Yes, a star can turn into a planet, but this transformation only happens for a very particular type of star known as a brown dwarf.

Yes, gravity can forms waves. Gravitational waves are ripples in spacetime that travel through the universe. If you think of gravity as a force acting at a distance, it is difficult to visualize how gravitational waves could form.

Sound does not travel at all in space. The vacuum of outer space has essentially zero air. Because sound is just vibrating air, space has no air to vibrate and therefore no sound

Despite the whimsical fantasies of fictional works and the confusion of supposed eye-witnesses, there is no credible scientific evidence that aliens have ever visited the earth.