Interia and gravity combine to make a planet stay in an orbit.
Orbit
There is no evidence that the moon has ever left its orbit since it formed, and there is no reason to expect that it will leave its orbit anytime in the forseeable future.
an object that moves fast enough can orbit another body in space. As a minimum, to orbit the Earth you need to travel 30,000 kph at 100 km altitude.
Nicolaus Copernicus formed the idea of a heliocentric universe, where all planets orbit around the sun.
Earth probably formed by accretion near the same orbit it is in now. Gravitational interaction from other bodies probably caused it to drift to its current orbit.
It has been orbiting the sun since it was formed.
the moon
the moon
The galaxy was formed from the attraction of stars which in our galaxy orbit a massive black hole.
It may have captured or formed moons in the distant past, but none remain. The closeness of Mercury's orbit to the Sun would likely prevent moons from maintaining a stable orbit.
When sea water evaporates it leaves the salt behind and takes the water into orbit
Kind of, yes. It's a balance between orbital velocity, which is the high speed relative to the ground, and gravity, which is the force keeping the object in orbit.