Despite the semi-colon error, your if's are all redundant anyway.

You have already established planetchoice is the required value using the case statement, you don't need to check again with an if, you just need to break like so:
switch ( planetChoice ) {
    case 1:
        printf ("The astronaut's weight on the planet Earth is %1.0f\n\n", floor (earthWeight));
    case 2:
        /* ... */