The History of Doom - S1.E16

Three aliens come to a ruined Earth. Travelling to the Great Hall of Justice, they find a series of memory tapes chronicling the Superfriends and the Legion of Doom. Although they find some origin tapes, they find nothing on the calamity that destroyed the Earth. They find the Legion of Doom HQ and discover that Luthor unleashed a solar flare that interacted with the Superfriends' planet-protecting shield, devastating all life on Earth. The aliens go back in time and alter the timeline with an eclipse so that the flare never makes it to Earth.