The Universe is a big place and our Milky Way galaxy is just one of many billions of galaxies. Because distances between galaxies are enormous, it is unrealistic to expect contact from extraterrestrials from neighbouring galaxies like Andromeda any time soon, if ever. So, we focus on our own galaxy.
The Milky Way is over 100,000 light-years in diameter, possibly 120,000 or 150,000, depending on estimates. It is shaped as a disk, 1,000 light-years thick. A light-year is of course a measure of distance that light passes in a year, where one light-year is 9,460,528,400,000 kilometres. That’s almost 10 trillion kilometres for just one light-year. Of course, these distances inside the Milky Way are nothing compared to distances between galaxies that are relatively near to us.
The Milky Way contains roughly 200 to 400 billion stars and it is estimated that on average, there’s more than one planet per star. This means there are between 100 and 400 billion planets in our galaxy, possibly many more. We are only beginning to realise just how common planets are. Our Solar system has eight, but we don’t know if our system is unique or common in our galaxy. There are some estimates that there are 40 billion Earth-like planets in the Milky Way, plenty of them in habitable zones around their stars. Many of them probably have life on them, intelligent or otherwise. With so many planets capable of supporting life, it is highly unlikely we are alone in our galaxy.
So, if there are so many potential alien civilisations in the Milky Way, why haven’t we heard from anyone yet?
There are several possible explanations.
First, until proven otherwise, it is possible that we are alone, however unlikely. This is unlikely simply because of the big numbers listed above. We “know” there are other civilisations out there, but since it has been only 20 years since we discovered the first exoplanet and there are about 2000 confirmed exoplanets to date, we have no idea how planets harbouring life are common in our galaxy.
Second, we are the most advanced civilisation in our neighbourhood and other civilisations are invisible to us at the moment. This is possible. Whether it is likely, depends on how common life in the Universe is. If there are only 10 or so planets with intelligent life within 50 light-years from us, it is quite possible that we are the only species capable of communicating with others.
Third, there are aliens who are aware of our presence, but choose to keep quiet. This is not unlikely, because we have no idea how other species may react, but if there are many alien species, then there is a galactic community. It is not far fetched that their relations resemble relations between nations on Earth. It would be unlikely that they all decided to keep quiet. Even if there was some general rule like the Prime Directive from Star Trek where only civilisations with faster-than-light means of travel are to be contacted, it is unlikely that all of them would follow this rule. Also, how come we haven’t picked up artificial signals coming from this community?
This is why the fourth possibility is the most likely one - there are many alien civilisations in the Milky Way, but still the majority of star systems is uninhabited. In our immediate neighbourhood of 50 light-years there is either nobody or at least our neighbours are too few and not advanced enough. Vast distances between stars make sure that to date, artificial radio transmissions from Earth reached only a handful of systems. At some frequencies, Earth “shines” brighter than the Sun. Even if someone received the signal from Earth and replied, it takes time for radio signal to travel. One light-year per year. Civilisations that send unintentional signals themselves are possibly too far away and their signals are too weak for us to receive.
Keeping in mind our galaxy is 100,000 light-years across and that nothing travels faster than the speed of light, according to Einstein, it will take a lot of patience to achieve the first contact with alien civilisations. The nearest star from Earth is Proxima Centauri, four light-years away. Some estimates put about 1,500 planets within 50 light-years from Earth. Again, depending on how common life in the Universe is, some civilisation on one of them may already be aware of our presence and have sent a signal that is yet to reach us.