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Why is it so difficult for someone to imagine the possibility that extra terrestrial beings might exist and may have visited this planet.We know gravity exist yet we can’t explain how it works. We know infinity exist yet we don’t understand that either. So why is it so hard to entertain the thought of alien beings visiting us. Oh, physics you say, impossible for it to happen. Physics can’t explain gravity nor infinity either. Maybe we don’t know all there is to know about interplanetary or intergalactic physics.

In addition to the obvious carbon-based-life-hospitable factors, such as a temperature suitable for liquid water and a suitably-sized sun, there are many more factors required for a planet to be fit for spaceship-constructing, carbon-based intelligent life (with more factors likely to be discovered):

Suitable gravity for atmosphere, with appropriate pressure, to retain oxygen and water vapor but not hydrogen; suitable axial rotation rate; appropriate tilt of planet axis for seasons; suitable orbital distance from star to prevent tidal locking; magnetic field from planet’s molten core to shield atmosphere from cosmic radiation; sufficient planetary plate tectonics for crustal mineral recycling; suitable distribution and shape of continents to sustain deep ocean currents; sufficient actinide radioisotopes in planet interior for decay heat; suitably-sized moon to create needed tides and stabilize planet rotation; giant outer gas planets to prevent large asteroids and comets from frequent collisions with planet; suburban galaxy location providing sufficient distance from recent supernovas; far enough away from gravitational disturbances by higher density of stars in inner area of galaxy; only minor orbital variations caused by small chaotic gravitational interactions between nearby planets.

With these (and likely even more) factors the number of planets in our galaxy capable of sustaining such carbon-based life is much smaller than the media and grant-seeking astronomers imply in their articles. The estimated probable number is likely very close to the experimentally known value of one.

As for non-carbon-based life forms, those can be found in numerous science fiction movies, books, and comics.