Water comes in different varieties. Not just fresh or salty, but also "heavy"—when one of its hydrogen atoms is replaced by deuterium, its heavier cousin. Such water is called HDO, and it's a true cosmic passport: the amount of deuterium in water reveals where and when that water was born.

A team of astronomers pointed the James Webb Space Telescope at WASP-39 b — a giant gas ball that orbits its star so closely that a year there lasts just four Earth days. The telescope captured starlight filtering through the planet's atmosphere as it transited its host star. In that light, like a colored shadow, the chemical makeup of the gas envelope was imprinted. Among the familiar lines of water and carbon dioxide, a faint but confident trace of HDO emerged.

The measured deuterium-to-hydrogen ratio in WASP-39 b's water is about 4 thousandths — tens of times higher than that of Jupiter or Saturn. Such numbers are more reminiscent of young stellar nurseries or the inner part of a protoplanetary disk, where our own Earth once formed. Perhaps the planet gathered deuterium-rich ice far from its star, beyond the snow line, and then migrated inward. Or perhaps its scorching atmosphere hosts processes we don't yet understand: layer mixing, photochemistry, atmospheric escape — all of which could alter the isotopic portrait.

The shadow of a distant planet told us about the composition of its water. And it turns out that this water remembers the cold outskirts of the system, where the planet once collected its icy harvest.