You need to run the program. That is the point of all this.
Enter the command
java HelloPlanets
to run the program. Actually, java starts the Java virtual machine, which then reads in the file of bytecode HelloPlanets.class, which the virtual machine starts executing.
Warning: The command is java with no "c" at the end.
Warning: Use the filename HelloPlanets with no ".class" at the end.
Well, that is one way to do it. Experienced users of Windows will be familiar with most of the steps. They will have found this process fairly easy. If you did not, it is merely a matter of practice (or finding a friend.)
Is Pluto a planet?