# Why do planets orbit?

All objects in the Universe that have mass emit gravity. Gravity causes celestial bodies to attract.

The more mass they have and the closer they are, the greater the attraction between them. The Sun is a massive body and its strong gravity attracts the planets and prevents them from escaping into outer space. Similarly, the Moon is attracted by the Earth's gravity.

But yes the gravity It's so strong, why do they stay in orbit and don't fall? Why have the planets not already rushed to the Sun and disappeared? The answer is the inertia.

The natural state of the planets is not rest, but a constant movement in a straight line. That is, if there were no gravity or any other force acting on them, the planets would move in a straight line and at a constant speed forever ... or until they collided with another body in space.

The force of gravity breaks that inertia and deflects the planet from its straight path. The Sun attracts the planet, pulls it and the planet does fall. We can calculate the distance that the planet falls every second: gravity is directly proportional to the mass of objects and inversely proportional to the square of the distance between them.

What happens is that, during that second, both objects have moved. The planet does not fall down in a straight line, but it draws a parable, because inertia pushed him forward while gravity pulled him down. And the Sun does not remain still, but it rotates and, consequently, its curvature under the planet changes.

The next second the Sun returns to pull the planet, which falls again tracing a new parable, and so on. Since he cannot escape gravity, he is trapped in a (almost) circular and closed orbit. The same happens to the Moon with respect to the Earth.

And we have learned it. The same system is used to keep artificial satellites in orbit. Any object close to Earth that moves at more than 8 km per second will be trapped in an orbit and will not fall. If it moves more slowly it will fall, since gravity will be stronger than inertia, and the pull of gravity will move it further down than straight inertia. If it moves faster, it will move away from Earth.

 Planet Orbital period (on land days) Mercury 88 Venus 224,7 land 365,26 Mars 687 Jupiter 4.331 Saturn 10.759 Uranus 30.799 Neptune 60.179 Pluto (dwarf planet) 90.613
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