Any animal that can move about will naturally have a front end and a back end. Sensory organs such as eyes will tend to evolve at the front of the body because they are most useful there. Lifting the body up on legs reduces friction with the ground and legs are easier to coordinate if the left and right sides are symmetrical.
If the creatures from other planets are sentient, then it’s reasonable to suppose that they also make and use tools to interact with their environment. That requires at least one limb to hold the object and another one to hold the tool. To keep their hands free while they move, these aliens would need at least another two limbs. Put all that together and you have a humanoid shape with two arms, two legs, a head and all the other vital internal organs in a central torso. But it’s easy to imagine lots of other possibilities too. A civilisation of sentient octopuses, for example. Or a race descended from starfish, which walks on two legs and has three others available to interact with the world.
Palaeontologist and evolutionary biologist Prof Stephen Jay Gould said that even if we “re-ran the tape” of evolution here on Earth, life could end up looking different to how it looks now. On an alien planet, with higher gravity, animals might need more legs to remain stable. And aliens that float in the clouds of a gas giant might be covered with eyes that can see gamma rays, to spot danger approaching from any direction.