Some of the most promising places to search for alien life aren't planets at all, but moons — because Europa and Enceladus keep oceans liquid beneath their ice partly through the gravitational squeezing of giant planets, an internal heat source that doesn't car

Some of the most promising places to search for alien life are moons, not planets, because Europa and Enceladus keep oceans liquid beneath their ice through the gravitational squeezing of giant planets, providing an internal heat source.

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