Hello, this is more of an invitation to a speculation rather than precise question about biology, but I will welcome any answer I can get. My primary concern is related to humans, but I digress to related organisms too.

I've been thinking about the way pop-culture portraits alien life
forms. The overwhelming majority of fictional aliens are humanoid in
shape. I see it as laziness on writers' part and also something to
appeal to humans who are limited in their tolerance and understanding
of the biological diversity. Designing intelligent species that are not humanoid must surely be very challenging and confusing to the audience. We are so used to seeing faces that we can hardly imagine not finding one in a potential extraterrestial (intelligent) life form.

Being an amateur hobbyist writer myself, I've tried to get better than
that, until I had a certain thought. So many surface organisms are similar in their form: a distinct head, limbs, torso. This applies to some invertebrates too. Could it be that the form of vertebrates, including humans who are just a bipedal variation, is so well-adjusted to living on a rock-type planet (Earth being one of them) that successful species eventually arrive at the body shape we know - torso, four limbs, and a head - through evolution? Is this shape so "good" that nature is likely to achieve it in an ecosystem similar to ours, a galaxy away?
Or is the fact that vertebrates are so successful on Earth just a
coincidence, and it is possible that if our ancestor met its demise
due to some random event, Earth would now be inhabited by, let's say,
intelligent snails rather than monkeys?

Short version: speculate - what are the odds that somewhere in the universe, there is a planet inhabited by humanoid organisms?

I realize that there are untold numbers of factors in this, such as
atmosphere, planet mass, liquids present on the surface other than
water. It also seems likely that potential organisms living on gas
giants or ice giants could follow a totally different path, more
adjusted to the environment. Still, any speculation is welcome.

Thank you and have a nice day
Tomasz G.
Poland

There is nothing especially wonderful about having four limbs, as any snake, insect, spider or millipede will tell you! The fact that so many of our Earth creatures to have four limbs is that so many of us are tetrapods: we inherited the four-limbed condition from a distant ancestor. Some have modified this either by standing upright (humans, kangaroos), changing one pair of limbs into wings (birds, bats) or losing them altogether (snakes, amphisbaenians), but all amphibians, reptiles, birds and mammals started out from a four limbed ancestor.

(If you've seen Avatar you'll remember that most of the wildlife on Pandora has six limbs. That's probably just as good an arrangement -- again, the wild success of insects suggests that it is.)

agreed and to address the questions directly "what are the odds that somewhere in the universe, there is a planet inhabited by humanoid organisms". I'd say reasonably high given the billions of planets that are thought to exist in the universe.

The real question is whether there is life anywhere else in the universe. (As someone has pointed out, but annoyingly I can't remember who, the great thing about that question is that either answer, "yes" or "no", is astounding.)

If there is life, and it has evolved complexity and intelligence, then it doesn't seem particularly unlikely that it's evolved a shape very broadly similar to ours: limbs for locomotion and more limbs for manipulation. Of course they might be built more like centaurs than humans. Or they might be more like an octopus. (Octopuses might be the best bet for intelligence evolving on Earth, if all the vertebrates were to suddenly disappear.)