Powerful flares from the sun may have provided the necessary heat. (discovermagazine.com)
Red dwarf stars are known to generate powerful flares, which can make them not much fun to be around. (science.howstuffworks.com)
That said, one caveat is that some red dwarfs are known to be violently active and to unleash powerful flares of radiation that could destroy any semblance of habitability on a planet. (astronomynow.com)