Not Too Hot, Not Too Cold - What Makes Earth Perfect for Life

Our home, the Earth. The only planet we know of in the universe to support life.

Earth is the only planet we know in the entire universe to support life. In a universe as big as ours, life likely exists out there somewhere by sheer odds alone. But for now, Earth is all we know and the only example of a habitable planet we can study. So why can Earth support life when so many other planets fail? In order to predict what other planets might support life, we first need to understand life on Earth. What is it that makes Earth perfect for life?

Life is incredibly complex, as are its requirements. But we can boil those requirements down to the most essential. Life, at least as we know it, needs energy, complex chemical structures, a stable environment, and liquid water. Earth provides all of this and more for its life.

The Energy to Live

The most fundamental of these needs is energy. Without energy, life of any form would be unable to function. On Earth, our energy comes from the Sun. Practically all energy on Earth, no matter what form it takes, originated from our star. In the case of biological energy, the bottom of the food chain is plants and other photosynthetic organisms. They are able to convert sunlight into usable energy. The plants are eaten by herbivores, which are in turn eaten by carnivores, moving the energy from the Sun up through the food chain.

For the most part, it’s easy to assume that most other planets supporting life would also derive energy from the stars they orbit. However, there are other possibilities as well. For example, if a planet (or moon, for that matter) has a molten core, then thermal vents releasing heat from the core could provide an alternate source of energy for life, even if the planet is too far from its star or the star is too dim.

The Chemistry of Life

Another requirement of life is complex chemistry. In the case of Earth life, we use carbon chemistry to perform all of our most vital functions. For this reason, carbon compounds are often referred to as organic in nature. Carbon is abundant on Earth, but it’s also common across the universe. In fact, the complex carbon molecules needed to start Earth-like life can be found free floating in space or buried in comets. For this reason, it’s not hard to imagine that the building blocks of life could be found on nearly any given planet, making Earth not so special in that regard.

It’s also conceivable that other forms of life could even use alternate forms of chemistry. Perhaps life in an environment completely different from Earth’s could rely on the chemically similar properties of silicon chemistry, resulting in aliens that are even more alien to us.

A Safe Place to Live

Of all the items on the list of requirements for life, a stable environment is the broadest topic. There’s no one single way to describe everything required for such an environment, and different forms of life might define stable in different ways. In its most basic form, the environment can’t be hostile to the life it supports. For example, if a planet is so hot that it denatures carbon compounds, then carbon-based life likely won’t exist there. Alternatively, if the environment is highly radioactive, then potential life would struggle to get off the ground. But really, the most important factor is a lack of extreme and rapid changes. If an environment changes too fast, then the life living there would never have the chance to adapt and survive, assuming it was even able to begin in the first place.

Earth has several factors that help it remain a safe and stable environment for life. The two most important factors would be our atmosphere and magnetic field. The atmosphere helps to trap heat from the Sun and distribute it across the planet, keeping things the right temperature for life. The Earth’s magnetic field, caused by the rotation of its molten metallic core, acts like a shield, protecting the Earth and its atmosphere from dangerous charged particles and radiation from the Sun by redirecting it. Without the field, the atmosphere would have been stripped away long ago.

The Aurora Borealis, or Northern Lights, are proof that the Earth's magnetic field is doing its job. Charged particles from the Sun that would be harmful to us are redirected to the North and South Poles where the collide with gas in the atmosphere to create the lights.

There are many other smaller factors as well that help stabilize Earth. For example, our Moon has likely stabilized the Earth’s axial tilt, the cause of seasons. Without it, the Earth would tilt far more drastically, leading to wilder swings in seasonal climate, which could be hindering, if not deadly, to life.

The Most Important Ingredient

Finally on our list, we reach what might be the most important ingredient of all for life: liquid water. Everything else discussed so far could conceivably exist on just about any planet. But water is a bit trickier. For life on Earth, water acts as a solvent. It’s what dissolves our nutrients and transports them throughout our bodies, among other important roles. All life on Earth, from single-celled bacteria to the largest of animals, requires water to survive. Alien life would need some sort of similarly powerful solvent, but water would be the most readily available across the universe.

But, most places in the universe don’t support liquid water. For example, if Earth were closer to the Sun, it would get hotter. If it gets too hot, the water will boil away. If it were farther, it would be colder, and too cold means the water freezes. But it turns out the Earth is located at the perfect spot where it’s not too hot, it’s not too cold, it’s just right. That’s why we say Earth exists in the Goldilocks zone of our Solar System. Any star will have a hypothetical Goldilocks zone, otherwise known as the habitable zone, but it does vary depending on the size and temperature of the star. You’d also need an atmosphere, both to regulate the heat and also stop the water from boiling away under low pressure, like it has on Mars.

An interesting possibility is the fact that a planet which can support liquid water on its surface will already have a source of energy, a stable temperature, carbon chemistry, and a significant protective atmosphere. This means that any planet that has liquid water on its surface has a good chance of supporting life in some form as is. Give a planet with abundant liquid water on the surface enough time, and it just might inevitably develop life eventually. This is part of the reason why we’re so interested in searching for life on Mars, given that it used to have liquid water on its surface.

When looking for life beyond Earth, there are many factors we need to consider. It’s possible that life on other planets may be nothing like life on Earth and will exist under completely different conditions. But for now, based on our only example, planets that are most like Earth are most likely to support life. In the end, if you want to find Earth-like life on another world, all you need to do is follow the water.