In our solar system, there are 8 planets orbiting a star, the Sun. And I understand that there are about 500 confirmed solar systems out there.
But why is it always planets orbiting stars? Why can't it be several stars orbiting a planet, or a star orbiting a star? Why is a star by definition stationary as opposed to planets which are moving.
Of course, in that case, it wouldn't make much sense to call it a solar system, but still.