i need help in HTML/JS Challenge: Create a solar system

idk what to start first 

but i made a little code

  var newPlanet = document.createElement("div");
document.body.appendChild(newPlanet);
newPlanet.className = "planet"

var moon = document.createElement("div");
document.body.appendChild(moon);
moon.className = "moon"

There is no explanation for this challenge..