The quick answer is “yes;” but despite the saucer “sightings,” abductions and rants of the conspiracy theorists, there is absolutely no proof that aliens have ever visited earth. However, most planetary scientists and astrophysicists believe that intelligent life is most likely abundant in the universe, but not in planets orbiting the sun. There are many billions of stars in our galaxy, the Milky Way, vast numbers of which have planetary systems in the “Goldilocks zone” (not too hot and not too cold). These also have the three essentials of life: A source of energy such as heat from their star, water and essential organics such as oxygen, hydrogen, carbon and nitrogen. Our three closest stars–the Centauri system–have at least one earth size planet; but it is about 25 trillion miles away, and visitors to earth would need to be immortal or in suspended animation to make the journey. If they came that far, they would not tease us with mere sightings.
The question of non-intelligent life in the universe is different. In our solar system, a minimum of Mars, Jupiter’s moon Europa and Saturn’s moon Enceladus could support microscopic life; as they have the aforementioned three necessities of energy, water and organics. We may have proof of such life before long, but don’t look heavenward expecting aliens any time soon.
