This is a paid asset, but now you can download Neutron Solar System Pack Free.
Detail this asset from Unity Store: Original Link
Neutron Solar System Pack v2.0
Neutron Solar System Pack provides you with the assets that can help you to quickly make a simple solar system model, or other space scene.
This pack includes:
– The textures of the Sun, 8 planets, 2 dwarf planet(Pluto and Ceres), and 9 moons(Moon, Io, Europa, Ganymede, Callisto, Titan, Rhea, Triton, Charon). Some less detailed surface textures (giant planet) are 1024×512, while most of the others are 2048×1024, the earth and the moon 4096×2048. The orginal textures of Pluto, Charon and Triton have black and unexplored regions, imaginary regions added on them.
– Planet shader and animated Earth shader
– Animated Sun shader
– A simple scene (object size to scale, distance not to scale).
The scene is mainly intended for Windows, but they can run on other platforms. You need to implement your own controls for moblie devices.