The DCU is set to unleash a core Kryptonian weakness that threatens David Corenswet's Superman one year after his own solo film. James Gunn's Superman, which premiered in Summer 2025, provided a notable tidbit about Kryptonian physiology, with the film's final moments showing Clark Kent telling Robot # 4 (Gary) that Kara used to go to planets with Red Suns to party and get drunk. While this throwaway line seemed to only serve as a fun nod to Kara's other lifestyle off planet, Supergirl provided more context on the impact of a Red Sun on the titular Kryptonian and, by extension, Superman.
Supergirl's CinemaCon footage unveiled the DCU's Red Sun planets and the weakening effects on Supergirl. The extended sequence showed Kara inside an interplanetary space bus traveling between worlds. The vessel was under the light of a red sun, immediately depowering her to near-human levels.
Supergirl was visibly not at full strength, and she even asked the pilots how close they were to a yellow sun and tried to suit up in a space helmet to reach one. This interaction confirmed that she was aware her powers were drained and that she needed yellow sunlight to recharge.
One of the pirates found Kara and blew her out of the airlock. An unconscious Kara was then seen floating helplessly, proving she's as vulnerable as any human without yellow solar energy. However, the nearby planet rotated, and the yellow sun rose over the horizon, restoring her powers instantly.
The Red Sun's complete weakening of Supergirl poses a threat to Superman as well, weakening his powers and putting him on par with a regular human. It effectively introduces a massive threat to Superman, since villains, most notably Lex Luthor, only knew that Kryptonite was his sole weakness.
This completely changes the game entirely because villains now have a clean, non-Kryptonite way to neutralize Kal-El, but it would take a large amount of effort to lure him to a red-sun planet or deploy red-sun radiation tech to reduce him to a near-death state.
It effectively raises the stakes for future encounters with more powerful villains, such as Brainiac in Man of Tomorrow, canonizing red-sun planets as a lived-in, exploitable Achilles' heel for the remaining Kryptonians in James Gunn's DCU.
Interestingly, in DC Comics, Supergirl is sent to a planet with a green sun, which further weakens her powers and nearly kills her. It is also stated that Superman only lasted a short time on the green sun planet before being saved by the Justice League.
If Supergirl or any future DCU project adapts this aspect from Tom King's Supergirl: Woman of Tomorrow comic, this would introduce an even bigger weakness for the two of them.
Supergirl arrives in theaters on June 26.
How the DCU Can Use the Red Sun Against Superman (Theory)
2025's Superman revealed the titular Kryptonian's first-ever loss to a villain fight after Ultraman (with the help of Lex Luthor) defeated him. This proves that Superman is beatable despite being powerful.
The impact of a red sun seems far more deadly than that of Kryptonite, mainly because it can turn a godlike hero like Superman into an ordinary person.
There is a strong chance that Brianiac already knows how vulnerable Superman really is to the red sun, and the Man of Tomorrow villain could have already prepared or reverse-engineered some red-sun tech arsenal ahead of his eventual arrival on Earth. By doing this, Brainiac is well-equipped to handle Superman's opposition.
In a way, this would also justify Superman's need for allies to defeat more powerful villains, which could eventually lead to the formation of the DCU's version of the Justice League.
