Artwork of the formation of the Solar System, according to a new theory by Tim Lichtenberg (and others) at Oxford University. The new theory proposes that planets formed in two stages - first the inner, rocky ones, and then the outer, ice- and gas-rich ones. The inner planets absorbed water as they formed, but outgassed most of it because they were hot from their formation. Previous theories held that the rocky planets developed after the gas- and ice giants, acquiring their water later via asteroid and comet impacts.