Janus Prime was a planet located on the edge of the Milky Way, twenty-seven light years from Earth. Janus Prime had a single moon and its orbit directly opposed the orbit of Menda. Janus Prime had glowing blue sand and radiation that destroyed lipids, turning humans into porridge. (PROSE: The Janus Conjunction)
The original Janusians were very advanced race, with great artistic and scientific achievement. They went to war with the inhabitants of Menda. After a long and bloody conflict, the two races overcame their differences and achieved peace.
To ensure that another war never started, they used their advanced technological skills to turn Janus GM2797's solar system into a gigantic bomb. The Eighth Doctor explained this process to Sam Jones:
"It was a terrific feat of planetary engineering. Firstly, they designed two special moons which each had the equivalent of one solar mass. The moons exist in normal space, whereas the masses coexist in hyperspace. By aligning the artificial moons with their sun and linking them via hyperspace, the sun's critical mass would be increased to the point where the nuclear reactions at its centre become unstable and it explodes in a supernova.'"
Another war broke out, but the doomsday weapon was not fully used. The inhabitants of Menda were slaughtered, although they managed to seed Janus Prime with isotope decay bombs. (PROSE: The Janus Conjunction)