With so much else going on, Avengers: Infinity War only gave us a quick sketch as to what happened in Thanos’ past to start him down the quest to destroy half of all life in the universe. It was revealed that his homeworld Titan had perished at some point in time due to overpopulation issues and though he attempted to pitch his extreme method of prevention to his people, he was shunned for his ideas.
The new Infinity War prequel novel, Thanos: Titan Consumed by Barry Lyga, has now filled in the blanks somewhat and clarifies what happened to the planet. Much like Jor-El in the tale of Superman’s origins (sorry, DC reference), Thanos calculated that Titan was ultimately fated to perish and attempted to save it with his radical, murderous notions.
He presented the idea to his father, A’lars, who wouldn’t listen to his obscene plans. Without the support of his dad, Thanos made his findings public and the knowledge that they were doomed naturally sent the Titan population into a frenzy. There were riots, massive suicides…total chaos. The ruling council obviously blamed Thanos for the mass panic and this is how he came to be exiled from his home.
But, as we know, without any measures taken to prevent its extinction, Titan and its people eventually died off, just as Thanos had foreseen. The planet ended up as the barren wasteland that became the site of the Avengers’ penultimate battle with the bad guy.
It’s worth pointing out that the novel isn’t strictly speaking part of official MCU canon. However, it’s certainly based on the narrative of Avengers: Infinity War and fits perfectly well with what we know about Thanos’ past. Until we hear any different in Avengers 4 or elsewhere then, this seems like a solid explanation for the villain’s past.