At URL: http://download.oracle.com/javase/tutorial/java/javaOO/enum.html
you use at example on Planet enum, two methods, that are unused in the exercise and that can create confusion in a student (like me :) ) that read the exercise:
infact, the:

private double mass() { return mass; }

private double radius() { return radius; }


are not used in the exercise, and the name of the methods is equal to:

private final double mass; // in kilograms

private final double radius; // in meters

that can generate confusion.