Compile package with java

I have these programs in java:

//file ../src/com/scjaexam/tutorial/GreetingsUniverse.java
package com.scjaexam.tutorial;
public class GreetingsUniverse {
    public static void main(String[] args) {
        System.out.println("Greetings, Universe!");
        Earth e = new Earth();
    }
}

//file ../src/com/scjaexam/tutorial/planets/Earth.java
package com.scjaexam.tutorial.planets;    
public class Earth {
    public Earth() {
        System.out.println("Hello from Earth!");
    }
}

I am able to compile with no error the second one using:

javac -d classes src/com/scjaexam/tutorial/planets/Earth.java

This puts the compiled file Earth.class in the ../classes/com/scjaexam/tutorial/planets/ folder as expected. Now I have to compile the main class GreetingsUniverse.java but this command fails:

javac -d classes -cp classes src/com/scjaexam/tutorial/GreetingsUniverse.java

src/com/scjaexam/tutorial/GreetingsUniverse.java:7: error: cannot find symbol
        Earth e = new Earth();
        ^
  symbol:   class Earth
  location: class GreetingsUniverse
src/com/scjaexam/tutorial/GreetingsUniverse.java:7: error: cannot find symbol
        Earth e = new Earth();
                      ^
  symbol:   class Earth
  location: class GreetingsUniverse

What is the right command to compile (and then tu run) this program?