By ‘habitable’ I mean planets that are neither too cold nor too hot but occupy a sweet spot that would be conducive to life as we know it existing. By combining actual data, from the Kepler space observatory launched by NASA in 2009 to look for Earth-like planets orbiting other stars, with statistical analysis, scientists have come up with an estimate of 15-30 billion habitable planets in our Milky Way galaxy alone. That seems like a lot, even if that number is tiny compared to the estimated 1011 stars in the galaxy.
When one factors in that there are about 1011 galaxies in our universe, the potential number of Earth-like planets is pretty large, around 1021, leading to the possibility that we (i.e., sentient life forms) are not alone in the universe.
So, to ask once again Enrico Fermi’s famous question first posed back in 1950: Where is everybody?
If life exists elsewhere, the reason we have not made contact with them may be because their technology is too primitive to make contact with us or the planet is too distant to do so, because of the limits posed by the speed of light.
Of course, there is always the possibility that the occurrence of the first self-replicating molecule that is necessary for natural selection to do its work and start the drive to more sophisticated life forms is so astoundingly tiny that our Earth is the only place in which the odds were overcome.
I personally feel (with no basis whatsoever) that there is a good chance that life in some form exists elsewhere is the universe but that we will never be able to overcome the difficulties posed by distance.
So while we may not be alone in the universe we will never truly know if we are or not.