Run it! https://rawgit.com/jakebrown58/GravitySimulator/master/particles.html
This is a gravity-particle sandbox. It comes with a default configuration for our solar system using masses, and distances I researched from from wikipedia. It also has a bunch of random celestial bodies, like comets and asteriods, and a cloud of space junk around Jupiter.
It uses newtonian gravity equations.
Download the locally open up particles.html in a browser.
It doesn't require a build, or a server, or any libraries.
It has a splash screen to tell you about all of the interactions it supports.
Play around with it and enjoy.
2.1 - flying, vectors, et cetera 2.0 - current (in the future new versions will list major features). - splash screen with basic instructions. 1.9 - cloud version .1 - JS Fiddle version.