# Why do Planets Orbit?

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#### Vishesh

##### Guest
I might sound Dumb here,but I am totally new in this field.Space caught my attention while going through some of the documentaries.Well here is what I want to ask.
Why is it necessary for the planets to Orbit?
and what makes them orbit?Gravitational force of the Sun?if Yes then why?
Does our whole Galaxy also Orbits?If it does then are there chances of collapsing with other star or galaxies in the future?

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#### MeteorWayne

##### Guest
A planet orbits because it's speed exactly balances the gravitation force of the object it orbits. If you look at an orbit from above, with the sun at the center and the planet's orbit an ellipse around it, the sideways motion of the planet is just enough to counteract how much the planet falls toward the sun. If it orbited slower, it would spiral in, if faster, it would spiral away.

The solar system orbits the center of the galaxy, and our galaxy orbits the center of mass of the local cluster of galaxies we reside in.

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#### Vishesh

##### Guest
Thanks for the answer MW you cleared my doubt.

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#### MeteorWayne

##### Guest

BTW, welcome to Space.com!

Wayne

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#### aphh

##### Guest
We also need to consider what set objects in motion and how they continue to keep moving. The answer to that is conservation of momentum.

At first there was an event that released energy (an explosion of star, for example). Objects and matter acquired some of the energy and were accelerated and set in motion. Motion only needs energy at first when you accelerate an object from 0 speed. Once you have reached a certain speed, your final velocity (like spacecraft reaching it's target orbit), no more energy is required to maintain that speed (unless there is something that resists your motion, like friction). There is some friction in space, so spacecrafts need to be boosted and accelerated back to their orbital speed every now and then.

In space little friction exists and objects that acquired speed from the initial event tend to maintain that speed, which is the orbital velocity or the speed required to orbit another object at certain distance. Planets and moons have conserved so much momentum, that they can not be easily slowed down and continue to orbit for millions or even billions of years.

http://en.wikipedia.org/wiki/Conservati ... r_momentum

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#### MeteorWayne

##### Guest
Yes, there are a hundred layers of complicated detail that can be added such as barycenter, relativistic effects, perterbations, etc.

Since the original poster stated he was new to the field, I tried to keep the explanation simple so he/she could get the general idea. Heck, I even debated using the word ellipse instead of circle

MW

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