In 1802, shortly after discovering 2 Pallas, Heinrich Olbers suggested to William Herschel that Ceres and Pallas were fragments of a much larger planet that once occupied the Mars–Jupiter region, this planet having suffered an internal explosion or a cometary impact many million years before. This hypothesis has fallen from favor. The large amount of energy needed to destroy a planet, and with the belt's low combined mass (only about 4% of the mass of the Moon) do not support the hypothesis. Also, the significant chemical differences between the asteroids are difficult to explain if they come from the same planet. Today, most scientists accept that the asteroids never formed a planet at all.