The short answer to the question is that we don’t know. The closest planets to Earth are located millions of miles away, so it’s very difficult to go off and find evidence directly.

However, we can use what we know about life on Earth to ask questions and test ideas with telescopes on the ground and in space.

The first thing we’ve noticed is that wherever we see liquid water on Earth, we find life. Humans, along with a vast array of other life forms, need water to survive. We can start looking for life on other planets by finding liquid water.

In the solar system, it turns out there aren’t many places where this is true. If a planet is too close to the sun, liquid water will evaporate, and if it’s too far away from the sun, water will freeze. Our Earth is located in the habitable zone around our sun, which is the region where liquid water can exist in a stable form.

Recent observations have shown that Mars likely had a lot of water on its surface in the past, although it doesn’t seem to right now.

Jupiter’s moon, Europa, appears to have liquid water on its surface right now buried under a huge crust of ice. NASA is currently working on a mission to send a probe to Europa to test these waters for life.

We also know that our solar system is not unique in the universe. Recent observations from the past decade have shown that most stars that you see in the sky—most of the stars in our galaxy— have planets orbiting around them. This is exciting, because it means there’s a vast array of planetary conditions out there for us to look at.

These stars are not all like our sun. Some are bigger or smaller, some hotter or colder, which makes the habitable zones around them different from our solar system. But the huge number of planets out there means there are likely some with conditions similar to Earth, and therefore life.