Throughout history, humanity has made a few scientific blunders where a scientific principle held as fact for hundreds of even thousands of years is found to be completely or mostly false.
Our solar system is of a finite size. I have an idea based on aliens making a scientific blunder that is obviously (to us) not true. The aliens believe that there is no possibility of alien life because they think that the universe functionally ends at the edge of their solar system. Stars and other visual and electromagnetic effects from outside their solar system are explained as complex reflections and optical illusions. Could an intelligent species form this hypothesis, or would other scientific principles mean that no thinking race would believe it by this technology level?
• Assume the aliens are identical to humans in terms of intelligence/curiosity
• The technology is close to 1950s Earth. (No artificial satellites or advanced computers, but electricity and flight)
I particularly want to know of any evidence that sharply contradicts the hypothesis that the universe ends at the solar system, or any reason a society could not have discovered that a star system is finite without first confirming that the universe is larger.