Our expert voices conversation on "How to look for alien life."

Earth is our best guide as we look for new forms of life. Our own planet hosts life in amazing complexity, from single-celled organisms to humans.

Asteroids and comets delivered the amino acids that form proteins to Earth over 4 billion years ago. Based on what we know from the evolution of life thereafter, we are looking for a liquid, probably water. We start out searching for places where liquid water is stable, either in the past, as on the surface of Mars, or currently beneath the icy crust on Europa and Enceladus. We then look for life's building block molecules such as amino acids. It took several billion years for complex life to emerge on Earth, so in our solar system we search for microbes, or fossils of simple organisms like algae or bacteria, the earliest forms of life.

Outside of our solar system, the first step is confirming habitability by analyzing extrasolar planet atmospheres, looking for gases like carbon dioxide and methane that are associated with life here. To confirm life, we need to build large enough space-based telescopes to image extrasolar planets, hopefully only a few decades from now.

Given how quickly life evolved here on this planet after conditions had stabilized, I am optimistic that where we have similar conditions, we will find evidence of life — or past life. The hard part may be finding life and not realizing it, if the life forms are very different from those here on Earth.

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