The habitable zone

The most important reason the Earth can support life is its distance from the Sun. It is at just the right distance for water to exist as a liquid.

Any closer and it would be too hot: the water would evaporate. Any farther and it would be too cold: the water would freeze. Either way, most living things, especially plants, animals and humans, couldn't grow and thrive.

Some microbes, such as certain bacteria, can survive frozen. But from what we know so far, liquid water is essential for life to develop. Earth's distance from the Sun keeps most of its water liquid, so life can begin and carry on in all its forms.

The range of distances from the Sun, or from any star, in which water stays liquid is called the habitable zone. It is also called the Goldilocks zone: not too hot, not too cold, just right.

As you've also learnt in Social Science, most of Earth's surface is covered with water. From space, all that water makes the Earth look blue, which is why it is called the blue planet.

Mars lies at the edge of the Sun's habitable zone. Several spacecraft have been sent there, and rovers have landed and explored its surface, but no proof of life has been found yet.

Scientists do think Mars may once have had liquid water, maybe even lakes, and conditions that could support simple life. That's one reason scientists stay so interested in Mars. It's also a reminder that science doesn't always have final answers: as we explore more we may find new clues, or even new kinds of life, and science stays open to change when we learn more.