How do moons comes into existence?

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Moons form in one of three ways, says Mark Sykes, director of the Planetary Sciences Institute in Arizona.

The oldest moons were born alongside the planets. Bits of gas and dust that didn't join to form the planets gathered on their own. Gravity, or a force that pulls objects together, attracted more and more material to these bunches and they became moons.

Some moons, like Mars's, were once asteroids or similar objects flying by. They were captured by the planet's gravitational pull and began orbiting around it. Earth's moon started with a dramatic twist. Something Mars-size hit Earth and pieces of the planet splashed into space. These fragments formed a ring around Earth and eventually clumped together into our moon.

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