I don't think thats true though. In 1903, Radioactivity had already been proposed as an explanation for the Sun's heat (2 years prior to Annus Mirabilis). The c

I don't think thats true though. In 1903, Radioactivity had already been proposed as an explanation for the Sun's heat (2 years prior to Annus Mirabilis). The connection between E=mc^2 and nuclear reactions doesn't become clear until you start losing mass (proton + neutron) in the reactions, which doesn't happen for the reactions involved in the Sun (Carbon Nitrogen Oxygen Cycle). The connection was recognized by Lise Meitner and Otto Hahn when they discovered fission, where a mass loss does occur. However, they didn't publish their discovery until January 1939; and Hans Bethe had already been working on the Carbon Nitrogen Oxygen Cycle, and didn't need fission to account for the Sun's processes (neutron + proton mass is conserved in the relevant reactions). He was just working off of observed empirical data to make his conclusions.