exomoons

Exomoons are moons orbiting planets beyond our solar system. None has been confirmed, despite two serious candidates, because their signals are tiny and easily mimicked. If they exist around giant planets, some could hold liquid water through tidal heating, even far from any star.

A second moon would change tides, eclipses, calendars, and possibly life itself. Here's exactly what would happen — based on real orbital physics — if we suddenly had two moons.