What are some alternatives?
When comparing Cosmonium and Gaia Sky, you can also consider the following products
Space Engine - Space Engine is a realistic virtual Universe you can explore on your computer.
Stellarium - Stellarium is a free open source planetarium for your computer.
Celestia - Celestia - real-time 3D visualization of space.
KStars - KStars is a Desktop Planetarium for KDE.
Solar System Scope - 3D model of the Solar System and night sky in real time
Universe Sandbox - Universe Sandbox ² is a physics-based space simulator.