What's your best guess of the probability that intelligent aliens are currently visiting Earth?
There are as many as 400 billion stars in our own Milky Way Galaxy, and many of them have planetary systems. Some subset of those billions of stars are sure to have water bearing planets orbiting them within their habitable zone (where temperatures allow for liquid water). Deep space scans by the Hubble Space Telescope tell us there are about 500 billion galaxies out there, some smaller than ours, but many larger.
Since we do not understand abiogenesis, we cannot fix any real probability for it happening anywhere. We can only say for certain that it either happened in our Earth’s case, or somewhere else then got transported somehow to Earth, because we couldn’t be here to say so if it had not. A reasonable guess (SWAG) is that the probability for life elsewhere in the Universe is extremely high, but not exactly 100%. If and when we figure out how abiogenesis occurs, we will be more able to fix a probability on its happening elsewhere, and have a better idea of the conditions necessary for it to occur.
The probability for intelligent life with advanced technologies would be much lower, though. For that to arise, a planet would need a stable, life-friendly environment lasting billions of years. Any nearby supernova, quasar, pulsar or gamma ray burst would bring the whole adventure to an end before intelligent life could evolve. A collision with a large space object, or such an object hitting the life-supporting planet’s star and causing a massive coronal ejection would also end life’s development there. The parent star can’t be a supermassive one, because it would supernova too quickly for life to begin. It has to be a main sequence star that will live many billions or ever trillions of years. Due to fluctuations in their parent star, many planets probably have environments capable of supporting simple single-cell life, but environments too extreme to support complex life-forms.
Then there is distance. Consider that Voyager 1 is now traveling at 35,000 MPH away from our Sun, and has been since 1977. At that speed, it will still take it another 40,000 years to reach the halfway point between the Sun and Centauri Proxima, our closest neighboring star at just 4.24 light years away. Our Milky Way is between 100,000 and 120,000 light years across. The known Universe is currently 94 billion light years across. Due to the accelerating expansion of empty space, distant parts of our Universe are now receding relative to us at speeds faster than light. If there are life forms out there, what is the mathematical probability that life developed advanced technology but did not destroy themselves with it, that they instead learned to travel thousands or billions of light years in a single lifetime, and that having managed all that, they set out to visit Earth of all possible places?
It would seem to me that given all that, the probabilities fall from near certain to near vanishingly small. Am I wrong? If I am, please convince me, because I would love to meet and interview another ET.