Earth as the planet for life

0:00- [Narrator] Why is Earth perfect for life? I mean, astronomers have spent decades, exploring the universe. And so far, Earth is the only place where we have clear evidence that life exists. So, what makes Earth so special? What conditions make a planet suitable for life?

0:20Answering that question is actually quite difficult, because all the life we currently know about comes just from one planet, Earth. That's why when scientists talk about the conditions needed for life, they are really talking about the conditions needed for life as we know it.

0:36I mean, it's totally possible that life somewhere else in the universe might turn out to be very different and it may require completely different conditions. But anyways, based on everything we've observed so far, we can still identify some major factors that make Earth suitable for life as we know it.

0:56Okay, so, what are these factors? Well, there are three major factors. The first one is the abundance of liquid water. The second is that Earth sits at just the right distance from sun. And third, Earth has an atmosphere with just the right composition to support life.

1:18So, let's explore each one in detail. Let's start with the first one, water. You probably know every living organism, whether you consider animals or plants, or even microorganisms, they all depend on water, right? But why? The answer has to do with chemistry.

1:35Life is essentially a huge network of chemical reactions. I mean, even a single cell is like a tiny chemical factory and there are thousands of reactions constantly happening. But guess what? For these reactions to happen fast enough, most of these chemicals have to be dissolved in a liquid.

1:54Otherwise, the reactions would happen far too slowly to support life. And guess what? Water is really good at dissolving many substances, which makes it a powerful solvent. And that's why water works so well for life. And guess what? Earth has plenty of it! About 70% of Earth's surface is covered in water, which is one of the main reasons our planet is so well suited for life.

2:28But remember, we are still talking about life as we know it. I mean, on other planets, different liquids could play the same role as water, right? I mean, think about an extremely cold world where methane or ammonia could exist as liquid. At Earth temperatures, these are gases.

2:48But on extremely cold worlds, they can exist as liquids. And then it's possible that alien life on such planets might depend on those liquids instead. But still, on Earth, water is the liquid that makes life possible. Speaking about liquid water, did you know that water can only exist as a liquid within a very tiny temperature range between zero degrees Celsius and a hundred degrees Celsius?

3:17I mean, below zero degrees Celsius, water tends to exist as solid ice. And above a hundred degrees Celsius, it tends to exist as gas, steam. This means that the temperature on a planet must fall within this very tiny range for liquid water to exist. And what does that temperature on the planet depend on?

3:39Well, it largely depends on the planet's distance from its star. If a planet is too close to the star, like say too close to our sun, the surface would be far too hot for water to stay liquid. Most of the water would just evaporate. In contrast, if a planet is very far away, then temperatures would drop so low that most of the water would just freeze into ice.

4:10But somewhere in between, you have a sweet spot. A planet at this distance would be not too hot, nor too cold, just perfect for water to stay liquid. Astronomers call this region around the star where liquid water can exist the habitable zone. You may also hear it called the Goldilocks zone.

4:34Why? Well, the name actually comes from the fairytale, the "Goldilocks and the Three Bears". In the story, Goldilocks tries three bowls of porridge. One is too hot, one is too cold, but one is just right, perfect. In the same way, planets can be too hot for liquid water to exist, or too cold for liquid water to exist, or it can be just right.

5:02And guess what? Earth sits in this just right zone around the sun! That's why large amounts of liquid water can exist on our planet, making it perfect for life as we know it. Again, remember, on very cold planets, liquids like methane might exist instead of water.

5:23So, methane-based life could potentially exist on planets there as well. Scientists are still exploring these possibilities. Anyways, coming back to Earth, another major reason Earth supports life is its atmosphere. An atmosphere is just the layer of gases that surround a planet.

5:44Earth's atmosphere contains many gases important for life. One of the most important one is oxygen. Humans and many animals use it for respiration. But interestingly, oxygen was not always present in large amounts. Turns out that early in the Earth's history, the atmosphere had very little oxygen, but still life existed.

6:06Early organisms like cyanobacteria lived, using other chemical processes. These organisms released oxygen as a waste product. And over millions of years, oxygen slowly got built up in our atmosphere. So, as the oxygen levels increased in our atmosphere, it allowed new kinds of life to evolve that could use this oxygen for energy.

6:36It's pretty cool, right? And Earth's atmosphere also contains other important gases, like nitrogen, essential for many biological molecules, carbon dioxide, which plants use during photosynthesis and our atmosphere also contains a special gas, called the ozone.

6:54The ozone forms a protective layer that absorbs much of the sun's harmful ultraviolet radiation. Without this prediction, life on land would struggle to survive. And there's another important thing. Earth's atmosphere helps regulate temperature through something called the greenhouse effect.

7:14What's that? Well, think of it this way. When sunlight reaches the Earth, it heats up the Earth's surface, right? Now, some of that heat actually tries to escape back into space. But certain gases in our atmosphere reflect that heat back to Earth's surface, trapping that heat, keeping our planet warm.

7:35So, look, these greenhouse gases act like a blanket around the Earth. Without this greenhouse effect, Earth would be much colder, making it very hard for life to survive. However, if you have too strong of a greenhouse effect, then you can imagine Earth would start getting cooked.

7:54So, that's also bad. In fact, on the planet Venus, the greenhouse gases trap so much heat that the surface temperatures are hot enough to melt lead. So, again, Earth's conditions are balanced just right. All right, so far, we've discussed three major factors responsible for life on Earth as we know it, right?

8:18The liquid water, the right distance from the sun and a protective atmosphere. But there are some other factors as well. For example, the sun is constantly releasing streams of charged particles called the solar wind and these particles can damage living cells.

8:35But fortunately, Earth's magnetic field that you can see over here deflects many of these particles, protecting the surface of our planet. And as a bonus, when these charged particles interact with the magnetic field, they light up the sky. We call them auroras.

8:54You have to be near the Poles to see them. But anyways, then there's our moon that helps stabilize Earth's tilt and rotation. Also, Earth's orbit around the sun is nearly circular, which keeps temperatures relatively stable throughout the year. And let's not forget about the abundance of carbon on our planet, which forms the complex molecules used in living organisms.

9:21So, when we combine all these factors, Earth is truly an extraordinary place. It has to do with the liquid water, being at just the right distance from the sun, a protective atmosphere, a magnetic field, a stable orbit and of course, the right chemical ingredients for life.

9:40Together, these features make Earth a planet just perfect for life as we know it.