The most promising places to look for life aren't planets at all. They're small icy moons hiding oceans bigger than all of Earth's seas combined.
Oceans in the dark
Far from the Sun, where sunlight is a hundredth of what Earth receives, liquid water should be impossible. Yet Jupiter's moon Europa almost certainly hides a global saltwater ocean beneath its cracked ice shell — likely holding more than twice the water of all Earth's oceans. The giveaways: a surface of fractured, rafted ice that looks like polar sea ice on Earth, and a magnetic signature that behaves exactly as a salty conducting ocean should.
The heat that isn't sunlight
The energy source is gravity itself. Europa's orbit is slightly eccentric, so Jupiter's enormous pull stretches and relaxes the moon on every lap — tidal kneading, like flexing a paperclip until it warms. The same mechanism keeps Saturn's small moon Enceladus so warm that it vents its ocean directly into space: Cassini flew through those geysers and tasted salt, silica, and organic molecules. An ocean sampled without ever drilling.
Why this changes the search for life
Life as we know it needs liquid water, chemistry, and energy — and ocean moons offer all three with no sunlight required. That widens the search enormously: habitable worlds may not need to sit in a star's 'Goldilocks zone' at all. It's why Europa Clipper is on its way to survey Europa's ice and ocean up close, and why Titan — with its methane rivers above and water ocean below — gets a rotorcraft named Dragonfly. The nearest alien sea is three years' flight away.