Official support for downloadable custom planetary systems is here! The website has been updated with pages to allow you to view and download celestial bodies and planetary systems that players have created and uploaded. When starting a new sandbox, you now have the option to select which planetary system to use.
This update is our first big step on the path to implementing 'Planet Studio', which is currently the most upvoted suggestion on our website. Currently, planet studio is only accessible from the developer console and is just a tool used for viewing celestial bodies and uploading custom bodies and systems. Actually creating and modifying custom content is still just an XML only capability at this time, but we are hard at work on a future update which will bring more in-game editing capability.
A few players have already been hard at work editing XML to create custom systems and bodies and the results are awesome. Please take a look at at their creations and feel free to toss them an upvote (both systems and bodies can be upvoted).
https://www.simplerockets.com/PlanetarySystems/Best
https://www.simplerockets.com/CelestialBodies/Best
For those of you interested in creating custom celestial bodies and/or planetary systems with XML right now, planet studio has received some updates in this release. Details on changes and some basic instructions on how to use planet studio in its current state can be found here.
The full release notes can be found here with a list of all bug fixes and tweaks in the new version.