I have written several times already on this subject, but there always seems to be something new to say. I am speaking, of course, about the possibility (probability?) of life on other worlds than ours.
Reading speculative fiction from the early 20th Century, it appears that there was at least some expectation of we would eventually discover extraterrestrial life right here in our own solar system. Mars was the perennial favorite for such speculation. From H.G. Wells to Edgar Rice Burroughs to Stanley G. Weinbaum to C.S. Lewis to Robert Heinlein to Arthur C. Clarke, it was practically taken for granted that the Red Planet would be absolutely crawling with life. But it wasn't just Mars. Stories were written about life on the Moon (First Men in the Moon) Venus (Parasite Planet), Jupiter (Call Me Joe), the moons of Jupiter (The Mad Moon), and even the asteroids (Garden in the Void).
I have to confess to a desperate optimism that the next robotic space probe would find some sort of life on one or another celestial body, only to have my inflated expectations dashed time and again by the grim reality of what was actually out there. I vividly recall making up a list (sometime around 1980) of the places that might conceivably harbor life of some sort. It's rather amusing today to look back and see what worlds I included: Mars (of course), Enceladus, Iapetus, Titan, Triton, Ceres, and Vesta, and one or two less likely places. I finally gave up all hope after the Dawn spacecraft arrived at Ceres in 2015, only to find it a battered, lifeless pile of rocks. I was actually seriously depressed for some weeks afterwards.
So now, all speculation must center on the so-called exoplanets - those circling other suns. The problem is that the chances for life "out there" aren't much better than for elsewhere in our own solar system. First off, we have to face reality about how many suitable stars are in the Milky Way. Not that many, it turns out. Oh, I know, people say "There are a hundred billion stars in our galaxy. There's just gotta be life on millions of them!" But not so fast. Remember that the first stars in the universe (which includes our galaxy) were what astronomers call "metal poor" - that is, deficient in elements heavier than Hydrogen and Helium. But you can't build planets without heavy elements - iron, nickel, carbon, and all the others. It takes at least three generations of stars before you get any that are sufficiently rich in heavy elements to suit our purposes. At least a third of the stars in our galaxy are "first generation" stars, so we have to eliminate them right off the bat as potential places to find life. That's 30 billion already. Next, the Milky Way is a far more inhospitable place than imagined even a few decades ago. A nearby supernova could conceivably sterilize a solar system, snuffing out whatever struggling organisms might have been there. And in the galactic core (comprising roughly a third of our galaxy's stars) the stars are perilously close to each other, absolutely inviting such periodic sterilizations. And we haven't even gotten to the issue of stellar mass. A full 80% of stars, not only in the Milky Way but in the entire universe, are red dwarfs, which are so "cool" that any planet hoping to harbor life would have to orbit so close to its star that it would be tidally locked, keeping the same hemisphere forever in daylight and the other in perpetual night. Any atmosphere such a world might have would over time eventually freeze out on the nightside, leaving the rest of the world to resemble Mercury more than the Earth.
At the other end of the size spectrum, stars with a mass several times greater than our sun have significantly shorter lifespans, only a few hundred million years. And when you think that it took our sun more than three billion years to bring forth just one planet (us) with complex, multicellular organisms, well... even "a few hundred million years" is simply not going to cut it.
We're now down to no more than 10 billion stars. Still a lot, you might think. But again, not so fast. Three quarters of those that remain belong to multiple star systems, making the probability of any planets being in stable orbits less than certain. Plus, we have to weed out red giants, variable stars, blue white super giants, carbon stars, stars inside globular clusters, and stars at the centers of so-called planetary nebulae. We're left with maybe a few hundreds of millions of candidate homes for extraterrestrial life. Yes, that's still a big number, but less than 1% of the more than 100 billion we started out with.
Now lets examine the many exo-solar systems we have so far discovered. One surprising discovery made since we began racking up the numbers of known solar systems is how many of them contain what are known as "Hot Jupiters", that is, a Jupiter sized planet sitting right where the inner planets (including the Earth) do in ours. Subsequent computer simulations have suggested that this may be the norm for solar systems, and that ours is a notable, and possibly rare, exception. It seems that the normal course of events is for a Jupiter sized planet to form in the outer solar system, and then for it to migrate over time into a much tighter orbit... right in the middle of a star's "goldilocks" zone (where conditions are "just right" for life to flourish).
So why have we been so lucky? Why has our own solar system's Jupiter remained where it is, safely out there in the outer solar system? The answer? Saturn! The orbits of the two largest planets are in a roughly 1:2 synchronicity, which means that they tug on each other with regular frequency. The end result is that Jupiter's tendency to migrate inwards is balanced by Saturn's pulling it back out (and thus saving the Earth from annihilation). Now what are the chances of a solar system possessing both a Jupiter and a Saturn? I honestly do not know, but I would imagine their not being very high.
So... we dodged that bullet. But how many other cartridges are there in nature's gun? Another is our Kuiper Belt (made up of objects composed largely of water ice). Many planetologists believe that our oceans originated not on the Earth, but rather out in the far reaches of the solar system, out beyond Neptune and Pluto. These KB Objects are occasionally disturbed by near misses with other stars, which cause them to plunge into the inner solar system as comets, and the occasional super comet. (There's one of those lurking near Jupiter, Comet 29P, right now. The last super comet was Hale Bopp, nearly 30 years ago.) So no Kuiper Belt, no oceans. No oceans, and you likely get a dead Earth. And even the presence of an extra-solar Kuiper Belt is no guarantee than any inner, rocky planets will have oceans. The water bearing object first has to impact the planet. Ours got hit (multiple times, it would seem), but Venus has likely missed out on such watery enrichment. The Earth is rightly called the "Blue Planet" (insert political joke here), but Venus is about the driest thing imaginable.
Now let's move on to the Moon? What? The lifeless Moon? How does that effect life here on Earth? Well, as it turns out, a great deal. First of all, its enormous size in relation to its parent planet makes it (almost) unique in our solar system. I say "almost" because Charon is larger, relative to Pluto. But many astronomers don't consider Charon to be a moon of Pluto at all, the two of them being rather a double planet - the only one we know of.
And why is the Moon's size significant? Because it has an outsized influence on the Earth. We all know about the oceanic tides, of course. But the Moon not only pulls on the Earth's water, it is also pulling on the very rocks under our feet. The effect is naturally infinitesimal, but nevertheless there. (And remember that the Moon was long ago far closer to the Earth than today.) Over the millennia, this twice daily tugging could well have fractured the Earth's crust into its tectonic plates, allowing for a continual replenishment of our planet's surface. No other body in our solar system experiences plate tectonics as ours does. Sure, many moons in the outer solar system appear to have seen such activity in the distant past, but it seems to have extinguished aeons ago. Plate tectonics are what prevented the Earth from winding up today like Mars or Venus, with their dead, dead, dead surfaces. So, thank you, Moon!
The Moon has also played a role in stabilizing the Earth's axial tilt, preventing wild gyrations in our rotation and smoothing out the seasons. An unstable axis could very well have resulted in geologic eras in which one hemisphere might be experiencing an ice age, while the oceans were boiling away in the opposite. Yikes!
Next, we have to consider our highly magnetic core. No other rocky planet has such a core. This is important to life, because it is our core which governs our world girdling Van Allen belts, which are basically our only defense against naked exposure to solar and cosmic radiation. It is believed that lack of such protection is what caused both Venus and Mars to lose whatever water they may have one had. Unfiltered solar radiation breaks water molecules down into their component atoms, which then fly off into interplanetary space. (This fact has caused me to speculate that maybe, just maybe, a significant portion of our planet's oceans originated on Mars! Who knows?}
I could go on, but by now you must realize that we live on a very unique planet indeed, one almost designed to support life. Or at least life as we know it (and we know of no other kind).
Bottom line (and personal opinion): We may very well be alone in the Milky Way. I can't speak for other galaxies.