Beneath the frozen shell of Europa — a moon smaller than our own — scientists believe an ocean twice the volume of all Earth's seas has remained liquid for billions of years, not through sunlight or surface warmth, but through the rhythmic gravitational choreography of neighboring moons. It is a reminder that life's prerequisites may be sustained by celestial mechanics as much as by proximity to a star. In 2030, NASA's Europa Clipper will arrive to listen for the moon's slow, tidal breathing — not yet searching for life, but asking the prior question: could it be possible?