This vox explainer tries to explain why we imagine aliens the way we do – largely humanoid, often bipedal, with the same number of eyes etc. Via Vox:
No one really knows what aliens look like, but we all have similar ideas about them. It’s often a creature with a big head, long arms and legs, and big, buggy eyes. We see these common images of aliens depicted in movies, books, and on TV shows — which are made by us.
Science fiction stories often explore the relationship between humans and aliens. So we find extraterrestrial creatures endowed with relatable human features. According to Charley Henley, a VFX supervisor who worked on Ridley Scott’s Alien series, “A lot of [Scott’s] designs are tied in with the human anatomy, and I think that is the common theme. We put a lot of humans into the aliens.” In our stories, we naturally anthropomorphize these creatures — so they end up looking, acting, and feeling just like us.
But when you talk to scientists actually looking for extraterrestrial life — and not imagining it — it’s a whole different story. Andrew Siemion, the director of SETI Research Center at UC Berkely, believes that if there’s any life out in space that’s similar to humans, they’d be using technology in a similar way as we do. So he and his colleagues conduct astronomy experiments to detect signs of technology out in space. But that doesn’t mean he has a better idea of what these creatures would look like. He said: