Cosmos Journeyer is designed with a clear separation of concerns in mind. If you are willing to contribute, this document will give you a good idea of where everything is and how it is organized.
The following diagram is not an inheritance diagram, but a composition diagram. Each arrow represents an "is part of" relationship.
For example, planets are part of the celestial bodies.
Even though you could argue the existence of an "is a" relationship between CelestialBody and OrbitalObject, I chose to avoid class inheritance for flexibility.
This means that in the previous diagram, in the Orbital Objects tree, are only classes the concrete types: Star, BlackHole, NeutronStar, TelluricPlanet, GasPlanet, and SpaceStation. The rest is only interfaces:
It all starts with simple Transformable and BoundingSphere:
those are simple objects that possess a BaylonJS TransformNode for their position, rotation, scaling and a bounding volume to make simple calculations for distances.
An OrbitalObject builds on top of this by adding the notion of orbit. They possess OrbitProperties that describes their motion around their parent which can be null in the case of some objects (think like stars).
OrbitalObject also possess PhysicalProperties that complement the OrbitProperties with information such as mass, axial tilting and rotation period.
To sum up, an OrbitalObject is an object that can rotate around another OrbitalObject and rotate on itself.
CelestialBody builds up on top of OrbitalObject by adding the notion of BODY_TYPE and radius that is expected from spherical objects.
CelestialBody are spherical orbital objects that encompasses both planets and stellar objects.
They can have specific post-processes applied to them (like atmosphere, clouds, rings...), which is why they also implement HasPostProcesses.
StellarObject builds on top of CelestialBody by adding a PointLight that is used to light up the scene. They also have a STELLAR_TYPE that describes their type (star, black hole, neutron star).
Planet builds on top of CelestialBody by adding new PlanetPhysicalProperties that describe their temperature and pressure. They also keep track of their number of moons.
The other nodes are the concrete implementations of all these abstractions.
They can be found in their respective folders (planets for TelluricPlanet and GasPlanet, stellarObjects for Star, BlackHole and NeutronStar, and spaceStations for SpaceStation).