How can i make a planet in unity 5?

This is a complicated question, with no short and easy answer. Quite frankly, if you're asking this question you probably don't have the base skills you need to do this yet. But since that isn't an answer and only a disclaimer, these links will introduce you to the basics:

I feel that this video here will give you an intro into some of the problems, and solutions for these sorts of things.

This link here is an ongoing tutorial series by Sebastian Lague covering general procedural terrain.

On top of that you will need to go here to get your hands on LibNoise. You need this because you will have to be able to use 3D perlin noise rather than 2D perlin noise. This is because it is rather difficult to create terrain along a sphere otherwise.

To sort of bring all this together, there is a blog here that does this. It is not a Unity based project, but all the theory that is presented is sound, and walks through many of the problem solving steps.

This is not an easy thing to do, especially if you are a beginner. If anything, you should do what you can to master procedural mesh generation first.