How do you calculate gravity from planets in a solar system?

It has been a long time since this has been answered, but maybe I can give some additional info.

The basic idea is to use Newton's Gravitational pull, as mentioned by other comments. Other answers give some links to basic code on how to solve that.

Solution

I have created a blog entry that solves this issue: Physics-based solar system lessons. It explains:

• The math used.

• The problems found (precision issues, orbit prediction, Speeding up the system)

• The implementation code (and basic setup)

• Where to find accurate planetary data.

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Here you can see an example of the applied code in the blog in a Earth-Moon system with a couple of satellites (orbiting Earth and Moon).

For any developers out there who would like to implement this really easily, try out:

World Physics System ~ http://nimbusgarden.com/worldphysicssystem

The World Physics System is a celestial body & point gravity scripting interface for Unity, intended as a replacement for the stock downward gravity. WPS can be used to simulate planetary orbits, body-body attraction, or "snowballing" effects. You may use it to create spherical worlds "out-of-the-box," but World Physics System is implemented robustly and is lightweight, providing you complete freedom in expressing your point gravitation creativity, complex and repulsive forces, allowing you to use the system as a subcomponent for other effects, such as spells, powerups, or goal-based Artificial Intelligences for NPC's. With the WPS, the sky is truly the limit!

Available from the Unity Asset Store, here.

More information, here.

Check out this script for a great example of simple planet gravity. http://www.unifycommunity.com/wiki/index.php?title=Simple_planetary_orbits

Take a look at this question: http://answers.unity3d.com/questions/196749/objects-acting-under-each-others-gravity.html

Additional: Search wikipedia for "Newton's law of universal gravitation". With an adjusted gravitational constant and mass (that fits your world´s metric system) you can loop between the nearest planets (and for example the sun with its high gravitational drag), and calculate the gravitational force, then the object´s acceleration and velocity. But you don`t want to calculate this for hundreds of objects each update, maybe per 5-10 th update? You can try and see what you get.