1 minute ago, Tramp Graphics said:For the first question that's easy, a sphincter . It's a common muscle in a digestive tract.
As for why Han would know that he didn't need a space suit, that too is relatively easy. The Falcon's sensors told him that it had an atmosphere, just one that wasn't breathable to humans. Any pilot would check his ship's sensors before debarking in order to know if it was safe to go outside or not. That's standard procedure .
Again, how is a hole that you can fly a spaceship through keeping in atmosphere and pressure? The Falcon's crew didn't notice and sphincter. They were concinced they were in a natural cave.
He migh have checked, but we don't know that he did. But if he would have checked, he would have noticed all that pressure and atmosphere that's not supposed to be there, and immediately would have taken off again, or not even have landed at all. Or at the very least would have found it odd and remarked upon it.