MCU Theory Explains How Thanos" Brother Can Exist & Fit Into The Marvel Villain"s Infinity War Mission

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Summary

A new theory suggests that Eros may have been adopted into Thanos' family on Titan, explaining their brotherly connection.
The theory speculates that Eros was sent to Titan along with other Eternals to ensure population growth and the emergence of a Celestial.
The theory also provides a possible explanation for Thanos' actions and the planetary shift of Titan in Avengers: Infinity War.

A major MCU theory may help to explain the connection between the Mad Titan Thanos and his brother, Eros, who debuted in Phase 4's Eternals, and could provide more context to Thanos' Avengers: Infinity War conquest. Even though Chloé Zhao's MCU directorial debut Eternals is viewed as one of the MCU's most divisive projects, the Phase 4 project teased a more in-depth exploration of Thanos' backstory by introducing Harry Styles as Eros, Thanos' brother. However, the connection between Eros and the iconic MCU supervillain has caused some confusion, especially since Marvel Studios altered the Eternals' origin story from Marvel Comics.
The Mad Titan Thanos first appeared in the post-credits scene of The Avengers, but was portrayed by Josh Brolin from Guardians of the Galaxy onwards. He wasn't properly introduced until 2018's Avengers: Infinity War, however, as the Phase 3 project explored Thanos' quest to acquire the six Infinity Stones and bring balance to the universe. Despite revealing more about Thanos' backstory, Infinity War failed to mention anything about his family on Titan, which made Eros' debut in Eternals all the more complicated. Even so, a new theory may have uncovered the exact connection between Thanos and Eros in the MCU.
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Related Eternals Teased 4 Major MCU Stories... And Only 1 Actually Matters 2 Years Later Although Chloé Zhao's Eternals set up four major storylines for the MCU's future, only one of them might be acknowledged during Phase 5. Marvel Studios Changed The Eternals’ Backstory, So How Can Eros & Thanos Be Brothers? Close One of the biggest points of confusion surrounding Thanos and Eros stems from the fact that Marvel Studios changed the Eternals' backstory for their live-action debut. In Marvel Comics, the Eternals are an offshoot of humanity, the result of Celestial experiments on proto-humans which also saw the birth of Deviants and mutants. Despite appearing human, the Eternals have elongated lifespans and superhuman abilities, though they are very much biological beings. In the MCU, however, the Eternals are synthetic beings created by the Celestial Arishem in the World Forge, so it's unclear how Eros, an artificial Eternal, can have a brother.
Eros, a.k.a. Starfox, was a controversial character in Marvel Comics, most notably as he was accused of using his ability to control others' emotions to seduce a married woman, leading to a sexual assault trial which he tried to evade.
Thanos and Eros' connection is much clearer in Marvel Comics, as Thanos was born as an Eternal, though carried the Deviant gene, contributing to his differing appearance to his peers. This meant that Thanos and Eros were biological brothers in Marvel Comics, but the distinction won't be so easy to explain with the MCU's Eternals' new origin story. Even though neither Thanos nor Eros have been confirmed to be returning to the MCU, recent speculation suggests that Thanos may be reappearing soon, so his family could be examined more closely, and one new theory might have already uncovered the truth.
Eros Was Adopted Into Thanos’ Family: Eternals Theory Explained

A new MCU theory shared on Reddit suggests that Eros may have actually been adopted into Titan's community, perhaps becoming Thanos' brother by legal means rather than biological. The theory poses that Eros was one of ten Eternals sent to Titan to ensure the population's growth and the emergence of a Celestial seed inside the planet, similarly to how Ajak's Eternal team was sent to Earth. Eros and his fellow Eternals would have remained on Titan for millennia, and likely would have integrated into Titan's society, so it makes sense that Eros could have been adopted by Thanos' own parents.
Since nothing is known about Thanos' family in the MCU, future Marvel Studios projects may prove or disprove this theory, especially if the Mad Titan returns at some point to wreak havoc again. There has also been no mention of Titan's history outside of Avengers: Infinity War's brief time on Thanos' homeworld, so it's unclear whether a Celestial really was growing inside the planet. However, this theory could provide the perfect explanation for Harry Styles' Eros being described as Thanos' brother, and could answer some major questions from Infinity War about Thanos' own motivations and the fate of his world.
Titan has only been visited in two MCU projects: 2018's Avengers: Infinity War and in a flashback in 2022's Doctor Strange in the Multiverse of Madness.
Titan’s Population Growth Answers Major Thanos Questions

The idea of Eros being integrated directly into Thanos' family on Titan, while also trying to expand the population of the world, may help to answer some huge questions about Thanos' own actions throughout the history of the MCU. Thanos spent years prior to his MCU debut becoming a universe-renowned warlord, as he traveled across the cosmos to invade worlds and half each planet's population. Ultimately, this led to his plan in Infinity War, which saw him collect the Infinity Stones and wipe out half the population of the universe, but his reasoning for this scheme wasn't ever properly explained.
Eros' entire purpose on Titan would have been to grow the population, allowing the Celestial within to one day emerge, but this would have worried Thanos. Thanos may have even learned of Eros' true purpose on Titan, yet was viewed as a madman when he suggested a random culling to stem the population growth and prevent the Celestial from emerging. Perhaps in an effort to stop any other world from suffering Titan's fate, Thanos devised his Infinity War plan, though while he may have been well-intentioned, his methods for thwarting the emergences of further Celestials were extreme and seriously flawed.
Titan’s Infinity War Mystery Can Be Explained By Eternals Close While nothing has yet been substantiated by Marvel Studios, the idea of a Celestial growing inside Titan, and the population growing so out of control that the Celestial was able to emerge, also explains one huge mystery from Avengers: Infinity War that hasn't been addressed. After Iron Man, Spider-Man, Doctor Strange and the Guardians of the Galaxy find themselves on Titan in Infinity War, Peter Quill's Star-Lord claims that the entire planet is eight degrees off its axis, which doesn't make sense as a symptom of overpopulation. Something more catastrophic must have happened to Titan to cause this planetary shift.
One of the few beings in the MCU that can impact the geology of an entire world is a Celestial, and a successful Celestial emergence from within Titan would explain why the planet is eight degrees off its axis, and why the world transformed from a lush and bustling society into a desolate and destroyed wasteland. Some would argue that a Celestial emergence would have torn the entire planet apart, but Tiamut's emergence from inside Earth didn't have global repercussions. It's unclear whether Eternals will get a sequel in the MCU's future, so these questions may, unfortunately, never be answered.