Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Space Exploration Engine Free.
Space Exploration Engine (webrip)
The Space Exploration Engine lets you break down the walls of your game world. Your players can be walking around on a planet’s surface one minute, and the next be stepping into a spacecraft and taking flight towards a nearby ring world. During their journey, the player might walk around inside their ship and interact with other physical objects. Or they might float outside their ship to carry out external repairs. All of this happens completely seamlessly.
Easy to Use
Drag and drop planets and ring worlds into your game world. Customize their size (no limit to how big you can make them), mass, terrain style, and more. A user guide explains how to use the asset, as well as explaining how it works for the curious.
Fully Customizable
With full source code and documentation, more advanced users can mold the engine to suit their exact requirements.
Target Any Hardware
The Space Exploration Engine can bring seamlessly traversable, life-size planets and ring worlds to just about any hardware that your players are likely to have. It doesn’t require a fancy GPU, nor even a fast CPU on low settings.
Demo
The planets in the demo are about the size of Earth, while the ring world has three times the radius of Earth.