In "Les époques de la nature" (1778) Buffon discussed the origins of the solar system, speculating that the planets had been created by a comet's collision with the sun. He also suggested that the Earth originated much earlier than 4004 BC, the date determined by Archbishop James Ussher. Basing his figures on the cooling rate of iron tested at his Laboratory the Petit Fontenet at Montbard, he calculated that the age of the earth was 75,000 years. Once again, his ideas were condemned by the Sorbonne, and once again he issued a retraction to avoid further problems.