Stuck on HTML/JS Challenge: Create a solar system

I can't seem to get past the first step of this challenge.  Here is the code I put in the <script>:

var planetEl = document.createElement("div");
planetEl.classname = "planet";
planetEl.backgroundColor = "pink";
planetEl.appendChild;

There's no error, I just didn't do it right.  I suspect there is something wrong with the last line, but I don't know what to do.  Help would be much appreciated!