function call

[Jasper]

This may sound like a simple question,but can u explain why we have to put
newOne=newPlanet(); ? Would it be ok if I just put newPlanet(); there?

void enterPlanets(List *list)
{
    Planet * newOne;
    do
    {
        newOne = newPlanet();

        if(newOne != NULL)
            lst_add(list, newOne);
        else
            printf("Unable to create new planet\n");

    } while(yes("Add more planets"));
}

[laserlight]

Probably not, since this newPlanet() is a function call that presumably returns a pointer that should be assigned to newOne.

[BEN10]

newOne=newPlanet(); ? Would it be ok if I just put newPlanet(); there?

It would have been ok if newplanet() would be returning void, which is probably not the case looking at your code.