Astronomers are ready to search for the fingerprints of life in faraway planetary atmospheres. But first, they need to know where to look — and that means figuring out which planets are likely to have atmospheres in the first place.

In the late 1970s, Saturn’s odd moon Titan, a hazy orange world, was expecting visitors — first, NASA’s Pioneer 11 probe, then the twin Voyager spacecraft.

Most moons are airless or boast little more than gauzy, gaseous veils. But Titan is cloaked in a blanket of nitrogen and methane so thick that, with a pair of wings and a running start, astronauts on the frosty satellite could fly just by flapping their arms.

A few years after the probes zipped past Titan, Kevin Zahnle(opens a new tab), a planetary scientist at NASA’s Ames Research Center, was mulling over the moon’s atmosphere when he found himself asking a deceptively simple question about how planets work: “Why is there air?”

Most scientists thought atmospheres around planets — and the odd moon like Titan — were a question of starting materials. If a growing planet gobbled up enough easily vaporized material, it would have an atmosphere. Otherwise, it wouldn’t. Scientists also knew that atmospheres cling to worlds because of gravity, and that the very smallest worlds lack the heft to hold onto air. But then observations of Mars suggested that, surprisingly, it too had lost substantial amounts of air.

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