WARNING: The following article contains spoilers for Superman #27 by Brian Michael Bendis, Ivan Reis, Danny Miki, Alen Sinclair, and Dave Sharpe, on sale now.
During Superman's fight with his newest enemy, Synmar, Superman realizes he is fighting something entirely different from what he is used to. Throughout the conflict, he is struck by just how alien Synmar is in comparison to previous threats against him. Sensing that his opponent wasn't motivated by malice, Superman compelled Synmar to lead him to the alien's home planet to learn why he was so angry. Unfortunately for Superman, this decision would make him the victim of ignorance, for upon arriving at Synmar, he was trapped in the orbit of an orange sun which not only reduced his powers, but also his intelligence.
The entire point of the fight was to show how out of his depth Superman was when dealing with Synmar. He had no idea what powered him, where he came from, or why he was here. And no support from the collective knowledge of Krypton of the United Planets gave Superman an advantage.
And in Superman #27, the Man of Steel took a leap of faith to resolve their conflict. But in doing so, he learned about another alien aspect of Synmar in the form of its orange sun, which has some debilitating side effects
While this may have been his first time encountering an orange sun in this universe, it is not the first time he has ever encountered one in the history of the DC Universe. In Action Comics #289 by Jerry Siegel and Jim Mooney, Superman traveled to a planet where an older version of Supergirl existed. This planet had an orange sun, but unlike in current continuity, this sun had no effect on Superman. Although, considering that this issue had Supergirl trying to play matchmaker by pairing Superman up with an older version of herself it isn't too surprising that this story is not considered canon.
Other forms of solar radiation also affect Superman. The most well known being red solar radiation, matching that of his birth world's sun, it robs Superman of his powers, in essence making him as human as can be. Another example it green solar radiation, introduced in Superman #155 by Bill Finger, Wayne Boring and Stan Kaye, this sun has a similar effect to red suns, taking away Superman's powers. And perhaps the most interesting is what happens when Superman is near a blue sun. Discovered on Bizarro's world of Htrae in Action Comics #855 by Geoff Johns, Richard Donner and Eric Powell, this sun gives Superman extra powers, a fitting notion considering the backward nature of Bizarro's world.
Typically, all these suns have some kind of physical effect on Superman, but this is the first time a sun has been implied to have a mental side effect for Superman as well. Reducing his intellectual capacity makes Superman significantly less dangerous. Many people assume Superman is all brawn, but he has demonstrated time and again that he can match even the greatest minds of the universe. How else could he have beaten Lex Luthor so many times?
But the concept of solar radiation affecting the Man of Steel's mental faculties has been a debate among fans for years. It even came up in a fun exchange in Seinfield, where Jerry and George debated if Superman's sense of humor changed based on what sun he lived under. While question remained unanswered, the introduction of the orange sun implies that while it may not affect his sense of humor, certain forms of radiations can actually impede Superman's ability to think effectively, if not alter his personality traits.