Something's not quite right. Our earth looks uninhabited. Unlike other planets in our solar system (to my knowledge), only earth has city lights that are visible from space on the night side of the planet. We have another texture of earth at night, called earth_night.png, that we can add to the Earth material. We will make this texture an emission, meaning it will produce its own light (which city lights would do on the dark side of the planet):

1. Import earth_night.png into the Project Assets/SolarSystem/Textures folder, if not already.
2. Select the Earth material. In Hierarchy, select Earth, and in Inspector, unfold its material (or navigate to Assets/SolarSystem/Materials/earth).

3. Check the Emission checkbox.
4. Drag the earth_night texture onto the square chip next to Emission Color.
5. Dial down the emission level (far right of Emission Color) to something like 0.5.
6. Save the scene and the project and click on Play.

I don't know whether you can see in the following screenshot, but the day side the planet is lit by the sunlight, and the back shows city lights' emission. Cool!