Earth’s Place in the Universe

Life on Earth

The blue planet

Where did Earth’s water come from?

That’s an important question, because after all, Earth is a very watery planet. There’s a reason it’s called the Blue Planet! And without water there would be no life…and no us. Water is also essential to life on earth and covers most of our planet.

Scientists think that much of Earth’s water arrived the hard way, by crashing into the planet’s surface inside icy meteorites during those first million or two years. There is also evidence that a lot of water may have come from within the solar nebula itself. That means a lot of Earth’s water was here from the very beginning.

The blue planet

When did life begin on Earth?

No one knows exactly how life began on earth. We do know that the first living creatures appeared at least 3.5 billion years ago. The first life was tiny microbes. Over time, those single-celled organisms evolved into plants and animals. Life changed the atmosphere. Blue-green algae converted carbon dioxide into oxygen. Without that oxygen, humans wouldn’t be able to breathe the air.

Our Goldilocks Planet

Earth is the only planet in our solar system that can support life. There may be other planets in the Milky Way or other galaxies that support life, but we have not discovered them yet.

Earth happens to be in just the right spot in the solar system. If we were closer to the sun, like Mercury or Venus, it might be too hot for life to survive. If we were farther away from the sun, like Saturn or Jupiter, our planet could be covered in ice all the time and too cold for life.

When astronomers search for planets in other solar systems, they think of what makes Earth so cozy and livable. They have a nickname for such a world: a Goldilocks planet. Not too hot. Not too cold. Just right for water and life. So far, Earth is the only Goldilocks planet we know about. Lucky us!

Written by Laura McCamy

Edited by Beth Geiger, MS Geology