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Jupiter has over 6000!
Gravity. As the planet rotates it's mass holds it's moons within it's orbit. The larger the planet the stronger is its gravitational pull.
the gravitational pull of the planet it orbits
the gravitational pull of the planet it orbits
the gravitational force holds its atmosphere together
The planet Mars is slightly smaller than the Earth and therefore has a weaker gravitational field. Gravity is what holds an atmosphere in place. In a weak gravitational field, an atmosphere will gradually leak away into outer space. That's what happened on Mars.
It's true that gravity holds planets in orbit. However, this is due to the gravitational influence of the sun, not the gravitational influence of other planets.
the sun
What exactly do you mean by "in space"? Everything is in space. If you meant why they are individually floating without getting attracted into the corresponding star, then that is due to the centrifugal force caused by the revolution of the planet. Please append the corrected question to this answer if you happen to read this again...:)
Gravitational force formed the earth and holds all things on its surface - including us!
a planet that holds life
Mercury has a surface pressure that is near enough zero (or a vacuum), the planet holds no atmosphere due to its small size. Its small size (and therefore mass) means that it has a low gravitational pull, not strong enough to hold gases close to its surface to create an atmosphere.