plane of Earth’s orbit around the Sun.
Without seasons, the diversity of species on Earth would be much smaller, or life might not have evolved at all. So thumbs up! Giant impacts eons ago gave our planet’s axis the perfect angle for seasons to appear.
But that’s not all they did. Remember that Mars-sized planet that hit Earth during the formation of the solar system? The impact was so violent that it sent huge chunks of both planets flying into space. This debris settled into the Earth’s orbit and eventually coalesced into the Moon.
Our satellite plays an important role in the evolution of life. First of all, its gravity prevents the Earth’s axis from wobbling. A stable axis makes for a more stable and moderate climate.
Without the Moon, the tides would be much smaller, Earth would rotate quicker making days shorter, and nights would be pitch black. The Moon played a major role in the diversification of species, and, according to some theories, even a vital role in the apparition of life itself.
Another great news for Earth: a faithful companion which makes the environment much more life-friendly.
This might come as a surprise, but there is another body out there that have had a decisive impact on life’s evolution: the planet Jupiter.
The king of the solar system, with its enormous gravitational field, acts as a gatekeeper. By deflecting comets and asteroids that could hit Earth, Jupiter has helped to create a more stable environment for life to evolve. Without it, Earth could have sustained many more impacts which could prevent advanced life from gaining a foothold.
On the other hand, Jupiter also sent some very important asteroids and comets in our direction. Those which we think brought water and organic molecules to our planet.
Not only do we have a loyal companion, but also a giant protector. Lucky us!
Finally, there’s a fundamental aspect which might seem self-evident, but should not be taken for granted. Time.
Life evolved on Earth because it had enough time to do so. Here again, we were lucky with our star. Although Sun is bigger than most stars, it is also a lot smaller than some other ones we’ve discovered.
The more massive the star, the shorter its lifespan.
While our Sun will live for about 10 billion years, some giant stars explode as supernovae after only a few million years. First organisms appeared on Earth after a billion years or so, and more complex ones evolved much, much later.
If our Sun was a huge massive star, our planet would be long gone before any life stood a chance.
Let’s recap!
TThe unique mix of conditions on Earth made it possible not only for life to appear, but also to evolve and thrive.
The breathtaking biodiversity we can admire today is firstly due to our star.
The Sun’s long lifespan gave life on Earth enough time to develop. Its heat and light regulate climate, temperatures and oxygen levels. The perfect distance from our star allows for liquid water on Earth’s surface.
Giant and small collisions eons ago gave rise to seasons and put the Moon on our sky which stabilizes our climate.
They also brought water and organic molecules to our world that allowed for oceans and a perfect blend of gasses in our atmosphere.
Earth’s volcanos and tectonic plates have enriched the atmosphere with carbon dioxide, thus retaining the heat on our planet.
The Earth’s magnetic field shields us from the charged particles sent by the Sun.
Finally, with its huge gravitational pull, Jupiter protects us from outer space bodies that could potentially hit us.
All these mechanics are not independent of each other — they interact, creating the perfect symphony.
If only one tiny detail in the 4.5 billion years of the evolution of the solar system and our planet changed, life would have taken on a completely different form. Or there might not have been any life at all.
Bottom line
Our planet is a lot more than rocks and water: it is an extremely complex and fragile living organism on its own.
This doesn’t mean that alien life, or even all lifeforms on Earth, require all of the above conditions to exist and thrive.
But we certainly do.
If want, or even need, to migrate beyond our planet, we will have an extremely hard time adapting to an environment we weren’t made for. And an even harder time finding another world with the same unique subtleties that make this pale blue dot perfect for our kind of life.
This is our only home. So let’s treat it with the respect it deserves.
Thank you for reading my story!
