It may surprise you, but it isn’t always obvious what’s alive and what’s not. And when we’re looking for life on distant planets, we can’t even get close enough to see it directly. So what does life look like from different distances?

Under a Microscope

A universe of fascinating life forms exists right under our noses. Most life is too small for us to see with the unaided eye. But through a microscope, a teeming world of living creatures comes into focus.

From Orbit

At the 200-mile altitude of the Space Shuttle, we can’t see individual creatures, but we can see plenty of evidence that Earth harbors life. There are oceans, clouds, patterns of vegetation that change over time, roads, wakes of ships, and city lights at night.

A shuttle astronaut’s view of Earth with the Moon in the distance. (NASA)

From a Nearby Planet

At the distance of Mars, Earth is a bright blue dot. With the right instruments, you could tell it has liquid water, an oxygen-rich atmosphere, and other signs of life.

While orbiting Mars, a spacecraft took this photograph of the Earth and Moon, 86 million miles away. ( NASA/JPL/Malin Space Science Systems)

From A Nearby Star

Imagine that you could look back at Earth from Alpha Centauri, the closest star system to our solar system. Our Sun would show up as a point of light. You would need instruments more sophisticated than any we have today to pick out Earth and find any trace of life. The key things to look for are oxygen, water vapor, methane, and a few other gases that indicate the possible presence of living organisms.